Genetics and Inheritance Overview

Hey everyone, welcome back. This is going to be another of the Crash Course series, but this is a new one. We're actually going to do the Crash Course Biology series, which I said that I would do because I've already done the physics and chemistry ones. So this one's going to be going through all of the relevant theory that you would need to be able to tackle the biology related questions in section three of the GAMSAT. It's mostly focused at people with a non-science background, but if you've come from a science degree and you need a refresher or you've never actually studied this part, of the science disciplines, then this is pretty much what you'd want to go through. The idea is it's just the key skills that you need to get you over the line for it. You can obviously do wider research on these topics as well, but realistically you really don't need a lot of content knowledge to begin with. a lot of it is actually about using it in order to problem solve with the questions that we get in GAMSA. So we're going to be looking at genetics and inheritance today but I'm also going to go through some other ones on things like cell structure, some of the more technical elements we might look at a little bit in terms of enzymes, membranes, some of the things that come up most commonly so that you've got a kind of intuitive understanding of how it all works so that you can use that in your problem solving and to answer the questions. So we'll get right into it basically and actually I'll mention as well on the resources page I forgot to actually mention this in a previous video I've put together a checklist which is up on my resources page now so you can go there it's a section 3 breakdown a few people asked about you know what's the kind of syllabus there is no real syllabus so this is just based on my findings from it what I think is most likely to come up and it's it's broken down into each of the three disciplines I've also put a short maths checklist in there as well and the stuff that's in green it's the stuff that's probably the highest value to you and you study and I'd probably start there where you can. All right, so what we're going to do is we'll go through, so genes, genetics, and inheritance. So this is a kind of contents map almost for what we're going to go through. One of the other things that I'm going to do a little bit differently in this series is actually write some other GAMSAT related or GAMSAT style questions. I do that in the section three kind of walkthrough videos, but I think previously what I did is mostly focused on the content in these because there's less content to really be focusing on. we're going to look at how to immediately apply it straight into GAMSAT style questions. So it'll be kind of a hybrid of both the theory element of the previous kind of crash courses along with the section three type videos as well, merged together. And hopefully that's really helpful for people. So the first thing we're going to look at is just what is a gene and what I might do here just to make this a little bit more organized. I'm just going to bring this out of the way now like that. So what is a gene? So basically... a gene is just a chunk of DNA. So again, I'll probably go into more of the detail of like what DNA is and all the rest of it and the coding and that kind of stuff in another video. But for now, we can just take DNA segments. of or that code for a particular protein for a particular protein so everything that we're looking at in biology is really just the product of proteins and the expression of a gene into that protein and then you get some physical trait or some kind of outcome so if we took all of the dna in a single cell even though it's not really all in one chunk it's split up across our chromosomes our 46 chromosomes we could just think of a piece of it like this and the idea is that this little piece here this would be a gene so we'll just call this gene A further along we'll have another chunk here and this might be gene B and so on and they each code for different different outcomes or different traits that is the concept of a gene whenever we hear it being used so then the next thing that we have is alleles as gene variants so an allele then there's a little bit of jargon and terminology coming up here but an allele is just a variant of a gene it's kind of in the point there so the idea is that it still is a version of the same gene but it codes for a slightly different outcome because it has a slightly different code or sequence in it in terms of its structure of that dna so if we had like a perfect example would be something like say hair color right so we have one gene that codes for hair color but then we have different variants of it we have brown hair blonde hair black hair red hair right and anything else in between i guess so that would mean that we would have one gene for simplicity and and that would be the hair color gene, hair color gene. But then we would have there, let's just say for simplicity, four variants. And those four variants of that one gene would be blonde, brown, black, and red. And other times you might get other types of combinations of those that bring you a new trait or a new outcome. But let's just say for simplicity at the moment, these are the variants of a particular gene. particular gene. So because we have pairs of chromosomes that means we also have pairs of genes or pairs of alleles so if this particular one here this gene were say located on the first of our 23 chromosome pairs so let's say it's on chromosome number one then we would have two of those chromosomes. We'll just draw them like this for now and not fuss too much about the detail. And let's say that this particular gene sits about here on that lower arm of each of those chromosomes. This one here might be carrying the... blonde allele and this one here might be carrying the brown allele and so we can see that we may be carrying two alleles of a particular gene for simplicity and the combination of those may then produce a particular trait or it may be that say the brown masks the effect of blonde or vice versa that's the idea so that's really what we're going to be looking at and using throughout this and so you can have any combination but obviously you're relying on inheriting those alleles from parents parents so the parents have to be carrying a particular allele to then pass that on to the offspring in each case so if neither parent is carrying say the red allele it's not possible for that red allele to make its way to the next offspring we will see though that you can have a particular trait that is not present but then that can kind of pop up in the offspring based on a particular and less common combination of alleles we'll come back to that though so that Chapman kind of leads us in to genotypes and phenotypes. So again, a little bit more jargon here. So genotypes is the actual name for the combination of alleles in your genetic makeup for a particular gene. So again, let's stick with hair color. Let's say we'll give, we usually give letter codes to each of these alleles. Let's do brown and red. So we'll do brown hair and red hair. And what's going to happen is brown is going to get capital. B and red is going to get lowercase b so just like that and so what this then comes into is these two alleles this particular one we usually give the capital letter to what's called the dominant allele and what that means is it's the allele that is more dominant than the other and it masks the effect of the other when they are paired together so if you have a mix of the two only the dominant genome or the only the dominant allele will express its particular trait and so the red is going to be what's called recessive so it recedes into the distance it disappears when there's a mix and so then these are the individual allele notations so our genotypes that are possible in this case for this particular trait is you could either have two of the capital b you could have one of the capital b's and one of the lowercase b's we could have both lowercase in this case right and again each of those letters is a contribution from the two parents in that line individual. So this particular one would be referred to as homozygous coming from the zygote homozygous same so both the same but both the same of the dominant trait so homozygous dominant and because both of those dominant alleles code for brown hair this individual would have brown hair let me just write that correctly brown hair whereas this individual over here this would be at what what's called a heterozygote. So heterozygote and hetero meaning different. So heterozygous and we don't have to say dominant. It's implied that because there's a mix of red and brown, the brown dominant allele is going to dominate over the red. And we're going to see this individual also have brown hair. They would look physically exactly the same as the homozygous dominant in terms of hair color. So brown hair. And then finally over here, we have the homozygous again because they're both the same. same but this time it's homozygous recessive because they're both the recessive allele and because there's no dominant allele masking anything or dominating that means both red contributions lead to red hair like that and so what we've got here is the gene genotypes. So these, I'll just box these, these would be the genotypes that are possible. So it's literally just the code or the mix or combination of alleles in the individual. And I'll do in green here, the phenotypes are the physical outcomes. would be the phenotypes in this particular case. And we can see there's a bit of an issue because two different genotypes here lead to the exact same phenotype. So we'll look a little bit more at that as well. But that is how we use those terms in that case. So a nice easy way to look at it is genotypes to do with the genes, phenotypes to do with the physical trait that we get or the physical outcome in that case. All right, and there we go. So then the next thing that we... have is Punnett squares. So Punnett squares now it's really just a problem-solving skill. It's not really a direct biology piece of content. It's a way of representing possible outcomes from particular matings or crossings of two individuals. And so the way that we do these, I'll get these out of the way. like that is we're kind of just looking at every possible combination so say we crossed an individual that was homozygous dominant like this for brown hair and then we mated them with a heterozygote individual like this we draw up a table and along the top we're going to put let's say that we have the male and the female that's not like that is it it's like that There we go. Haven't done those symbols in a while. So male and female. And so the male individual, let's say that this is the male individual over here, and this is the female individual, the heterozygote. So the male, when they actually then produce sperm cells, they can only contribute one of their two alleles per sperm cell, right? And again, we might get into more of that in detail, but it's not 100% relevant. We'll do a little bit more on cell division in another video though. So the idea is that we can only have one or the other. so we either have the capital B, let's say the one on the left, or we have the capital B from here, but they're actually identical. We'll leave them in there for now though. Then with the female, they can produce two different egg cells. So they could either produce the capital B carrying the brown allele, or they could have the lowercase b carrying the red. And now what we've got is every possible combination of offspring. So if it were to be this particular sperm that were to meet this particular egg, then we end up with... the homozygous dominant and you can now see that you've got this matrix or this array of different possibilities and we've now got every possible possibility so we can see that out of the four different options two of them are actually identical these would be homozygous dominant and they would have brown hair right these two down here would be heterozygous but they would also have brown hair because remember that the brown dominant allele is going to mask the effects of the lowercase red in both cases So what that would mean is that we would expect that in that individual, 100% of the offspring should have brown hair, and we should see no red hair in that particular case. In terms of the ratios of the homozygous versus the carriers, it's a 50-50 split. And to make that a little bit easier, wherever you have double ups on the sperm or egg, you can pretty much just cross it out. And it just brings it down in this case to a two by one table. And you can see it's a one to one ratio or 100% brown hair. like that now if we change that up a little bit though let me just save the table and i'll get rid of this here so say now that we've got the the male as well is also going to be a carrier so they've both got brown hair technically both parents but they're both carriers of the red so brown and brown and so now all of a sudden we've got different sperm and egg and so if we do that combination now we get a couple of heterozygous carriers and for the first time now we get lowercase b and lowercase b there is no interference of the dominant allele so we have a brown haired individual here a brown haired carrier possible here a brown haired carrier here and this would be a red haired individual remember that this is not actually for children or for offspring it would be for possible outcomes for that particular gene in a potential offspring and so what that shows now is that that there's a three to one ratio of the dominant to the recessive or the brown to the red and that would mean that there is a 25 chance of red hair in this particular crossing and this is what i was talking about before where you can have both parents neither of them have red hair yet their child might have red hair we see this all the time like blue eyes and that kind of thing where you have recessive traits um it can pop up if both parents were actually kind of uh unknowingly carriers of that particular trait and there we go and we can expect a 25 chance of that occurring and it starts to get into a little bit of probability as well that's where we can bring in some basic maths principles here and so this here is referred to as a mono hybrid cross mono because it's one gene hybrid because you're mixing the two different genotypes together and across just to say a mating of two individuals we can also do that for dihybrid crosses where we're considering two genes at the same time so let's also do that but what I'll do is leave that up for the moment because I might need to come back to it so this time we'll do the dihybrid cross and the class This one is just like... The pea plants from the Mendelian genetics where he talked about the wrinkled and the smooth and the green and the yellow. I'm not going to go with that one because it's like the classic textbook one. This time what I'm going to do is go with flowers. And we're going to look at two different things. We're going to look at petal shape and then petal color. And so the idea is this is gene number one and this is gene number two. And so at petal shape, we're going to have two different types of shape. We're going to have pointed, which is going to be capital P, and then we're going to have rounded which is going to be lowercase p, which is going to be a little bit annoying. In terms of notation, normally the letter is actually chosen by the recessive trait, and then the dominant just gets the capital of that letter, but just watch out for whatever ace are actually doing in the question. Sometimes they might choose to have the letter determined by the dominant trait as well. The capital letter is always indicating the dominant trait. And then gene two. with color let's say we have red and I'll do capital R for that one and then we'll do white as lowercase R like this these are the different possible outcomes and then you can get any kind of mixing and matching of four different outcomes there so again let's make both the parents double heterozygotes meaning that they're carriers of at both particular genes so what that means now is we would have the genotypes of parent one would be capital P lowercase p and we put a little semicolon between them you're not going to have to do much writing though so like it's multi-choice so don't fuss too much about notation and then same thing the other parent will be carriers at both loci like that. I'll keep that on screen there. So if we do the table, it becomes a four by four table now. And so really every possible combination is we have to consider the contributions at both genes. So I like to just do it almost like an expansion like this, like from maths like that. And so therefore the first possible possibility is both capital. So I'll put a capital P, I'll put a little colon in between capital R. Then I could have capital P lowercase R then I could have lowercase P I'll try to exaggerate that as well so it's clear capital R and then I could have both lowercase as well it's a bit messy lowercase P lowercase R and it would be the exact same in the other parent because it's the same combination in this case so we'd have capital P, capital R, capital P, lowercase R, lowercase P, capital R, lowercase P, lowercase R. And there we go. And so now we get our four by four. And so you could go through, you could do the whole thing like this, and I might just fast forward this because otherwise it'll be kind of boring to watch through it. So give me just a second. And so from there now there's a couple of different patterns that you can get from this that help fill this in although again you're probably never going to actually have to fill one of these in it's just good to know how they work because you may get presented with something similar but you can see that the double heterozygote just like the parents runs diagonally like this and if you actually track all the phenotypes you'll find that the if you do a big triangle all of these individuals will all have exactly the same phenotypes as each other they'll all have the double dominant phenotype so pointed and red whereas then we get a little triangle going this individual this individual and this individual these all have the same phenotype as each other but they're slightly different and i think i just mucked that one up i did that's why i see this is where knowing the patterns is really helpful because you catch mistakes there we go so these would be uh pointed for the shape but the color would be white you can see they've all got the lowercase r And so then again you can see it kind of points out a little triangle But I'll get rid of that one and then finally we get another triangle so these three Here and see that little triangle forming there these ones all have the recessive trait at the first But then the dominant at the second so that is rounded but red and then finally you have in the corner the double recessive rounded and white like that so you get these triangles kind of banking towards the bottom right corner so it's good to know some of those patterns in there but this is it and so then when you count them out you'll find as well there's some ratios. So the double dominant is going to be nine of the different outcomes. Then the single dominant to recessive is going to be three of the outcomes. That's the yellow. And the green is recessive to dominant, or recessive and dominant, which is three. And then we have the double recessive, which is one. We get this ratio of phenotypes nine to three to three to one. It's not really all that helpful, but in terms of probability, the probability of, say, the double recessive is going to be one out of 16 different outcomes. outcomes like that you can start calculating probabilities the chances of them getting you to fill in a punnett square though is very unlikely that's really time consuming they'll probably not do uh like a dihybrid they may get you to use that for a monohybrid like we did before and just do simple calculations out of four and stuff like that but there we go so that is how the dihybrid cross works and this is you can see it's not really content it's more so an application of it it's problem solving So understanding how they work can be quite useful. Then we have dominance patterns. So this whole time we've been assuming that the dominant trait completely masks the effect of the recessive one. And so this is what we've been talking about here with complete dominance. But you can also have incomplete dominance. And so what this means is that you kind of have the dominant trait not fully able to dominate the other trait. So instead it kind of blends with it. And a really good example here is... is to talk about like paint right and paint colors so if you put you know black paint over the top of white paint then it'll pretty much be able to completely mask it or dominate it right and it'll just turn the wall black if you put green paint over blue paint it'll look greenish right that concept of mixing colors so you get a new outcome or a new phenotype because of the fact that there was not an ability to completely dominate the other and so incomplete dominance is exactly that that is where you get a new phenotype and you instead get a paint mixing or a blending of the two original phenotypes so if we use the petal color for example red and white would make pink if there was incomplete dominance of the red and then co-dominance this is where they cooperate so this is where you'll see a bit of both so instead in the you Petal color, for example, you would see spots of red and spots of white. You wouldn't see pink, though. So they actually both get to compete in that sense. So they cooperate and you see both. And they'll always explain which type of dominance pattern. it is or they'll give indications as to which type it is based on that information then we have where it's linked so we've mostly been looking at what are called autosomal traits meaning they're on the chromosome pairs 1 to 22 and they are all what's called autosomes meaning they're not related to the sex of the individual at all whereas sex-linked chromosomes or sex chromosomes are the 23rd pair and in males males have an x and a y chromosome whereas females have just both x's and this has pretty significant impacts on the likelihood of a particular trait if that gene is located on one of those chromosomes so if it's x-linked means that it is on the x chromosome and what that means as well if i just move this bit out of the way oh didn't take all of it with it oh so there we go so x-linked traits if we have something on an x chromosome let's say we have a female and it's sitting here and here if they were to have say a capital and a lowercase like that then they would be uh they would be a heterozygote carrier and if it's dominant then they would have the a trait the capital a trait that is right if we had a male though because the second chromosome is not an x chromosome it can't have that particular gene because it doesn't have the information for it so it means that it becomes more likely for them to actually get get the recessive trait so if we have lowercase a normally you would need two of the lowercase a to have that particular phenotype but in this case because there's only one they actually do express just that trait because there is no dominant capital a to dominate over it and so this makes this more likely at that point it kind of lowers the bar in a sense and so these individuals are called hemizygous if you ever see that word you Just means they're not really a homozygous individual because they don't have two of that. They're not heterozygous because they don't have one of each. They're hemizygous. They've got half. They've got just one of it. And the same thing if you had a male that had the capital A variant. that location then they would have they would express that particular phenotype and there would be hemizygous for that as well so that you can imagine has particular consequences on the likelihood of that in male versus female individuals Y-linked, obviously not possible in a female at all. These are going to be genes that are on the Y chromosome, so they are male-specific in that case. And then finally we have test crosses and hypothesis testing. And this is where now it becomes a little bit less about biology and more about using logic and using things like Punnett squares and a basic understanding of inheritance. to be able to answer new complex fresh questions. So a test cross is the mating or the crossing of a homozygous recessive individual to another individual where we don't know their genotype and from the offspring we can start to determine what the genotype of that unknown individual is. So say we have someone who we don't know what their genotype is so I'm just going to put two question marks but we're looking at a particular particular particular gene that has these alleles right and if we cross that with a homozygous recessive so both lowercase a we can see what the outcome of that crossing is so if for example that individual were to be capital capital we would know that they would be either the dominant or the recessive let's say that this is albinism Alright, so no skin pigmentation, pale skin, like kind of silvery white hair and red eyes. So this individual here maybe does not have albinism, right? Because albinism is a recessive trait. So in order to have it, you have to be... homozygous recessive. Say this individual here is non-albinism so they are expressing pigmentation but we don't know if they are a homozygous dominant so capital capital or if they're just a carrier instead so we have to consider both outcomes. If they were this individual and we do our Punnett square You can see that each individual can only produce one type of sperm or egg. So this is a double up and this is a double up here. And we can just look at this combo. Everything should just be carrier individuals. So what that would mean is... that it would be 100% non-Albinism because even though they're a carrier themselves, the offspring, they still need to have both lowercase in order to actually express Albinism. And so therefore, if we crossed those... two individuals a hundred times and then we had a hundred offspring then all those offspring should be non-albinism if the unknown parent is homozygous dominant. If however they were actually homozygous or heterozygous I should say and we crossed them with that homozygous recessive individual then we've got a couple of different options. We can see now their gametes or the cells that they're producing are different, so we consider both. In the other individual, they're both the same, so we can just cross out the bottom row. And we can see now there's two different outcomes. This here would be non-albinism, but this individual would be a homozygous recessive, and these would express albinism. And it would be a 50-50 split if we were to use a large sample size. So again, we would expect roughly 50 of those 100 outcomes to be albino and then the other 50 to be non at that point. So based on the outcome, we can then determine which of these cases occurred. for the genotype of the unknown individual. And this is the concept of a test cross. So hypothesis testing, this would be like, it's nothing specific. It would just be if they give you a bunch of information about a particular trait. and then they ask you, you know, which of the following experiments would test a particular hypothesis. And a lot of the time you've got to think about the logic of whether or not it would produce a discernible difference in each case. That's all I'll really say on that one there. We kind of go into that a little bit more in the questions now anyway, and that's what we'll jump to. So if I come back up, I think it was quite a way up now. Here we go. I'll just zoom that out a little bit. So this question that I've put together, just like all my other Section 3... sample questions and everything we've got coat color in cats is the result of a complex combination of a number of genes and gene variants the browning gene codes for tyrp1 an enzyme involved in the metabolic pathway for eumelanin pigment production its dominant form b will produce black eumelanin it has two recessive variants that's just a little typo from me there b lowercase b which is going to be the chocolate variant and then bi which which is going to be the cinnamon variant. So we've got a little bit more complexity this time. With B.I. being recessive to both B and lowercase b. Chocolate is a rich brown colour and is referred to as chestnut in some breeds. Cinnamon is a lighter reddish brown. And then we've also got the sex-linked orange locus determines whether a cat will produce eumelanin. In cats, the orange fur, or with orange fur, pheomelanin, I'm hoping I'm saying that right, or red pigment, completely replaces eumelanin, black or brown. This gene is located on the X chromosome. The orange allele is capital O and is co-dominant with non-orange O. So we can see all of that kind of terminology and jargon is now coming up. The heterozygote produces a tortoiseshell coat and this... masks the effect of the browning gene. So there's a lot of interaction going on here. I've made this particularly difficult. I always categorize the difficulty of these and this will go up on my resources page. I've put this in the very difficult category. I was umming and ahhing about even putting it in the nightmare. category that I've got on there as well which I've not yet used. So a study was done on a population of a particular breed of cat by crossing a cinnamon female with a black male and it says how many different coat types are possible in this in the offspring of this particular crossing one two three or four so whenever you're doing these i always kind of just look for all right what are the possible genotypes and i'm going to do little mini crosses on the paper in each case so the first thing is i look to the parents right so it says a particular breed of cat crossing a cinnamon female so cinnamon was uh recessive to both so if it's recessive it means in order to have a cinnamon cat they must be homozygous recessive so i'm just going to do that now i know that genotype. The next thing is there was also the orange element to it but there's no mention of orange in this particular question so I get to just ignore that. That means that gene is irrelevant and I don't have to overthink it and we're crossing that with a black male cat. Now black on the other hand that was dominant to both brown and also to cinnamon so we don't really know what the possible outcomes are here. So the black individual it could have been homozygous dominant black it could have been dominant over brown it could have also been dominant over cinnamon as well so i have to actually look through all of these because it just says which are possible with the information given so the good thing is the female cat cannot give more than a cinnamon right that's the only egg cell that they can produce so therefore all of the combinations are going to come from what goes with that from the father and so if the father was this individual individual here, they can only produce capital B, and so therefore you'd get capital B, lowercase B, and cinnamon, I should say. So that would be black dominating over cinnamon, so we get a black cat. That is one possible coat type. The next is the heterozygous carrier male. We've already considered what happens if the dominant mixes, but there they could contribute the lowercase b, and lowercase b, that would give us lowercase b and the cinnamon, and lowercase b was chocolate. that is also dominant over cinnamon so that would give us a chocolate cat and then finally down the bottom here we would have again we've already considered the capital but it could have been a cinnamon carrier cat and if we put those together we get a cinnamon cat and so in that case we've got three different outcomes and so there are three possible coat types there we go all right cool i'll just lock that so i can you Move some of that writing. Just move that down there. All right. And so then question two says a male cat that is the offspring of a tortoise shell colored female and a chocolate male cannot be. Which of the following? So again, breaking down where is this information coming from? So. a tortoise shell female so tortoise shell was when you had a heterozygous co-dominance between capital o and lowercase o so we know that they're that the other thing it said was that the tortoise shell masks the effects of the browning gene. So they could be underlying with anything from black to brown to cinnamon. We have no idea there. And then we're crossing that with a chocolate male. So the chocolate male, they have not any orange or anything in them. So what that means is, and it is also X-linked, if the male does not have any orange color to them, then that would mean that they have O, the lowercase O, and then a Y chromosome, which is not controlled. contribute we mix that then they're chocolate so chocolate could either be uh not capital b it could be lowercase b b like that or it could be dominant to a cinnamon as well these are the both the possibilities for the male and then for the female let's go back there again it didn't say about what it is so it could have been this it could have been this could have been this could have been this or it could have been cinnamon being masked as well Lots and lots of different possibilities. But what it's really focusing on is a male cat that is produced by these two individuals. So we don't really want to have to go through all of those. That's clearly too much information. But remember that male cats, first of all, are XY. So that means that they cannot be homozygous or heterozygous, for that matter, for the orange trait. So I'd start there at that point. That will actually lead me to the answer, because if I looked at the orange trait, I can see tortoise shell is mentioned here, but that would have to be capital O lowercase o, like the mother, but male cats don't have two X chromosomes. So that immediately. is going to be my answer. The other thing though is you can imagine that the male cat a chocolate chocolate colored if that were to be the case there that is possible because we could have both non-orange and non-orange go together at that point or actually no sorry it would be non-orange and y because it has to make a male cat like this so we can get mix and match from there and there without a punnet square so there we go a non-orange male cat and then in terms of making chocolate that's possible because we could just get like a lowercase b from here and maybe a lowercase b from there and then it would be chocolate so that one's out a homozygous chocolate we just proved that could get that from one from each of the parents so that's also out and then a carrier of bi the cinnamon and that is also possible because you could get that capital b and then maybe a cinnamon from there and then the cinnamon would be masked so it was easier just to go for the correct answer but we could also rule it out as well and then question three if the genotype of a black cat is to be determined via repeated crossing with another cat what would be the best option so we've got a black cat that is dominant for the color right and we want to know is it homozygous or is it heterozygous that's hinting at maybe a test cross so with a test cross you you want to go with a recessive trait. So therefore black is not recessive. So you don't want to go with that. Neither is chocolate, because even though it's recessive to black, it's dominant to cinnamon. Cinnamon is the true recessive, so that would be hinting at the answer. And tortoiseshell is no good, because it masks what the actual browning gene is doing altogether. And so it would be a complete wild card in that sense. So therefore the cinnamon cat would be the best option. Notice though it's not using the... the word test cross so i don't think that asa would ever really expect people to know technical words unless they introduce them in the stem so instead of just set it up as a problem solving thing and then question four a mutation that occurs in approximately one in every 350 000 live births of cats involves the orange gene producing the orange phenotype in the heterozygous carrier that is instead of the tortoise shell you don't get the co-dominance you then get complete dominance of the orange gene. So what that means is normally this would be the tortoise shell type. And what's happening now is it's leaning towards just orange as if it were to be homozygous dominant. And so if a tortoise shell cat were crossed with a chocolate male carrying the cinnamon allele, the chance of producing an orange female cat is what? Now, again, that's a lot of information to take on, right? So it's a probability question. so the first thing is i would look at what the genotypes of the parents are going to be so a tortoiseshell cat uh were crossed with a chocolate male now remember tortoiseshell is female only anyway so i didn't even have to say male and you kind of already know which the genders or which of the sexes of the cats would be so if a tortoiseshell cat so what that means is they are like this on each of the X chromosomes. And let's see, that would be masking what color they are. So we don't know what the browning gene is doing. And we're crossing that with a chocolate male. is carrying the cinnamon so they don't have any orange so they would be lowercase o and then a y chromosome not contributing and we're told that they're chocolate so they've got lowercase b but they're carrying cinnamon so that was bi and then it says what's the chance of producing an orange female cat so clearly this is involving the the mutation otherwise why mention it but we first have to look at what's the chance of the female orange cat occurring right so to get a female orange cat you can see without the mutation it's actually not going to be possible because you need both capital o's and the father does not have a capital o to produce so the best that we could get is we could get this mixed with This capital mixed with this lowercase which would produce tortoiseshell and then the chance carrying a cinnamon, the chance of producing an orange female, that's really it there. And then it doesn't really matter what's happening. at the other spots because then it will mask it right so from that we would probably want something recessive or actually it wouldn't matter so the chances of this occurring is one in four because we've got either if i write them out like this It could be this mixed with this, this mixed with this, this mixed with this, or this mixed with that. And so the only one that gave us that outcome was this one here, that was one out of four possibilities. We multiply that then, because that has to happen, and... a completely independent event the second thing that has to happen is we have to have the mutation so that then it converts this and makes that into an orange phenotype and that was one in 350 000 like that and then that was be it this would be the whole thing so then from there if we do that that would then be 1 out of and then we have to just do 4 times 350,000 so we double that to 700,000 double it again to 1.4 million. Just like that, we get one out of 1.4 million. So the answer then becomes C. There we go. Cool. Now you might be overthinking it and think, well, what about the chance of a male or a female offspring? We've actually already factored that in here because... the combinations of this is an x this is an x this is an x this is a y the combinations of x and y is already taking care of the sex of the offspring in that case so we don't have the times by half as well and there we go so we get one out of one point formula million and then that's the answer so that's a bit more of a mathsy type question but notice that again you wouldn't be able to really work with that if you didn't have a working understanding of how inheritance works and what all of those words mean i will point out that is not the level of difficulty that you should always expect that would absolutely be up the top end of a difficult question i'm quite proud of it because i always like when i write really difficult questions that work because it's bloody hard to do it but um yeah if that kind of makes sense then you're on to a good start at that point hopefully this has all kind of wrapped everything up for you in terms of inheritance like i said i try to keep this just to the stuff that you actually need rather than going into a bunch of irrelevant stuff if there was anything that was mildly irrelevant it might be the dihybrid cross i wouldn't be too fussed about that understanding how to do a monohybrid cross and a punnett square is useful but that's really it so i think i'll leave that one there if you've got any questions for me or any recommendations for what you'd like to see let me know as always always but i think biology kind of links one to the other so i think the next one that i'll do is probably going through some basics of cell biology and the cell membrane going through the structure of dna as well because that one does come up quite frequently and then we'll be able to move to some other things like the different body systems i'm probably just going to do a big gigantic video on that one that just goes through all of them because it may actually be quite short it's really not that much that you need to know about them but that should be a pretty fun one because that's probably my favorite part of biology and I always like teaching it as well so I'll get to that one as well and we'll do it like this with the bio ones every time where I'll have some sample questions to work through so you can kind of see how the theory gets applied as well perfect all right easy well hopefully this is all helpful as always leave me some feedback like the video subscribe if you haven't already because I can see that people are watching I try to put up as much stuff as possible and that's probably the quickest way to see what I'm putting out not missing anything because you video gets buried pretty quickly. Anyway, I will see you in the next one and yeah, that's it.

So this one's going to be going through all of the relevant theory that you would need to be able to tackle the biology related questions in section three of the GAMSAT. It's mostly focused at people with a non-science background, but if you've come from a science degree and you need a refresher or you've never actually studied this part, of the science disciplines, then this is pretty much what you'd want to go through. The idea is it's just the key skills that you need to get you over the line for it. You can obviously do wider research on these topics as well, but realistically you really don't need a lot of content knowledge to begin with.

a lot of it is actually about using it in order to problem solve with the questions that we get in GAMSA. So we're going to be looking at genetics and inheritance today but I'm also going to go through some other ones on things like cell structure, some of the more technical elements we might look at a little bit in terms of enzymes, membranes, some of the things that come up most commonly so that you've got a kind of intuitive understanding of how it all works so that you can use that in your problem solving and to answer the questions. So we'll get right into it basically and actually I'll mention as well on the resources page I forgot to actually mention this in a previous video I've put together a checklist which is up on my resources page now so you can go there it's a section 3 breakdown a few people asked about you know what's the kind of syllabus there is no real syllabus so this is just based on my findings from it what I think is most likely to come up and it's it's broken down into each of the three disciplines I've also put a short maths checklist in there as well and the stuff that's in green it's the stuff that's probably the highest value to you and you study and I'd probably start there where you can. All right, so what we're going to do is we'll go through, so genes, genetics, and inheritance.

So this is a kind of contents map almost for what we're going to go through. One of the other things that I'm going to do a little bit differently in this series is actually write some other GAMSAT related or GAMSAT style questions. I do that in the section three kind of walkthrough videos, but I think previously what I did is mostly focused on the content in these because there's less content to really be focusing on.

we're going to look at how to immediately apply it straight into GAMSAT style questions. So it'll be kind of a hybrid of both the theory element of the previous kind of crash courses along with the section three type videos as well, merged together. And hopefully that's really helpful for people. So the first thing we're going to look at is just what is a gene and what I might do here just to make this a little bit more organized. I'm just going to bring this out of the way now like that.

So what is a gene? So basically... a gene is just a chunk of DNA. So again, I'll probably go into more of the detail of like what DNA is and all the rest of it and the coding and that kind of stuff in another video. But for now, we can just take DNA segments.

of or that code for a particular protein for a particular protein so everything that we're looking at in biology is really just the product of proteins and the expression of a gene into that protein and then you get some physical trait or some kind of outcome so if we took all of the dna in a single cell even though it's not really all in one chunk it's split up across our chromosomes our 46 chromosomes we could just think of a piece of it like this and the idea is that this little piece here this would be a gene so we'll just call this gene A further along we'll have another chunk here and this might be gene B and so on and they each code for different different outcomes or different traits that is the concept of a gene whenever we hear it being used so then the next thing that we have is alleles as gene variants so an allele then there's a little bit of jargon and terminology coming up here but an allele is just a variant of a gene it's kind of in the point there so the idea is that it still is a version of the same gene but it codes for a slightly different outcome because it has a slightly different code or sequence in it in terms of its structure of that dna so if we had like a perfect example would be something like say hair color right so we have one gene that codes for hair color but then we have different variants of it we have brown hair blonde hair black hair red hair right and anything else in between i guess so that would mean that we would have one gene for simplicity and and that would be the hair color gene, hair color gene. But then we would have there, let's just say for simplicity, four variants. And those four variants of that one gene would be blonde, brown, black, and red. And other times you might get other types of combinations of those that bring you a new trait or a new outcome. But let's just say for simplicity at the moment, these are the variants of a particular gene.

particular gene. So because we have pairs of chromosomes that means we also have pairs of genes or pairs of alleles so if this particular one here this gene were say located on the first of our 23 chromosome pairs so let's say it's on chromosome number one then we would have two of those chromosomes. We'll just draw them like this for now and not fuss too much about the detail. And let's say that this particular gene sits about here on that lower arm of each of those chromosomes.

This one here might be carrying the... blonde allele and this one here might be carrying the brown allele and so we can see that we may be carrying two alleles of a particular gene for simplicity and the combination of those may then produce a particular trait or it may be that say the brown masks the effect of blonde or vice versa that's the idea so that's really what we're going to be looking at and using throughout this and so you can have any combination but obviously you're relying on inheriting those alleles from parents parents so the parents have to be carrying a particular allele to then pass that on to the offspring in each case so if neither parent is carrying say the red allele it's not possible for that red allele to make its way to the next offspring we will see though that you can have a particular trait that is not present but then that can kind of pop up in the offspring based on a particular and less common combination of alleles we'll come back to that though so that Chapman kind of leads us in to genotypes and phenotypes. So again, a little bit more jargon here.

So genotypes is the actual name for the combination of alleles in your genetic makeup for a particular gene. So again, let's stick with hair color. Let's say we'll give, we usually give letter codes to each of these alleles.

Let's do brown and red. So we'll do brown hair and red hair. And what's going to happen is brown is going to get capital. B and red is going to get lowercase b so just like that and so what this then comes into is these two alleles this particular one we usually give the capital letter to what's called the dominant allele and what that means is it's the allele that is more dominant than the other and it masks the effect of the other when they are paired together so if you have a mix of the two only the dominant genome or the only the dominant allele will express its particular trait and so the red is going to be what's called recessive so it recedes into the distance it disappears when there's a mix and so then these are the individual allele notations so our genotypes that are possible in this case for this particular trait is you could either have two of the capital b you could have one of the capital b's and one of the lowercase b's we could have both lowercase in this case right and again each of those letters is a contribution from the two parents in that line individual. So this particular one would be referred to as homozygous coming from the zygote homozygous same so both the same but both the same of the dominant trait so homozygous dominant and because both of those dominant alleles code for brown hair this individual would have brown hair let me just write that correctly brown hair whereas this individual over here this would be at what what's called a heterozygote.

So heterozygote and hetero meaning different. So heterozygous and we don't have to say dominant. It's implied that because there's a mix of red and brown, the brown dominant allele is going to dominate over the red.

And we're going to see this individual also have brown hair. They would look physically exactly the same as the homozygous dominant in terms of hair color. So brown hair.

And then finally over here, we have the homozygous again because they're both the same. same but this time it's homozygous recessive because they're both the recessive allele and because there's no dominant allele masking anything or dominating that means both red contributions lead to red hair like that and so what we've got here is the gene genotypes. So these, I'll just box these, these would be the genotypes that are possible.

So it's literally just the code or the mix or combination of alleles in the individual. And I'll do in green here, the phenotypes are the physical outcomes. would be the phenotypes in this particular case. And we can see there's a bit of an issue because two different genotypes here lead to the exact same phenotype.

So we'll look a little bit more at that as well. But that is how we use those terms in that case. So a nice easy way to look at it is genotypes to do with the genes, phenotypes to do with the physical trait that we get or the physical outcome in that case.

All right, and there we go. So then the next thing that we... have is Punnett squares. So Punnett squares now it's really just a problem-solving skill.

It's not really a direct biology piece of content. It's a way of representing possible outcomes from particular matings or crossings of two individuals. And so the way that we do these, I'll get these out of the way. like that is we're kind of just looking at every possible combination so say we crossed an individual that was homozygous dominant like this for brown hair and then we mated them with a heterozygote individual like this we draw up a table and along the top we're going to put let's say that we have the male and the female that's not like that is it it's like that There we go. Haven't done those symbols in a while.

So male and female. And so the male individual, let's say that this is the male individual over here, and this is the female individual, the heterozygote. So the male, when they actually then produce sperm cells, they can only contribute one of their two alleles per sperm cell, right?

And again, we might get into more of that in detail, but it's not 100% relevant. We'll do a little bit more on cell division in another video though. So the idea is that we can only have one or the other. so we either have the capital B, let's say the one on the left, or we have the capital B from here, but they're actually identical.

We'll leave them in there for now though. Then with the female, they can produce two different egg cells. So they could either produce the capital B carrying the brown allele, or they could have the lowercase b carrying the red.

And now what we've got is every possible combination of offspring. So if it were to be this particular sperm that were to meet this particular egg, then we end up with... the homozygous dominant and you can now see that you've got this matrix or this array of different possibilities and we've now got every possible possibility so we can see that out of the four different options two of them are actually identical these would be homozygous dominant and they would have brown hair right these two down here would be heterozygous but they would also have brown hair because remember that the brown dominant allele is going to mask the effects of the lowercase red in both cases So what that would mean is that we would expect that in that individual, 100% of the offspring should have brown hair, and we should see no red hair in that particular case. In terms of the ratios of the homozygous versus the carriers, it's a 50-50 split. And to make that a little bit easier, wherever you have double ups on the sperm or egg, you can pretty much just cross it out.

And it just brings it down in this case to a two by one table. And you can see it's a one to one ratio or 100% brown hair. like that now if we change that up a little bit though let me just save the table and i'll get rid of this here so say now that we've got the the male as well is also going to be a carrier so they've both got brown hair technically both parents but they're both carriers of the red so brown and brown and so now all of a sudden we've got different sperm and egg and so if we do that combination now we get a couple of heterozygous carriers and for the first time now we get lowercase b and lowercase b there is no interference of the dominant allele so we have a brown haired individual here a brown haired carrier possible here a brown haired carrier here and this would be a red haired individual remember that this is not actually for children or for offspring it would be for possible outcomes for that particular gene in a potential offspring and so what that shows now is that that there's a three to one ratio of the dominant to the recessive or the brown to the red and that would mean that there is a 25 chance of red hair in this particular crossing and this is what i was talking about before where you can have both parents neither of them have red hair yet their child might have red hair we see this all the time like blue eyes and that kind of thing where you have recessive traits um it can pop up if both parents were actually kind of uh unknowingly carriers of that particular trait and there we go and we can expect a 25 chance of that occurring and it starts to get into a little bit of probability as well that's where we can bring in some basic maths principles here and so this here is referred to as a mono hybrid cross mono because it's one gene hybrid because you're mixing the two different genotypes together and across just to say a mating of two individuals we can also do that for dihybrid crosses where we're considering two genes at the same time so let's also do that but what I'll do is leave that up for the moment because I might need to come back to it so this time we'll do the dihybrid cross and the class This one is just like...

The pea plants from the Mendelian genetics where he talked about the wrinkled and the smooth and the green and the yellow. I'm not going to go with that one because it's like the classic textbook one. This time what I'm going to do is go with flowers.

And we're going to look at two different things. We're going to look at petal shape and then petal color. And so the idea is this is gene number one and this is gene number two.

And so at petal shape, we're going to have two different types of shape. We're going to have pointed, which is going to be capital P, and then we're going to have rounded which is going to be lowercase p, which is going to be a little bit annoying. In terms of notation, normally the letter is actually chosen by the recessive trait, and then the dominant just gets the capital of that letter, but just watch out for whatever ace are actually doing in the question.

Sometimes they might choose to have the letter determined by the dominant trait as well. The capital letter is always indicating the dominant trait. And then gene two.

with color let's say we have red and I'll do capital R for that one and then we'll do white as lowercase R like this these are the different possible outcomes and then you can get any kind of mixing and matching of four different outcomes there so again let's make both the parents double heterozygotes meaning that they're carriers of at both particular genes so what that means now is we would have the genotypes of parent one would be capital P lowercase p and we put a little semicolon between them you're not going to have to do much writing though so like it's multi-choice so don't fuss too much about notation and then same thing the other parent will be carriers at both loci like that. I'll keep that on screen there. So if we do the table, it becomes a four by four table now.

And so really every possible combination is we have to consider the contributions at both genes. So I like to just do it almost like an expansion like this, like from maths like that. And so therefore the first possible possibility is both capital. So I'll put a capital P, I'll put a little colon in between capital R.

Then I could have capital P lowercase R then I could have lowercase P I'll try to exaggerate that as well so it's clear capital R and then I could have both lowercase as well it's a bit messy lowercase P lowercase R and it would be the exact same in the other parent because it's the same combination in this case so we'd have capital P, capital R, capital P, lowercase R, lowercase P, capital R, lowercase P, lowercase R. And there we go. And so now we get our four by four.

And so you could go through, you could do the whole thing like this, and I might just fast forward this because otherwise it'll be kind of boring to watch through it. So give me just a second. And so from there now there's a couple of different patterns that you can get from this that help fill this in although again you're probably never going to actually have to fill one of these in it's just good to know how they work because you may get presented with something similar but you can see that the double heterozygote just like the parents runs diagonally like this and if you actually track all the phenotypes you'll find that the if you do a big triangle all of these individuals will all have exactly the same phenotypes as each other they'll all have the double dominant phenotype so pointed and red whereas then we get a little triangle going this individual this individual and this individual these all have the same phenotype as each other but they're slightly different and i think i just mucked that one up i did that's why i see this is where knowing the patterns is really helpful because you catch mistakes there we go so these would be uh pointed for the shape but the color would be white you can see they've all got the lowercase r And so then again you can see it kind of points out a little triangle But I'll get rid of that one and then finally we get another triangle so these three Here and see that little triangle forming there these ones all have the recessive trait at the first But then the dominant at the second so that is rounded but red and then finally you have in the corner the double recessive rounded and white like that so you get these triangles kind of banking towards the bottom right corner so it's good to know some of those patterns in there but this is it and so then when you count them out you'll find as well there's some ratios.

So the double dominant is going to be nine of the different outcomes. Then the single dominant to recessive is going to be three of the outcomes. That's the yellow. And the green is recessive to dominant, or recessive and dominant, which is three. And then we have the double recessive, which is one.

We get this ratio of phenotypes nine to three to three to one. It's not really all that helpful, but in terms of probability, the probability of, say, the double recessive is going to be one out of 16 different outcomes. outcomes like that you can start calculating probabilities the chances of them getting you to fill in a punnett square though is very unlikely that's really time consuming they'll probably not do uh like a dihybrid they may get you to use that for a monohybrid like we did before and just do simple calculations out of four and stuff like that but there we go so that is how the dihybrid cross works and this is you can see it's not really content it's more so an application of it it's problem solving So understanding how they work can be quite useful. Then we have dominance patterns. So this whole time we've been assuming that the dominant trait completely masks the effect of the recessive one.

And so this is what we've been talking about here with complete dominance. But you can also have incomplete dominance. And so what this means is that you kind of have the dominant trait not fully able to dominate the other trait.

So instead it kind of blends with it. And a really good example here is... is to talk about like paint right and paint colors so if you put you know black paint over the top of white paint then it'll pretty much be able to completely mask it or dominate it right and it'll just turn the wall black if you put green paint over blue paint it'll look greenish right that concept of mixing colors so you get a new outcome or a new phenotype because of the fact that there was not an ability to completely dominate the other and so incomplete dominance is exactly that that is where you get a new phenotype and you instead get a paint mixing or a blending of the two original phenotypes so if we use the petal color for example red and white would make pink if there was incomplete dominance of the red and then co-dominance this is where they cooperate so this is where you'll see a bit of both so instead in the you Petal color, for example, you would see spots of red and spots of white.

You wouldn't see pink, though. So they actually both get to compete in that sense. So they cooperate and you see both.

And they'll always explain which type of dominance pattern. it is or they'll give indications as to which type it is based on that information then we have where it's linked so we've mostly been looking at what are called autosomal traits meaning they're on the chromosome pairs 1 to 22 and they are all what's called autosomes meaning they're not related to the sex of the individual at all whereas sex-linked chromosomes or sex chromosomes are the 23rd pair and in males males have an x and a y chromosome whereas females have just both x's and this has pretty significant impacts on the likelihood of a particular trait if that gene is located on one of those chromosomes so if it's x-linked means that it is on the x chromosome and what that means as well if i just move this bit out of the way oh didn't take all of it with it oh so there we go so x-linked traits if we have something on an x chromosome let's say we have a female and it's sitting here and here if they were to have say a capital and a lowercase like that then they would be uh they would be a heterozygote carrier and if it's dominant then they would have the a trait the capital a trait that is right if we had a male though because the second chromosome is not an x chromosome it can't have that particular gene because it doesn't have the information for it so it means that it becomes more likely for them to actually get get the recessive trait so if we have lowercase a normally you would need two of the lowercase a to have that particular phenotype but in this case because there's only one they actually do express just that trait because there is no dominant capital a to dominate over it and so this makes this more likely at that point it kind of lowers the bar in a sense and so these individuals are called hemizygous if you ever see that word you Just means they're not really a homozygous individual because they don't have two of that. They're not heterozygous because they don't have one of each.

They're hemizygous. They've got half. They've got just one of it. And the same thing if you had a male that had the capital A variant. that location then they would have they would express that particular phenotype and there would be hemizygous for that as well so that you can imagine has particular consequences on the likelihood of that in male versus female individuals Y-linked, obviously not possible in a female at all.

These are going to be genes that are on the Y chromosome, so they are male-specific in that case. And then finally we have test crosses and hypothesis testing. And this is where now it becomes a little bit less about biology and more about using logic and using things like Punnett squares and a basic understanding of inheritance. to be able to answer new complex fresh questions.

So a test cross is the mating or the crossing of a homozygous recessive individual to another individual where we don't know their genotype and from the offspring we can start to determine what the genotype of that unknown individual is. So say we have someone who we don't know what their genotype is so I'm just going to put two question marks but we're looking at a particular particular particular gene that has these alleles right and if we cross that with a homozygous recessive so both lowercase a we can see what the outcome of that crossing is so if for example that individual were to be capital capital we would know that they would be either the dominant or the recessive let's say that this is albinism Alright, so no skin pigmentation, pale skin, like kind of silvery white hair and red eyes. So this individual here maybe does not have albinism, right? Because albinism is a recessive trait. So in order to have it, you have to be...

homozygous recessive. Say this individual here is non-albinism so they are expressing pigmentation but we don't know if they are a homozygous dominant so capital capital or if they're just a carrier instead so we have to consider both outcomes. If they were this individual and we do our Punnett square You can see that each individual can only produce one type of sperm or egg. So this is a double up and this is a double up here.

And we can just look at this combo. Everything should just be carrier individuals. So what that would mean is...

that it would be 100% non-Albinism because even though they're a carrier themselves, the offspring, they still need to have both lowercase in order to actually express Albinism. And so therefore, if we crossed those... two individuals a hundred times and then we had a hundred offspring then all those offspring should be non-albinism if the unknown parent is homozygous dominant. If however they were actually homozygous or heterozygous I should say and we crossed them with that homozygous recessive individual then we've got a couple of different options. We can see now their gametes or the cells that they're producing are different, so we consider both.

In the other individual, they're both the same, so we can just cross out the bottom row. And we can see now there's two different outcomes. This here would be non-albinism, but this individual would be a homozygous recessive, and these would express albinism. And it would be a 50-50 split if we were to use a large sample size.

So again, we would expect roughly 50 of those 100 outcomes to be albino and then the other 50 to be non at that point. So based on the outcome, we can then determine which of these cases occurred. for the genotype of the unknown individual. And this is the concept of a test cross. So hypothesis testing, this would be like, it's nothing specific.

It would just be if they give you a bunch of information about a particular trait. and then they ask you, you know, which of the following experiments would test a particular hypothesis. And a lot of the time you've got to think about the logic of whether or not it would produce a discernible difference in each case. That's all I'll really say on that one there. We kind of go into that a little bit more in the questions now anyway, and that's what we'll jump to.

So if I come back up, I think it was quite a way up now. Here we go. I'll just zoom that out a little bit.

So this question that I've put together, just like all my other Section 3... sample questions and everything we've got coat color in cats is the result of a complex combination of a number of genes and gene variants the browning gene codes for tyrp1 an enzyme involved in the metabolic pathway for eumelanin pigment production its dominant form b will produce black eumelanin it has two recessive variants that's just a little typo from me there b lowercase b which is going to be the chocolate variant and then bi which which is going to be the cinnamon variant. So we've got a little bit more complexity this time. With B.I.

being recessive to both B and lowercase b. Chocolate is a rich brown colour and is referred to as chestnut in some breeds. Cinnamon is a lighter reddish brown. And then we've also got the sex-linked orange locus determines whether a cat will produce eumelanin. In cats, the orange fur, or with orange fur, pheomelanin, I'm hoping I'm saying that right, or red pigment, completely replaces eumelanin, black or brown.

This gene is located on the X chromosome. The orange allele is capital O and is co-dominant with non-orange O. So we can see all of that kind of terminology and jargon is now coming up. The heterozygote produces a tortoiseshell coat and this... masks the effect of the browning gene.

So there's a lot of interaction going on here. I've made this particularly difficult. I always categorize the difficulty of these and this will go up on my resources page.

I've put this in the very difficult category. I was umming and ahhing about even putting it in the nightmare. category that I've got on there as well which I've not yet used. So a study was done on a population of a particular breed of cat by crossing a cinnamon female with a black male and it says how many different coat types are possible in this in the offspring of this particular crossing one two three or four so whenever you're doing these i always kind of just look for all right what are the possible genotypes and i'm going to do little mini crosses on the paper in each case so the first thing is i look to the parents right so it says a particular breed of cat crossing a cinnamon female so cinnamon was uh recessive to both so if it's recessive it means in order to have a cinnamon cat they must be homozygous recessive so i'm just going to do that now i know that genotype. The next thing is there was also the orange element to it but there's no mention of orange in this particular question so I get to just ignore that.

That means that gene is irrelevant and I don't have to overthink it and we're crossing that with a black male cat. Now black on the other hand that was dominant to both brown and also to cinnamon so we don't really know what the possible outcomes are here. So the black individual it could have been homozygous dominant black it could have been dominant over brown it could have also been dominant over cinnamon as well so i have to actually look through all of these because it just says which are possible with the information given so the good thing is the female cat cannot give more than a cinnamon right that's the only egg cell that they can produce so therefore all of the combinations are going to come from what goes with that from the father and so if the father was this individual individual here, they can only produce capital B, and so therefore you'd get capital B, lowercase B, and cinnamon, I should say.

So that would be black dominating over cinnamon, so we get a black cat. That is one possible coat type. The next is the heterozygous carrier male. We've already considered what happens if the dominant mixes, but there they could contribute the lowercase b, and lowercase b, that would give us lowercase b and the cinnamon, and lowercase b was chocolate.

that is also dominant over cinnamon so that would give us a chocolate cat and then finally down the bottom here we would have again we've already considered the capital but it could have been a cinnamon carrier cat and if we put those together we get a cinnamon cat and so in that case we've got three different outcomes and so there are three possible coat types there we go all right cool i'll just lock that so i can you Move some of that writing. Just move that down there. All right.

And so then question two says a male cat that is the offspring of a tortoise shell colored female and a chocolate male cannot be. Which of the following? So again, breaking down where is this information coming from?

So. a tortoise shell female so tortoise shell was when you had a heterozygous co-dominance between capital o and lowercase o so we know that they're that the other thing it said was that the tortoise shell masks the effects of the browning gene. So they could be underlying with anything from black to brown to cinnamon. We have no idea there.

And then we're crossing that with a chocolate male. So the chocolate male, they have not any orange or anything in them. So what that means is, and it is also X-linked, if the male does not have any orange color to them, then that would mean that they have O, the lowercase O, and then a Y chromosome, which is not controlled. contribute we mix that then they're chocolate so chocolate could either be uh not capital b it could be lowercase b b like that or it could be dominant to a cinnamon as well these are the both the possibilities for the male and then for the female let's go back there again it didn't say about what it is so it could have been this it could have been this could have been this could have been this or it could have been cinnamon being masked as well Lots and lots of different possibilities. But what it's really focusing on is a male cat that is produced by these two individuals.

So we don't really want to have to go through all of those. That's clearly too much information. But remember that male cats, first of all, are XY.

So that means that they cannot be homozygous or heterozygous, for that matter, for the orange trait. So I'd start there at that point. That will actually lead me to the answer, because if I looked at the orange trait, I can see tortoise shell is mentioned here, but that would have to be capital O lowercase o, like the mother, but male cats don't have two X chromosomes. So that immediately.

is going to be my answer. The other thing though is you can imagine that the male cat a chocolate chocolate colored if that were to be the case there that is possible because we could have both non-orange and non-orange go together at that point or actually no sorry it would be non-orange and y because it has to make a male cat like this so we can get mix and match from there and there without a punnet square so there we go a non-orange male cat and then in terms of making chocolate that's possible because we could just get like a lowercase b from here and maybe a lowercase b from there and then it would be chocolate so that one's out a homozygous chocolate we just proved that could get that from one from each of the parents so that's also out and then a carrier of bi the cinnamon and that is also possible because you could get that capital b and then maybe a cinnamon from there and then the cinnamon would be masked so it was easier just to go for the correct answer but we could also rule it out as well and then question three if the genotype of a black cat is to be determined via repeated crossing with another cat what would be the best option so we've got a black cat that is dominant for the color right and we want to know is it homozygous or is it heterozygous that's hinting at maybe a test cross so with a test cross you you want to go with a recessive trait. So therefore black is not recessive.

So you don't want to go with that. Neither is chocolate, because even though it's recessive to black, it's dominant to cinnamon. Cinnamon is the true recessive, so that would be hinting at the answer. And tortoiseshell is no good, because it masks what the actual browning gene is doing altogether.

And so it would be a complete wild card in that sense. So therefore the cinnamon cat would be the best option. Notice though it's not using the... the word test cross so i don't think that asa would ever really expect people to know technical words unless they introduce them in the stem so instead of just set it up as a problem solving thing and then question four a mutation that occurs in approximately one in every 350 000 live births of cats involves the orange gene producing the orange phenotype in the heterozygous carrier that is instead of the tortoise shell you don't get the co-dominance you then get complete dominance of the orange gene.

So what that means is normally this would be the tortoise shell type. And what's happening now is it's leaning towards just orange as if it were to be homozygous dominant. And so if a tortoise shell cat were crossed with a chocolate male carrying the cinnamon allele, the chance of producing an orange female cat is what?

Now, again, that's a lot of information to take on, right? So it's a probability question. so the first thing is i would look at what the genotypes of the parents are going to be so a tortoiseshell cat uh were crossed with a chocolate male now remember tortoiseshell is female only anyway so i didn't even have to say male and you kind of already know which the genders or which of the sexes of the cats would be so if a tortoiseshell cat so what that means is they are like this on each of the X chromosomes. And let's see, that would be masking what color they are.

So we don't know what the browning gene is doing. And we're crossing that with a chocolate male. is carrying the cinnamon so they don't have any orange so they would be lowercase o and then a y chromosome not contributing and we're told that they're chocolate so they've got lowercase b but they're carrying cinnamon so that was bi and then it says what's the chance of producing an orange female cat so clearly this is involving the the mutation otherwise why mention it but we first have to look at what's the chance of the female orange cat occurring right so to get a female orange cat you can see without the mutation it's actually not going to be possible because you need both capital o's and the father does not have a capital o to produce so the best that we could get is we could get this mixed with This capital mixed with this lowercase which would produce tortoiseshell and then the chance carrying a cinnamon, the chance of producing an orange female, that's really it there. And then it doesn't really matter what's happening.

at the other spots because then it will mask it right so from that we would probably want something recessive or actually it wouldn't matter so the chances of this occurring is one in four because we've got either if i write them out like this It could be this mixed with this, this mixed with this, this mixed with this, or this mixed with that. And so the only one that gave us that outcome was this one here, that was one out of four possibilities. We multiply that then, because that has to happen, and...

a completely independent event the second thing that has to happen is we have to have the mutation so that then it converts this and makes that into an orange phenotype and that was one in 350 000 like that and then that was be it this would be the whole thing so then from there if we do that that would then be 1 out of and then we have to just do 4 times 350,000 so we double that to 700,000 double it again to 1.4 million. Just like that, we get one out of 1.4 million. So the answer then becomes C. There we go. Cool.

Now you might be overthinking it and think, well, what about the chance of a male or a female offspring? We've actually already factored that in here because... the combinations of this is an x this is an x this is an x this is a y the combinations of x and y is already taking care of the sex of the offspring in that case so we don't have the times by half as well and there we go so we get one out of one point formula million and then that's the answer so that's a bit more of a mathsy type question but notice that again you wouldn't be able to really work with that if you didn't have a working understanding of how inheritance works and what all of those words mean i will point out that is not the level of difficulty that you should always expect that would absolutely be up the top end of a difficult question i'm quite proud of it because i always like when i write really difficult questions that work because it's bloody hard to do it but um yeah if that kind of makes sense then you're on to a good start at that point hopefully this has all kind of wrapped everything up for you in terms of inheritance like i said i try to keep this just to the stuff that you actually need rather than going into a bunch of irrelevant stuff if there was anything that was mildly irrelevant it might be the dihybrid cross i wouldn't be too fussed about that understanding how to do a monohybrid cross and a punnett square is useful but that's really it so i think i'll leave that one there if you've got any questions for me or any recommendations for what you'd like to see let me know as always always but i think biology kind of links one to the other so i think the next one that i'll do is probably going through some basics of cell biology and the cell membrane going through the structure of dna as well because that one does come up quite frequently and then we'll be able to move to some other things like the different body systems i'm probably just going to do a big gigantic video on that one that just goes through all of them because it may actually be quite short it's really not that much that you need to know about them but that should be a pretty fun one because that's probably my favorite part of biology and I always like teaching it as well so I'll get to that one as well and we'll do it like this with the bio ones every time where I'll have some sample questions to work through so you can kind of see how the theory gets applied as well perfect all right easy well hopefully this is all helpful as always leave me some feedback like the video subscribe if you haven't already because I can see that people are watching I try to put up as much stuff as possible and that's probably the quickest way to see what I'm putting out not missing anything because you video gets buried pretty quickly. Anyway, I will see you in the next one and yeah, that's it.

Transcript for:Genetics and Inheritance Overview

Transcript for:
Genetics and Inheritance Overview