hi everyone and welcome to miss estc biology and in this video we're going through topic 16 one of my favorites which is inheritance so make sure if you do want any extra help fully understanding all the theory knowing the key marking points that come up and key examiners tips then check out my AEV notes which I've Linked In the description below but for now let's get into my favorite topic inheritance topic 16 which is looking at the passage of information from parents to offspring or another words inheritance and we're going to start by looking at these key terms for inheritance so hloy also represented by the letter N means that you have one set of chromosomes in those cells deployed cells or twoed means you have two sets of chromosomes and homologous pairs of chromosomes are two chromosomes that contain the same genes in the same position now we're going to look at meiosis first of all and meiosis is a type of cell division it's known as reduction division because it results in the production of gametes meaning you end up with half of the number of chromosomes compared to the parental cell this is crucial because when two gametes fuse together during fertilization the resulting zygote will then have the correct diploid chromosome number and without that reduction division the chromosome number would double every single generation when fertilization happened leading to an exponential increaseing chromosomes and the number of genetic material now unlike in mitosis there are two nuclear divisions that's why we end up with reduction in the chromosome number and we get four genetically different hloy cells so how that happens then as we said we've got two nuclear divisions we get four genetically different hloy cells we have two rounds of division that are referred to meiosis 1 for the first round and meiosis 2 for the second round both stages include the same stages prephase metaphase anaphase tase and then cyto Kinesis to split the cytoplasm and the cell membrane to create two new cells and here's just a reminder of these two key terms hloy and diploid that's going to come up as we go through the stages of meiosis now the key stages that you need to know about actually happen in meiosis 1 so the way we get these genetically different hloy cells the genetic difference is introduced by independent assortment of homologous chromosomes and crossing over both of which happen in meiosis 1 so crossing over this happens in meiosis 1 when the homologous chromosomes pair up at the equator and when they pair up at the equator opposite each other we call that a b ve valence so this here would be a balence it's two homologous chromosomes aligned next to each other what can happen not all the time but sometimes when that balent forms the nonsister chromatids which means the chromatid from the two different chromosomes can cross over when they're in that balence and the exact position where they cross over is known as a kayma where those noncy chromatids cross over it can put tension on that chromatid causing parts of the chromatids to break and then Swap and they're exchanged and because they've now swapped a section of that chromatid they have exactly the same genes because they're homologous chromosomes but they might have had different versions or different alals of those genes and that results then in new combinations of Al in the resulting gametes that would be made from these chromosomes independent assortment also happens in meiosis 1 and it's in metaphase 1 when the homologous pairs are lining up the at the equator to make those balence but this time the concept is it's random which side of the Equator the homologous pairs align so whether you have the maternal or paternal on the left or the right for all of those homologous pairs so we can see here we've got one option where you've got all of the blue on one side all of the red but we've got a different combination in this option and a different combination here and a different combination there and because it's random which side of the Equator they go you're going to end up with different combinations of maternal and paternal chromosomes in the gamet which introduces genetic variation and you can actually calculate the number of possible combinations you could get of how those homologous pairs could align and therefore how many different possible gametes you could create by using the formula 2 to the power of n where n is the number of homologous Pairs and it's two because we have two chromosomes in that pair so for humans we have 23 homologous pairs so to work out how many possible different gametes we could create because of independent assortment of homologous chromosomes it' be 2 to^ 23 which gives us over 8 million different combinations next then we go on to the roles of genes in determining the Fe genotype here's a selection of essential key terms you need to know for inheritance so we've got genotype and phenotype the genetic constitution of or an organism so it's alos it has for a gene whereas the phenotype is the expression of the genes and its interaction with the environment homozygous means a pair of homologous chromones carrying the same alals for a single Gene where as heterozy is when you have two different Al for a single Gene we've got a recessive dominant and co-dominant alals recessive Al are only expressed if there are no dominant ones present dominant Al are always expressed and co-dominance is when you have two Al are equally dominant and therefore if you have a copy of both in your genotype both will be expressed in the phenotype multiple alals is when there is more than two possible Al for a particular Gene sex linkage is when a gene is located on the chromosome autosomal linkage is not about the sex chromosomes but it's when genes are located on the same chromosome but not a sex chromosome epistasis is when one gene modifies or masks the expression of another Gene monohybrid inheritance is when we look at the inheritance of one particular Gene and dihybrid inheritance is when we look at genetic crosses for two particular genes so these would be good to turn into flash cards to test yourself and to learn now you commonly have to do genetic diagrams to determine the probability of different either genotypes or phenotypes in The Offspring based on Parental genotypes or phenotypes you're given and we use different coding to represent the alals so this is just a summary of the accepted coding in monohybrid inheritance which is what you would have done at gcsc when you're looking at the inheritance of of either a dominant or aess of Al for one particular Gene we pick one letter and that represents the Gene and you have the capital letter which represents the dominant Al lowercase letter represents the recessive Ali in co-dominance because both Ali are dominant it wouldn't work to use one letter that were capital or lowercase because they' both have to be capital and therefore it would look the same so that's why we change the coding system and we use a BAS letter that represents the Gene and then we do a superscript letter which represents the Al and we use a two different letters both capitals to represent the two different Cod dominant alals we have to do the same thing in multiple Al because you have more than two Al for a gene therefore we can't just do capital and lowercase with the same letter so that's why here we can see we've got the base letter for the Gene and we've got the three possible alals for blood group shown there sex linkage we do the base letter showing the chromosome and then the alil goes superscript and you don't get the sex link genes on the Y chromosome so you never have an Al written next to the Y you only have an Al written next to the X autosomal linkage this is just using the same system as the monohybrid but that will always be a dihybrid example so that's the inheritance of two genes that's that's why we've got two different letters but it should be capital or lower case for dominant and recessive same with epistasis that's always a dihybrid example because one gene can mask the expression of the other Gene but you'd pick one letter for one of the genes another letter for the other capital and lowercase to represent dominance and recessive so let's go through some genetic cross examples so look at how this all works starts with monoh hybrid this one we've got syst fibrosis is caused by a recessive alil and we've got two carriers reproduce what is the probability that they'll have a child cystic fibrosis so here we have the parental genotypes they're both carriers they need a capital and a lowercase a dominant and a recessive I've then done my punet Square showing the possible gametes for both parents join those gamet together to show the four possible genotypes of The Offspring and then I've put in what the um genotype for someone who is a cystic fibrosis sufferer must be if it is caused by a recessive Al so from that we can see we have 25% probability of a child having cystic fibrosis um if they then asked what is the probability of it being a girl which they did here we then need to Times by a half because it's always 50% probability of having a boy 50% probability of having a girl or male or female then if we have a look at this Cod dominance example cows can be red white or R we call this color red when we're describing cows red and white are both dominant and you only get ran which is this mixture of white and red if an individual has the co-dominant genotype so they have a red and a white alial so both are expressed in the phenotype and in this example we're told that we have two ran cows reproducing and we asked is the probability they'll produce red Offspring so here are the parents genotypes if they're R they must have one of each Al a red one and a white one we've then got our Panet Square showing the parental gametes on the outside just as a single letter so you have one Al for each gene in a gamt joining together those gtes to show the four possible genotypes and I've written in the Box the four matching pheno types and from that we can see we have 25% probability of having a red cow with those parents blood groups is an example of codominance and multiple Al there are three alals blood group The alal for blood group a an Ali for blood group b an alel for blood group O But A and B are both dominant so if an individual has a and b they'll have the fourth phenotype which is blood group a blood group O or the AL that codes for blood group O is a recessive Al so you're only blood group O if you have two copies of that recessive Al and that's why you could have alel B and alel O and still be blood group O because B is dominant over o and the same with blood group a alil i a is dominant over alil IO so here's our potential genotypes there's our potential phenotypes and in this example we've got parents with blood group a b and blood group O reproduce I a and IB are co-dominant and IO is recessive what's the probability they'll have an offspring with blood group a so first of all we need to work out the parents genotypes and if they're blood group a one parent must be I a ib and blood group O one parent must be IO i o so then if we split it up to see the gamits that both parents would be able to produce we can then do our pet Square to see the four possible genotypes then in the Box I've also written the corresponding phenotype and from that we can see we've got 50% probability of blood group a we're now moving on to sex linkage we're going to look at the example of color blindness color blindness is caused by a recessive alel found only on the X chromosome and we've got the example of if a non-color blind male reproduces with a female carrier of the AL what's the probability of their children being color blind so the female is a carrier which means they must have two different Al so the capital and the lowercase and recessive is color blindness so that is our color blindness alel and they're reproducing with a male so they are x y and the male does not have color blindness so the males only have one version of the Gene and the version they have is the capital r for non-color blind so if we have a look at the planet Square we can see the gami on the outside the four possible genotypes and the matching phenotypes so this would be a biological female with color vision CU it's a dominant alal biological female with color vision and that would be a color blind biological male missed off the biological male there and then the biological male with color vision so the probability of having a child being color blind find is 25% next is epistasis and that's was the definition we said is where one gene influences the expression of another Gene an example of this is coat color in mice or coat color in Labradors fruit color and vegetables is another one so color is quite a common example so if we go through the cat color and labrador's example one of the genes controls whether pigment will be expressed so produced the dominant alial for that Gene means the pigment will be made the recessive Al codes for NO production of pigments and in which case for a Labrador if they have two copies of this recessive Al it will mask the expression of the second Al and they will always be yellow in color or golden whatever you want to call it Gene 2 is the gene responsible for what color the pigment will be so capital B would be a dominant Al and that one Coes for black fur recessive alil is for brown fur so here's are three possible phenotypes but Gene one can mask the expression of Gene 2 because if an individual is lowercase lowercase e meaning two recessive Al it doesn't matter what combination of Ali they have for Gene 2 Gene 2 is not going to be expressed so if we go through an example here we have two parents heterozygous for both genes and because I've got at least one capital E that means the pigment will be expressed and because I've got a capital B that means the pigment is black so this would be two black Labradors the possible gametes that they could both produce would be all of the options of the capital E lowercase e capital B lowercase b in your gamt cuz you only have one copy of each gene in each gamt so here's our four possible gametes for both parents that then gives us this planet Square it's a large one there four gametes for all of them and we can then put in what the possible genotypes would be and the possible phenotypes so we can see here any that have a capital E are going to be either black or brown any that have two recessive e Al will always be yellow and it masks the expression of that second Gene so in this instance we've got a 9 to 4 to3 ratio of those phenotypes so a DI hybrid cross is where the inheritance of two genes is considered at the same time which we actually just saw there in that epistasis example a common example linked to this is mendal and his P so he looked at P plants considering the gene for color and the gene for whether it was round or wrinkled only he didn't know genes at that point he was just talking or noting down the color and texture and when we do genetic crosses we should always show and label where possible the parental phenotype parental genotype possible gametes The Offspring genotype correctly match up the phenotype to those genotypes and give the proportion of each phenotype and that's why often in the exam they give you this space to fill in each of those criteria so we've got a round yellow and a wrinkled um green and in this example we are being told that the round yellow is um homozygous dominant for both of those genes wrinkled green would have to be homozygous recessive for both because wrinkled and green are recessive now you aren't expected to know that that's information you would have had to have been told then we work out the possible gametes and in a dihybrid example every gam should have one copy of each letter so one of the RS one of the Y's to indicate one copy of each gene now for this parent they can only produce gametes with a capital r capital Y which is the two dominant Ali and this parent can only produce gametes with the two recessive so recessive R recessive y we can then do a pet square but actually for this one we don't need to do a punet square because if this parent can only donate this gamet this parent can only donate that one if we join them together every single option on the pet Square would come out with this heterozygous Offspring genotype so 100% would have that genotype which would mean 100% would be yellow and round but heterozygous so you can actually cross those together now this commonly is shown if you were then to cross F1 which is the first generation which is what we just created together we've now got two parents who are both ter azygous for both genes for this we now have these four possible gametes for both parents and now we're going to end up with one of these large Panet squares again where we'd have to put in all of the possible genotypes combining those gametes and then we can put in all of those possible corresponding phenotypes and then we can work out the ratio or proportion of the phenotypes and we have nine yellow and round three three green and round three yellow and wrinkled and one green and wrinkled so a 9331 ratio and in any di hybrid cross if you are given a scenario where both parents are heterozygous for both genes you will always get a 9: 3 to 3: 1 ratio unless there is autosomal linkage which we can to have a look at or if there's crossing over in meiosis or if epistasis has occurred as well so crossing over in meiosis that creates new combinations of alos and gametes so that means what you predicted in your planet Square may not happen and this links back to what we said right at the start of the video crossing over introduces new possible combinations of Ali and the gametes and therefore we don't get what we expected from our planet square and we going to come back to that concept once we've gone through autosomal linkage and we said autosomal linkage is when two different genes are on the same nonsex chromosome so we can see here homologous pair of chromosomes Gene one and Gene two are on the same chromosome so they are linked autosomally we're going to apply the example we've just done so Gene one is for whether it's um yellow or green Gene two is for whether it's wrinkled or smooth and we said that the parents were both heterozygous for both genes which we can see this is the homologous pair of chromosomes for one of the parents and they've got the capital and the recessive the capital on the recessive it just so happens to be because they're linked the two dominant so the two capitals are on this chromosome of the homologous pair and the two recessive Ali are on the other chromosome in the homologous pair and that means we can't actually make four possible gametes now we can only make two possible gametes because in meiosis when these homologous pairs are separated that will mean this will go on to make one gamt which will have to have the capital r and the capital Y and this chromosome will go on to make another gamet which means that one will always have lowercase R and lowercase Y and therefore these are the only two gamuts that this parent can make rather than the four possible ones that we looked at initially so here were the four possible ones that we said initially if there was no autosomal linkage but because there is autosomal linkage these are the only gametes that the parent can produce and if we have two parents who are the same reproducing with those two possible gametes these are the four possible genotypes we get and the corresponding phenotypes so our Rao is now three yellow and round to one green and wrinkled so with two heterozygous parents you would expect to get a 9:3 to 3:1 ratio unless there's autosomal linkage when you could get a different ratio and for this particular example we got 3:1 ratio now we did also say unless there's crossing over that happens so sometimes you might get autosomal linkage exam question where you're LED through all the information we've just gone through to work out you'd expect a 3:1 ratio but they then might say however scientists did this cross and what they actually observed was these phenotypes in this proportion and you could be asked to explain their results and it's only possible to get those two extra phenotypes if crossing over occurred in meiosis between these homologous pairs to create new combinations of gtes so here we can see those linked gen these were the two recessive ones on one of the homologous pairs here were the two dominant ones on one of the homologous pairs they now look like double structures this is showing after DNA replication and in meiosis one when they line up at the equated form of by veilance we can see the noncy chromatids crossing over to form a Kay Asma break and Swap and that then means when they do then separate in meiosis 1 and meiosis 2 ultimately we get four gametes which have new combinations of Al in the gametes so to explain these results then we'd have to say yellow and round and wrinkled and green are the most commonly observed because those are the genotypes that are a result of fusing the capital r and Y and the lowercase R and Y together and those were the ones that are most common because they were not formed by the crossing over and crossing over is actually quite now out of the yellow round and green wrinkled we've got fewer green and wrinkled because that's the recessive combination so you'd have to have two copies of the RS two copies of the lowercase y's to be green and wrinkled so that's why there fewer green and wrinkled compared to yellow and rounds then while we don't have very many at all of the middle two those two are a result of crossing over to create the capital r lowercase Y and the lowercase r capital Y gametes and crossing over doesn't happen all the time it's rare and therefore we wouldn't get very many of these two genotypes which is how we'd get those two phenotypes so that would be your explanation of those results combining the concept of autosomal linkage and crossing over now throughout that I was talking about what we expected to see versus what we actually observed and that leads us into using the statistic of Kai squar in in inheritance so Ki squ is the statistic to investigate differences between frequencies and it can be used to determine is there a significant difference between the frequency you expect and the frequency You observe and in inheritance that links to what we expect from the planet square and we can compare that to what we actually observe if you count literally the phenotypes that are there and here's an example of this corn can be purple or yellow or smooth and wrinkled in terms of color and texture using the results of the pet square and then counting the number of actual corn kernels of particular colors and textures you can look at whether the frequency expected is significantly different to the frequency you observed so we're told here that purple is dominant yellow is recessive two heterozygous parents were cross this is just di hybrid simple genetics there's no a linkage and the expected ratio is three purple to one yellow so that is our expected ratio and we're told that a student then counted them and they had 21 purple kernels and 13 yellow kernels does this follow the expected ratio so what we would then need to do is calculate the kai squar but I'm actually jumping straight to the result here so you can see how we'd use this as um a conclusion so our null hypothesis would be for this example there's no significant difference between the expected and observed frequency of the color of the corn kernels the kai squ value was then calculated to be this we've got one degree of Freedom there were two categories so if we read across to one degree of Freedom at P = 0.05 that means our Kai squar value is lower than the critical value and therefore we have more than 5% probability that the difference between the observed and the expected is due to chance and therefore we have to accept the null hypothesis meaning there is no significant difference between what we observed and what we expected so in other words the frequency of the core kernels that we observed does match what we expected from our planet square and it confirms that men dad in genetics ratio of 3:1 test cross is the final thing we're going to look at and these are used to determine the genotype of an individual expressing a dominant phenotype by Crossing it with a homozygous recessive IND individual and this helps in revealing whether the dominant phenotype an individual has is determined by a genotype as heterozygous or homozygous dominance because if you have the dominant phenotype that could be someone who's homozygous dominant or heterozygous so if we use an example of Mel in genetics or inheritance specifically the inheritance of flower colors and pea plants where purple is dominant and white is recessive we could perform a test cross to determine the genotype of a purple flower so what we would need to do is cross the purple flower plant that we're trying to identify the genotype of with a white plant because we know that white plant has to be homozygous recessive from the results of the Panet square or the spring breeding together we can then work out whether the parent was homozygous dominant or whe they were heterozygous because in this ponet Square we've got we've crossed the purple flower with a white flower and because we do have a white flower here that would only be possible if the purple parent was heterozygous CU to get a white flower both parents have to have a white adial so in these test crosses if you get any offspring that have the recessive phenotype it confirms the parent that was the dominant trait must be heterozygous and that's what we've just got here explained so you can pause read through that bit but it's wellth just talking through on that last slide so that takes us to the end of this topic hope you found it helpful if you did don't forget to hit that subscribe button so you don't miss out on any of the latest videos a