i'm dr d dr d dr d dr d dr d dr d dr d explain stuff hey everyone dr d here and in this video we are going to be covering chapter 4 from our genetics essentials concepts and connections textbook 4th edition this chapter deals with extensions and modifications of basic principles of genetics what this chapter is essentially going to show us is that not all traits are inherited in a mendelian fashion with dominant a clear dominant recessive pattern where you would do a monohybrid cross and you would see three to one phenotypic ratio a one to two to one genotypic ratio every time there are exceptions and these exceptions are called non-mendelian inherited traits and there are good reasons for this these exceptions this does not disprove mendel but it shows exceptions and we are going to explore in this chapter what those exceptions are and how they arise and one of the main ways they arise is by dealing with the sex chromosomes because the sex chromosomes are different in the different sexes of a species right so you've heard of the x and y chromosome the x chromosome has to do with female this and y chromosome with male ness well the y chromosome is actually a very stubby short chromosome as you can see here on the left in this image 4.1 there's only a handful of useful genes on this chromosome and the main one being sroi which directs maleness now the x chromosome is much longer and it has a host of different genes so it has a lot more useful genetic information now if there are and if there are these genes if there are problems with these genes on the x chromosome well they are going to impact females differently than males okay so we're going to get into that in this chapter and there's other reasons that you would have non-mendelian inherited traits as well but a lot of them have to do with the sex chromosomes so again uh you have sex chromosomes the x and y during meiosis the x and y chromosomes can pair even though they're not homologous they can still pair at the telomeric regions there's these regions where they can still pair then during sexual reproductions uh during sexual reproduction uh when you have meiosis to form the gametes remember the gametes are haploid those gametes come together during fertilization to form the diploid zygote and the life cycle continues this way remember that males have x and y chromosomes females of different species have xx chromosomes right so it is the male who dictates the uh the sex uh determination of the offspring because it's the male that has either an x or a y to donate to the sperm the females have two x chromosomes so they cannot direct a maleness through a y chromosome so it's the sperm essentially it's the sperm that can dictate a male or a female embryo or zygote so concept check time concept check number one what process causes the genetic variation seen in offspring produced by sexual reproduction well we we touched on this in a previous chapter right we said that that it's meiosis that in uh that gives you genetic variability right remember that by way of crossing over an independent assortment that's where the genetic variation is introduced in populations it's during meiosis so meiosis is the answer here now um not all creatures use not all eukaryotic creatures use the x y system for determining sex uh determining sex uh some have either two x chromosomes for female and an x by itself uh they call it standalone x for maleness so for example in grasshoppers you would see two x chromosomes uh directs the sex determination for female and one single x chromosome would direct the grasshopper to be male now in humans we have the x x x y system two x chromosomes uh female and an x y male and you would see this in most mammals and again the y chromosome is very short and stubby lacking very many genes at all whereas the x chromosome is much longer and it has a lot more actual useful genetic information on it and by the way you know how chromosomes are numbered one through 23 and then you have two sets of one through 23. it's the 23rd chromosome that refers to the x and y chromosomes so the 23rd chromosome is what what we're talking about here okay and again remember they do form kind of tetrads they do they do pair up during meiosis remember tetrads are formed during meiosis well tetrads will form here but it's because of these pseudo-autosomal regions at the tips that that allows them to to uh to form up to to form a tetrad not because they actually share a lot of uh uh regions of homologous regions okay now there's even more interesting systems out there for example in in birds snakes butterflies some amphibian fishes you'll have uh the it's the male that has two of the same chromosome and it's the female that has two different chromosomes so you know again in in humans uh x y the two different chromosomes were xy and that gives you males but in these different creatures the two different chromosomes would give you femaleness right so it's kind of an interesting fun fact to know and again from figure 4.5 we see the inheritance of sex in organisms with x y chromosomes results in equal numbers of male and female offspring y because again the the male is the one with the x and y chromosomes and so the gametes can either be x or y and that is a 50 50 chance of an x or y uh gamete when the x gamete sperm fertilizes the female egg which has to be x you get a female when the y gamete sperm fertilizes the female egg you will get a male so it's a 50 50 chance you always have a 50 50 chance to have a male or a female but it's dictated by the sperm the sperm dictates the sex of the offspring now there are some interesting creatures out there as well including some plants some fungi protozoans fish that don't have sex chromosomes at all and they call these the genic sex determination system where there are no sex chromosomes there's no x and y or z w or z z it's only the only sex determining genes are found at on autosomal they're found on autosomal genes and i don't know if i've defined this before but obviously autosomal chromosomes are the non-sex chromosomes so humans for humans the autosomal chromosomes refer to chromosomes 1 through 22. the 23rd chromosome is called the sex chromosome so in gene sex determination system uh there are no sex chromosomes only autosomal chromosomes and it's a genes on the autosomal chromosomes that direct sex determination now some creatures actually it's very interesting but it's temperature can direct temperature can direct sex determination these are called environmental factors and their role in sex determination uh so for example turtles uh warmer incubation temperatures produce more females during certain times of the year but for alligators the reverse is true isn't that interesting remember the plot line of jurassic park that they should have all been females but life found a way as the doctor said uh and that was due to temperature differences and the spontaneous uh change in sex okay concept check number two how do chromosomal genic and environmental sex determination differ we remember we just covered all this chromosomal sex determination there's a difference between the chromosome alleles of males and females you have the xy for example in humans and the xx in females genic there are no sex chromosomes uh that remember there are only autosomal chromosomes and sex determination occurs on those autosomal chromosomes and then environmental remember environmental factors such as incubation temperature and time of year can affect sex determination now again in humans you have the xx or xy system right for sex determination the sry gene which i told you about this is the important gene er which which gives maleness right this is the important gene on the y chromosome that lends maleness to two males uh so and it lives on the y chromosome uh now what's interesting is that humans can be born with different numbers of x and y chromosomes you could have uh in in turner syndrome you could have just an x chromosome so you could be born with only an x chromosome and that's it no y chromosome no other x chromosome to complement and this occurs in about one in three thousand female births klein felter syndrome you have extra x chromosomes you can have one extra two extra three extra you could have an extra y and an extra x and this happens in one in one thousand male births notice the presence of the y chromosome directs male maleness and that's due to the presence of the sry gene on the y chromosome the absence of a y chromosome directs for femaleness and that's why you would get the female births now you can also have poly x females and that's when you have a bunch of x's you could have three four or five x's uh isn't that interesting and that can happen at one in one thousand female births so here's an outline let's just go through this sex chromosomes associated sexual phenotypes and humans xx for female typically uh which which the characteristics are female traits x y for male male traits then you can have like i said before klinefelter syndrome x x y x x y y x x x y uh this would give you the characteristics male traits tall small testes reduced facial and pubic hair now you can also have turner syndrome where like i mentioned before you could just be born with one x chromosome and that's it in this case you would have female traits you'd be short a low hairline broad chest and neck folds now remember what else you could have uh polyex females three x's four x's five x's uh these are known as polyx females female traits tall and thin interesting then you can have males with an extra y this is also very interesting you can have x y y male male traits tall it's a very very interesting stuff and again i was telling you about this sry gene you can see it here on the y chromosome it lives on the short arm of the y chromosome that this gene is y linked because it is found only on the y chromosome and it directs maleness concept check number three in humans what will be the phenotype of a person with x x x y sex chromosomes um what would be the phenotype that's klein felter klein felter we we touched on that so these would be will demonstrate y characteristics uh maleness maleness due to the presence of that y chromosome and the presence of that sry gene so now let's talk a little bit about these x link characteristics what happens when you have genes that are on the x chromosome and those genes may you know develop a mutation or something how does that affect males and females differently and how would that differ from mendel's patterns of inheritance uh you know his his typical monohybrid cross predictions so let's look into this this all of these studies really started with this gentleman thomas uh hunt morgan with his work in drosophila melanogaster where he had labs and he would grow up his fruit flies and he would look at the fruit flies and tried to unravel the mysteries of heritability and genetics using these fruit flies remember fruit flies are a very good model organism what he actually found were he found a very rare mutation among the fruit flies he found white eyed fruit flies which is very interesting because the wild type or normal phenotype is a red eye fruit fly i mean just think about it when was the last time you saw a white eyed fruit fly it's very rare so when he found these white eye fruit flies he was very excited and he wanted to learn more about it he wanted to know well is this a dominant trait is this a recessive trait and he would set up these uh monohybrid crosses and when he set up the monohybrid crosses he was very shocked to see uh a pattern of ratios uh inheritance pattern that differed from what mendel predicted what he predicted was a three to one phenotypic ratio a one to two to one genotypic ratio correct uh but that's not what he saw so let's take a look at what he saw so what he eventually tried was separating these out by sexes where you had the the the female with the wild type you know they called this x plus x plus right x plus meaning the x chromosome has the wild type allele and the other x chromosome has the wild type allele so this is a true breeder wild type female and then crossed it with a true breeder mutant male so this is x w w is for the white eye mutation so and obviously the y chromosome doesn't have the mutation because the y chromosome doesn't have an eye color gene remember it's the x chromosome that has all these different genes the y chromosome has very few genes and the main gene that's important for directing maleness is that sry gene so uh the when you when you have meiosis and the gametes form what kind of gametes can this female make this female can only make gametes that are wild type so the x chromosome has the wild type red eye allele now the male fly remember male flies males direct either y chromosomes or x chromosomes into their gametes at a 50 50 chance so their x chromosome has a w for the white eye and the y chromosome obviously doesn't have the mutant mutation at all so let's just let's just follow this down first and we'll then we'll come back and look at the reciprocal reciprocal cross here so let's take a look so what happened that wild type uh egg from the mom met either a wild type oh sorry a white mutant from sperm from the male and that would give you this genotype here that would give you red eyed females right on the on the converse on the converse the the um the wild type female egg met a y uh chromosome from the male sperm and that would also give you red eyed males that would also give you the wild-type male so all all of the offspring were wild-type does that make sense all of the offspring were wild type but let's look at the reciprocal cross over here when thomas hunt morgan did this in reverse with the white eyed female flies this is this would mean they have xw xw genotypes and across that with the wild type male x plus y remember what after meiosis what kind of gametes could this true breeder make well all the gametes have to be xw and the male you can have x plus or y so after fertilization what could what could happen if the x w egg meets the x plus sperm you have x w x plus offspring which are red eye females however if the xw egg met the y sperm you would have white eyed males do you see how this would totally differ from mendel's expected ratios for the p1 to f1 cross the p to f1 cross you see so in this case you would have half your fruit flies with red eyes half your fruit flies with white eyes and a hundred percent of them with the red eyes are female and a hundred percent of them with the white eyes are male you see that so that should tell you right away that this is a sex linked they call these sex linked or x linked characteristics that means that the gene or allele you are studying lives on the x chromosome and typically that means that it cannot be complemented by a gene on the y chromosome so even if it's a recessive allele it acts as a dominant allele it's going to it it can't be masked in males okay very interesting stuff right very interesting stuff and then they went on let's let's see they went on to do the rest of the monohybrid cross so let's start with this one on the left again uh this female was x plus xw that means she could make uh eggs with x plus or xw this male was x plus and y so the sperm could be x plus or y so what did we see you put the sperm information on top and this punnett square the egg information on the side on this punnett square and it looked like you had one red eye female that was x plus x plus one red eyed female that was x plus y one uh red eye female that was x plus w and then you had one wild type male so this looks to be a three to one phenotype ratio which actually makes sense um you know mendelian this would follow mendelian genetics right three to one phenotype ratio but when you do the converse when when you look at the converse cross where this female was x plus xw the eggs she could produce are either x plus or x w the male could produce x w or y when you plot this out you would see again um half of the female sorry half of the uh the flies have red eyes half have white so this is a one to one ratio of red to white eyes and that totally deviates from mendel's expected ratios and again once again shows that this is not a typical dominant recessive situation instead this is likely an x-linked or sex-linked inheritance pattern and again why the fruit fly the fruit fly is great because it is a great model organism it's small size it has a great reproduction time it has a lot of offspring etc etc this is the life cycle of a fly you don't really need to know this in too much detail but again this just highlights why we use flies so so commonly in population genetics in all kinds of mutation research i told you in stem cell research chromosome variation behavior studying chromosomes uh genetic control of pattern formation you know body segment formation behavioral genetics so many different aspects of life that we could study flies and directly relate those results to humans and the whole reason we know about sex-linked or excellent chromosomes is because of these flies very very interesting stuff and we can further we can look at an example of x-linked inheritance in in humans by looking at color blindness i mean most of you probably know that color blindness is more common in males than in females in humans and here's why it's an x-linked trait so you have a normal color vision female a human female this she would have x plus x plus remember the plus means that it's a wild type allele on the x chromosome her gametes can be x plus or x plus the colorblind male can have x with the c allele which means colorblind or y so the gametes the sperms could be xc or y and then after fertilization you you only have normal vision in the offspring normal vision why because if you have x plus xc you have a wild type x chromosome in the females and if you have x plus y you have normal wild type so the wild type is dominant and the wild type will compensate for the color blindness so conclusion both males and females have normal color vision but in the reciprocal cross this is where it gets interesting in the reciprocal cross so you have a colorblind female this time a true breeder color blind female so she would have the two x chromosomes that have the c allele for color blind so her eggs can only form that can only possess the x chromosome with the colorblind allele the normal vision male can make a you know wild type x chromosome sperm or a y sperm and when you do that punnett square cross you will see that all of the females have normal vision but all of the males are colorblind so do you see why out there you know in amongst populations of humans you see now why there are so many more males that uh you know have this colorblind trait than females this because it's x-linked and x-linked traits tend to affect males more than females because they don't have two x chromosomes with the ability of one of those x chromosomes uh masking or the the effect of the other right they don't have a compliment to help them out as females do see here in this female the wild type uh complement was able to mask the color blind but in males the colorblind x aldel is the only one they have there's nothing to complement on the y chromosome so concept check number four and then we'll take a quick little break time with gizmo and wicket so let's get started concept check number four hemophilia which is reduced blood clotting is an x linked recessive disease in humans kind of like color blindness then a woman with hemophilia mates with a man who exhibits normal blood clotting what is the probability that their child will have hemophilia we just covered this didn't we isn't it identical to this situation here instead of color blind female in this case it would be hemophilia right in the female so wouldn't it be 50 50 and all of the females would be normal for blood clotting and all of the males would be abnormal yep exactly right so again on that note let's take a quick little break time and we'll be right back [Music] hey everyone welcome back from break time with gizmo and wicket where we left off we were talking about the x chromosome and one thing you should realize is something very interesting with respect to the x chromosomes and that is when you have more than one x chromosome inside of a cell the excess x chromosomes uh inactivate themselves so if you know if you're a male you only have one x chromosome so the x chromosome is read by the cell but if you're a female and you have two x chromosomes well then one of those two x chromosomes will inactivate itself it will form what is called as a bar body a bar body is an inactivated x chromosome and you can actually see it in the nucleus of female cells and you don't see a bar body in male cells and why again because they don't have to inactivate an x chromosome so it doesn't matter how many x chromosomes you have all but one becomes a bar body all but one gets inactivated so let's look at this if you have two x chromosomes this is your female no no syndrome and you have one bar body if you're a male no syndrome uh no bar bodies if you've got turner syndrome uh that you've got zero bar bodies if you're kind felter with one extra x you have one bar body klein felter two extra x's i'm sorry one extra x and an extra y you have one bar body klein filter two extra x's two bar bodies and so on and so forth you see so it doesn't matter how many x's you have all but one gets inactivated and becomes a bar body this actually explains the patchy distribution of color on tortoise shell cats this is due to random x inactivation what does that mean that means in the cat's body certain cells have inactivated one x chromosome which leads to one coke color and other cells in the cats have activated the other x chromosome which leads to a different coat color so and this usually happens earlier in development this x inactivation and that's why you see these patchy areas it's not that one cell is different than the cell adjacent to it is that you've got this patchy uh orientation due to this x inactivation being random and happening a little bit earlier in development isn't that interesting so here's concept check time how many bar bodies will a male with x x x y y so kind filter with two extra x's and an extra y chromosomes have in each of his cells what are those bar bodies okay so you're gonna have two extra bar bodies each body each bar body is an inactive x chromosome why chromosomes do not need to inactivate themselves because they they don't have a lot of genetic information there's the sry gene mainly and uh the y the y chromosomes uh do not need to be inactivated now let's talk a little bit about the y linked characteristics what's going on with the y chromosome these y link characteristics are also known as hollandric traits they're only present in males from y chromosome which is inherited by the father remember your y chromosome comes from your father and it's the father's sperm that dictates whether you have the x y male or x x female offspring all male offspring will exhibit the trait the y chromosome is lost has lost dna over time and the y chromosome is mostly non-functional so it accumulates mutations which serve as good genetic markers or snips the y chromosome has lost a lot of dna over time a lot of the y chromosome is non uh in non coding information it's and so it accumulates mutations quickly because it's not information that's genic it's not information that's being used for anything so you can actually look at those genetic markers these snips short nucleotide uh polymorphisms and you can use that to trace your lineage back through your paternal past if you will it's pretty interesting stuff and it's also important for sex determination remember that sry gene is important for sex determination it's the important gene that directs maleness in the y chromosome for males so now moving on from uh the sex-linked or x-linked uh inheritance let's look at other types of inheritance that deviate or that that appear to deviate from mendel's uh mode of inheritance let's look at concepts like dominance uh like complete dominance incomplete dominance and codominance what mendel was studying uh in his monohybrid and dihybrid crosses would be complete dominance you have one trait that completely masks another but you can also have uh incidence of incomplete dominance and codominance and this can confuse the pattern of heredity that you can see so let's let's ex let's explore this so again complete dominance is when the phenotype of the heterozygous is the same as the phenotype of one of the homozygotes so this is what mendel saw if you had purple flower plants and purple is the dominant color well it would always mask the white flower plant and the f1 generation would always be purple flower but incomplete dominance what about that phenotype of the heterozygous is an intermediate it falls within the range between the phenotypes of the two homozygotes now keep in mind i'm going to show you this in more detail a little bit but this is not the same thing as blending the remember blending hypothesis was disproven incomplete dominance is not blending and you'll see why in a little bit let's look at codominance codominance the phenotype of the heterozygote includes the phenotypes of both homozygotes again this is not blending either and a lot of times you can confuse incomplete dominance with codominance so we're going to go through and talk a little bit more about the difference between the two so here's complete dominance at the top left again if you have a red flower plant and a white flower plant well if red is dominant to white then the offspring are going to be 100 percent red right now um oh and not only will the offspring be 100 red the only variability you will find in the offspring are either red flowers or white you're not going to find pink or anything like that now with incomplete dominance you will find if the phenotype of the heterozygotes fall between the phenotypes of the two heterozygotes dominance is incomplete so in this case you would find red you would find white and you would find everything in between you would find different shades okay so concept check time how does incomplete dominance incomplete complete dominance incomplete dominance and codominance differ which is what we just touched on but again with complete dominance the heterozygote expresses the same phenotype as one of the homozygotes so you either have red flowers or white flowers with incomplete dominance the heterozygote has a phenotype that is intermediate between the two homozygotes and with codominance the heterozygote has a phenotype that simultaneously expresses the phenotypes of both homozygotes again i know this is confusing but one way to think about it is with codominance uh let's talk about incomplete dominance first with incomplete dominance you're going to have some kind of intermediate intermediate uh phenotype so again like pink flowers let's talk about codominance let's say there is a um there is a gene for an extra finger and then there's a gene for a split finger well it wouldn't be some mix of the two it would be an extra and split finger you would see both of those traits together does that make sense they they both express themselves and they both present themselves uh if that makes any sense so with penetrance here's another concept penetrance this is the percentage of individuals that have a particular uh that show a particular phenotype that have a genotype so just because you have a genotype like you have an allele for something doesn't mean you're going to express it even if it's a dominant allele even if it's dominantly inherited it doesn't necessarily mean you will you know 100 percent of people will express that phenotype this is penetrance and then there's expressivity which is the degree to which the character is expressed so you can have traits that have variable penetrance so some people don't show the trait even though they have the genotype for the trait whereas some people show the trait because they have the the allele for the trait and then you can have variable expressivity as well which means that you know how defined is that trait in some people it might not be defined very well defined at all it might be a minor phenotype and in others it might be a major phenotype so let's look at an example with human polydactyly with human polydactyly you have an extra finger and the penetrance is about 90 that means of the people with this uh this uh human polydactyly uh gene only about 90 percent show polydactyly and of the ones that show it of the 90 that show polydactyly there's variable expressivity which means uh you could have everything from a tiny little nub as the extra finger or a tiny little skin tag to an actual extra finger so you see the difference between penetrance and expressivity penetrance is how what percent of people with that allele express it like show it and and and expressivity is of those that show the phenotype how pronounced is that phenotype so concept check number seven how does incomplete dominance differ from how does incomplete dominance differ from incomplete penetrance so we talked about this i'm not going to read through all of these but look let's look at the answer in incomplete dominance the heterozygote is intermediate between the homozygotes so you've got for example the pink flower in incomplete penetrance some individuals do not express the ex the expected phenotype at all right so that that's a completely different uh outcome there here's another situation that you need to be aware of that can really affect the mendelian ratios that you expect and this is the case of lethal alleles believe it or not yellow coke color in mice can be lethal and result in uh abortion you know basically a miscarriage in the mouse in the mouse so look at look at yellow uh fur color if you're homozygous dominant for big y homozygous big y uh that's actually a lethal allele it's considered a lethal allele it results in a dead pup right and this is a miscarriage you would not this would not be a living birth right this would not be a viable birth so what you would need to do for this punnett square you would take a non-true breeder a heterozygous yellow mouse with a big y little y and you would breed it with a heterozygous non-true breeder yellow mouse big y little y and after fertilization you would expect a quarter to be big y big y a quarter to a half to be big y little y and a quarter to be little y little y but if you were just focused on the offspring if you were just focused on on the live births and you didn't know this concept of lethal alleles what would you say the outcome is you would look at the pups and you would say well about two-thirds are yellow and one third is white so there's a two to one ratio of yellow to white and that does not agree with mendel's pattern of inheritance but if you know the concept of the lethal allele you would be able to factor in the quarter of the of the fetuses that didn't make it to birth to fruition so again lethal alleles exist there are combinations of of alleles that you could inherit and that would result in you know miscarriage and you need to factor those in you need to be aware that they at least exist because that could mess up your your studies if you're all your if all you're studying or the live births you're going to miss the lethal effects of the lethal alleles okay so keep in mind that there can be multiple alleles for a given locus within a population so for example you can have the blood group you can have not just blue or brown eyes but you could have hazel eyes green eyes there could be more than just two alleles within a population also keep in mind that frequently genes exhibit independent assortment themselves but they don't necessarily act independently in their phenotypic expression the reason for this is gene interaction gene interaction this can confound heredity studies because the effects of one gene can actually affect the phenotype of a different gene at a different locus and there are two examples of this gene interaction which i'll explain just a minute and epistasis which i'll also explain after that so here's an example of gene interaction and by the way if wiki's come to say hi and help me to do the presentation he's just hanging out over here hi wiki all right now with his support we will get through this example of gene interaction with the pepper capsicum annum now here's an example of what i said gene interaction there's going to be a gene interaction that affects the color of this pepper notice that this is the pea generation these are the true breeders for red peppers and cream peppers the reason this pepper is red is because it has two different pigment genes two different dominant pigment genes capital y and capital c okay and there's a gene interaction between those two pigments that gives the pepper the red color and the cream color pepper the reason it's cream color is because it's missing both of those genes both of those pigment genes or pigment producing genes i should say now when you cross the cream color pepper with the red pepper you're gonna see a hundred percent red phenotype in the f1 generation that's because these are hybrids but they both have the capital y and the capital c they all have the capital y the capital c so all of the offspring are going to be red because of the gene interaction between y and c but when you self-fertilize when you self-fertilize the f1 generation and you look at the f2 generation something interesting happens you it reveals the consequence of the gene interaction some of these are capital y capital c in this case you're going to have red peppers because of the gene interaction between y and c in some of these in 1 and 16 you're going to have no y and no c right no capital y no capital c and these are going to be cream like the original parents however if you only have a dominant y and only recessive c you're going to see the phenotype of just allele y and the phenotype of allele y by itself without the gene interaction with c is a peach color plant a peach color pepper now if you have just c and no y then you're gonna see the phenotype of just allele c which is an orange pepper does that make sense so this shows you that allele y gives you a peach color pepper allele c gives you an orange color pepper and the combination of the two gives you a new phenotype this is a gene interaction to give you a novel phenotype and that's exactly the definition of gene interaction it produces a novel phenotype a new phenotype which is red and the lack of either of those pigment producing enzymes is cream cream color okay now let's take a look at a different example let's look at epistasis this is where one gene masks not works with not interacts with but one gene masks the effect of another gene the hypostatic gene so the epistatic gene is the one that does the masking the hypostatic gene is the one that gets masked and you can have recessive epistasis or you could have dominant epistasis let's take a look at dominant epistasis here you're dealing with the y i'm sorry the w allele this is the epistatic allele and you have the y allele this is the hypostatic allele notice the uh notice what uh y does first y converts compound b a green color compound to compound c by making an enzyme enzyme two but what if compound b wasn't there think about it what if compound b was not there if there's no compound b then the enzyme that's produced by allele y wouldn't be able to convert green to yellow and that's exactly what this epistatic gene does you see this capital w now yeah this capital w epistatic gene it produces enzyme one which converts compound a a colorless compound to compound b a green color compound so we could say that w w is a capital w i should say is epistatic to y because y can only produce its phenotype yellow squash if w capital w is not around does that make sense because when capital w is not around you're able to produce compound b and then capital y can convert compound b to compound c so what is the conclusion here genotypes big w big y and big w little y do not produce enzyme one it makes sense because that's the that's the epistatic gene dominant epistemic so you only need one by the way that's why it's dominant epistasis you only need one copy of capital w if this was recessive you'd need two copies of that epistatic gene genotypes big w big y and big w little y do not produce enzyme one little w little w little y little y produces enzyme one but not enzyme two so you're gonna end up with uh green green squash uh and little w little w big y produces both enzyme one and enzyme two that's the only way to get yellow squash does that make sense here's just a table outlining the different combinations we were talking about now let's move on to sex influence and sex limited characteristics and define these terms here sex influence characteristics are those determined by autosomal genes not the sex-linked genes not the x-linked genes these are determined by autosomal genes and are inherited according to mendel's principles but they get expressed differently in males and females so usually it's the the sex chromosomes have an effect on these genes but these genes do not reside on those sex chromosomes then there are sex limited characteristics these are encoded by autosomal genes that are expressed in only one sex and these are these these have zero penetrance in any other sex so they're only found in the one sex even though again they are autosomal genes which means they don't actually reside their locus is not on the sex genes or is this x chromosomes x or y and then there's cytoplasmic inheritance this is where characteristics encoded by genes located in cytoplasm occurs so you could have genes in the cytoplasm and that can give you a phenotype genes in the cytoplasm can also include genes of the mitochondria so let's do a concept check how do sex-influenced and sex-limited characters differ from sex-linked characteristics well we just touched on that remember sex-linked characteristics are found on the sex chromosomes whereas the sex-influenced and sex-limited characteristics are found on the autosomes so let's double check our our logic here both sex influenced and sex limited characteristics are included are encoded by autosomal genes whose expression is affected by the sex of the individual who possesses the gene sex linked characteristics are encoded by genes on the actual sex chromosomes so here's an example of cytoplasmically inherited characteristics with regard to the mitochondria remember mitochondria have their own genes and their own chromosome and that is considered part of your cytoplasmically inherited genes now sometimes mutations can arise in these mitochondria and over time with each subsequent cell division those mutated mitochondria can accumulate in particular cells so you could end up with one cell with all mutated mitochondria and one cell with healthy mitochondria and then you could end up passing on this mutated mitochondria to your offspring so for example what if this became an egg right what if this became an egg that was fertilized in that case this person would have mutated mitochondria and there are actual genetic diseases genetic disorders that you can inherit from such a case from from you know mis mutated mitochondrial genes and this can have lifelong debilitating effects on on the child so again cytoplasmically inherited traits present in males and females usually inherited from one parent typically the maternal parent especially if you're talking about through the mitochondria the mitochondria is only inherited by the mom's egg through the mom's egg so all of the mitochondria in your body by the way are from your mom because the egg was the only gamete with mitochondria the sperm didn't bring any mitochondria with it so all of the mitochondria in your body or from your mom and so if there was a mutation in those mitochondrial genes then you're going to have problems but it's because of the mutation that originated in your mom not from the dead reciprocal crosses give different results obviously because usually you you inherit that cytoplasmic dna from one parent usually and it's usually the mom so a reciprocal cross would result in no effect and exhibit extensive phenotypic variation even within a single family again that makes sense because if you if you think about it your your sibling could have inherited all bad mitochondria might have mutated mitochondrial genetics and you may have inherited wild-type mitochondria so you might be relatively healthy whereas your sibling might be relatively sick because of the proportion of the uh mutated mitochondria that you versus your sibling inherited pretty interesting stuff so let's get back to sex influenced and sex-linked uh characteristics going into some examples of each so we got genetic maternal effect this is where the phenotype of the offspring is determined by the genotype of the mother we're going to give you an example of that in a minute then there's genomic imprinting where differential expression of gene material depending on whether it is inherited by the male or the female parent so you might inherit a allele but if you inherit it from your mom it might get silenced or if you inherit it from your dad that gene might be silenced and what does that mean there's different ways of silencing genes for example you could methylate a gene and so you add a methyl group to the gene and that could cause the gene to silence itself okay that's an example of genomic imprinting and then epigenetics where uh due to alternations to the alterations to the dna again through methylation or to the histones so that they they shut down gene expression that do not include changes to the base sequence this can affect the way the dna sequences are expressed when people talk epigenetics they're usually talking about um turning on or off genes but not by changing the gene but by changing the expression of the gene usually by methylating the gene or by altering the histones the hit if you if you modify the histones remember the histones in the in the chromatin you can modify the little histone tails to either open up the gene so it can be red or close up the gene so it can't be read to silence the genes so let's go through these again and define each one we we we've touched on these before but sex link characteristics these are genes located on the x uh chromosome the sex chromosome sex influenced characteristic these are genes on autosomes remember autosome means any chromosome that's not the sex chromosome that are more readily expressed in one sex sex limited characteristics very similar to sex influence however in this case it's an autosomal gene whose expression is limited to one sex not just more readily expressed in one sex it's only expressed in one sex then there's genetic maternal effect remember this is where nuclear genotype of the mom determines the phenotype of the offspring cytoplasmic inheritance this is where cytoplasmic genes uh where which are usually inherited entirely from only one parent for example remember the the mitochondrial genes are an example of cytoplasmic genes any genes any genomic material that's outside of the nucleus is called cytoplasmic genes so if it's not part of your genome proper if it's not one of your 46 chromosomes it's a cytoplasmic gene and all of us have dna a whole chromosome a circularized chromosome of dna inside of our mitochondria and that's not considered part of our genome that's cytoplasmic dna and we inherited that cytoplasmic dna from our moms and remember if you have mutations in that mitochondrial dna you can actually have severe uh uh heritable issues um and they have to do with the misfunction of the of the mitochondria okay and genomic imprinting this is where genes whose expression is affected by the sex of transmitting parents so again like i said you could inherit an allele from your mom but it might be silenced due to a methylation or something just by by way of being inherited by that parent so here is an example an elegant example of maternal effect in the snail you can have left handed sinistral snails shells and dextral right-handed coil on the snail shell in this case you've got males with right-handed coils right-handed is dextral and female with left-handed sinistral shell now remember what maternal effect means it means that the genotype of the mom determines the phenotype of the offspring okay so follow that the genotype of the mom dictates the phenotype of the offspring just remember that it'll make much more sense as we go through here so here's what here's what we're seeing uh in dextrol which is right-handed coiled uh uh father you have s plus s plus dextral a right-handed coil results from an autosomal allele s plus that is dominant remember s plus is dominant the female in this case is sinistro which means left-handed coil which codes for sinistral s which is the recessive for left-handed coil now when these parents mate their gametes s plus and s fertilize fuse to form offspring f1 offspring and get this all of the f1 offspring our sinistral left-handed coil even though all of them are s plus and s what if you were just looking at the genotype what should the phenotype be if you were just looking off of this genotype it's s plus s remember s plus is dominant for dextrol this offspring is s plus s so shouldn't this offspring be dextral it should it should be dextro because dextral is the dominant allele however it's sinistral and the reason is like i just said maternal effect the genotype of the mother determines the phenotype of the offspring so let's see what it says here all the f1 are heterozygous s plus s because the genotype of the mother determines the phenotype of the offspring all of the f1 have a sinistral shell so let's look at this in closer detail here again the dextrol dominant father cross with the sinistral recessive mother to give you heterozygotes that should be dextrol if you're looking at it genetically but are actually sinister if you're looking at maternal effect if you're factoring in maternal effect now what's the next step self-fertilization which means that uh this this snail will have to make with its own genotype so it can make g it can make uh gametes with s plus or s and here's what happens you have offspring but guess what all of the offspring are dextral all of the offspring are dextrol now look at this a quarter of the offspring are s plus s plus that makes sense that they're dextrol right because s plus s plus is dominant and s plus is codes for dextrol the heterozygotes are also dextral because s plus and s well s plus is dominant and it codes for dextrol so why then think about this why is this shell dextrol why is this shell dextrol little s little s should code for sinistral but it's showing dextral well remember what maternal effect means the genotype of the mom not the phenotype of the mom the genotype of the mom dictates the phenotype of the offspring so even though the offspring one quarter of the offspring should be sinister it shows dextro because of maternal effect very interesting stuff so again conclusion because the mother of the f2 progeny has genotype s plus s all of the f2 snails are dextral very interesting stuff now moving on to genomic imprinting again remember this is when you inherit an allele from one parent but that allele might be turned off switched off due to methylation or some other kind of imprinting on the on the genome so in this case it's igf-2 it's a growth factor for mice the paternal allele has igf2 the maternal allele has igf2 the paternal allele is active and its protein products stimulates fetal growth the maternal allele is silent it's switched off it's imprinted off the absence of its protein product does not further stimulate fetal growth and then the size of the fetus is determined by the combined effects of both alleles in that interesting so here's an example of genomic imprinting where you're inheriting a gene but it is switched off depending on which parent you inherited from humans also have an igf allele and it is also imprinted and it can affect fetal growth so very interesting another example of how animals can lend to human studies now this one's really a favorite of mine it's very interesting and that's other types of phenotypes that are perplexing until further researched and these are here's an example temperature sensitive alleles this is an allele whose product is functional only at certain temperatures notice this bunny isn't it black and white but notice where is it black where is this bunny where does it have black fur on its ears on its nose on its little feet and it's on its little tail right aren't those the coolest parts of the bunny temperature wise the the ears are cool the nose is cool the hands the paws the paws the tail here's the thing the enzyme the enzyme that produces this pigment in the fur is only functional at below 25 degrees c so around around room temperature room temperature is 24 degrees c does that make sense so room temperature is around 24 degrees c your body's temperature well for humans anyways about 37 degrees c so uh you know if we had a similar enzyme yeah the tips of our ears might turn black too you know or the hairs might turn black and that's because of the cooler uh tips of your fingers tips of your nose tips of your ears very interesting stuff at any warmer than that and the enzyme unfolds right so the these are temperature sensitive uh mutations temperature sensitive genes so when when the body temperature warms up a bit the protein that's the result of this gene the protein might unfold a little and that protein unfolds it may not function and so it can't produce the pigment in the rest of the bunny isn't that interesting all right i believe this is the last concept of the chapter polygenic characteristics versus pliotropy polygenic characteristics simply put are characteristics encoded by genes at many loci so for example people often ask me you know dr d what's what's the gene for height for example you know like uh my brother's six five and i'm only five five you know what what happened there what gene did i not get um well here's the thing height for example is an example of a polygenic characteristics there are characteristics encoded by genes at many loci there are many many many different genes that dictate your height does that make sense some of them dictate how long your bones will get some of them dictate how long you know how much growth there is during development right there's growth factors involved there's bone stimulating factors bone morphogenetic proteins there's all kinds of different genes that need to work in concert for you to be tall or short it's not one gene it's not like you know sickle cell that has one genie of sickle cell there are many different genes involved here and then plyotrophy where one gene can affect multiple characteristics you could have one gene play a role let's say in heart formation and then it can play a role later on maybe in blood vessel formation too so you have one gene and it might have multiple different roles in development and then in the function of the body so again what is the difference between polygenic and plyotrophy uh polygenic refers to influence of multiple genes on one characteristics plyotropy is the effect of a single gene on multiple characteristics all right and that leads us to the end of this chapter i hope it made sense this is a little bit of a confusing chapter but nothing you can't get through i hope you guys learned a lot and i'll catch you guys next time dr d dr d dr d dr d dr d dr d a dr d dr d dr dr [Music] d