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Video transcript

well before we even knew what DNA was much less how it was structured or was replicated or even before we could look in and and see meiosis happening in cells we had the general sense that offspring were the products of some traits that their parents had that you know if I had a guy with blue eyes let me say this is the blue-eyed guy right here and then if he were to marry a a brown-eyed girl let's say this is the brown-eyed girl maybe make it a little bit more like a girl if he were to marry the brown-eyed girl there that most of the time or maybe in all cases where we're dealing with the brown-eyed girl maybe their kids are brown-eyed let me do it so they have a little brown-eyed baby here all right and this is just something I mean there's obviously thousands of generations of human beings and we've observed this we've observed that kids look like their parents that they inherit some traits and that some traits seem to dominate other traits and one example of that tends to be a darker pigmentation and maybe the hair or the eyes even if the other if the other parent has light pigmentation the darker one seems to dominate or sometimes it actually ends up being a mix and we've seen that all around us now this study of what gets passed on and how it gets passed on it's much older than the study of DNA which was really kind of discovered in the or became a big deal in the middle of the 20th century this was studied a long time and and kind of the the father of classical genetics and heredity is Gregor Mendel Gregor Mendel who's actually a monk and he would mess around with with plants and across them and see which traits got passed and which trace didn't get passed and try to get understanding of how how traits are passed from one generation to another so when we do this when we studied these this you know I'll call it classical classical genetics I'm going to make a bunch of simplifying assumptions and because we know that most of these don't hold for most of our genes but let give us a little bit of sense of how to predict what might happen in in future in future generations so the first simplifying assumption I'll make is that is that some traits have kind of this all-or-nothing property and we know that a lot of traits don't that let's say that there in the world and this is a gross oversimplification let's say for I color eye color let's say that there are two alleles now remember what an allele was an allele is a specific version of a gene so let's say that there's the you could have blue eye color or you could have brown eye color or you could have brown-eyed color that we live in a universe where someone could only have one of these two versions of the eye color gene we know that eye color is far more complex than that so this is just a simplification and let me just make up another one let me say that I don't know maybe a for for for teeth for tooth size tooth size that's a trait you won't see in any traditional biology textbook and let's say that there's one trait for big teeth big teeth and that there's another allele for small teeth small teeth and I want to make very clear this distinction between a gene and an allele L allele so let's let's you know I talked about Gregor Mendel and he was doing this in the 1850s well before we knew what DNA was or how or what even chromosomes were and how DNA was passed on and etcetera but let's think a little bit about let's go into the into the into the microbiology of it to understand the difference so I have a chromosome let's say on some chromosome let me pick a let me pick some chromosome here is it say this is some chromosome let's say I got that from my dad and on this chromosome there's some location here we could call that the locus on this chromosome that where the eye color gene is that's the location of the eye color gene now I have two chromosomes one from my father and one from my mother so let's say that this is the chromosome from my mother and they you know we know that when they're normally in the cell they aren't night nice and neatly organized like this in a chromosome but this is just to kind of show you the idea and let's say these are homologous chromosomes so they code for the same genes so on this gene from my mother on that same location or locus there is also the eye color gene now I might have the same the same version of the gene and I'm saying that there's only two versions of this gene in the world now if I have the same version of the gene I'm going to make a little shorthand notation and I'm going to write Big B actually let me do it the other way I'm going to write little B for blue and I'm going to write Big B for brown this if I there's a situation where this could be a little B and this could be a Big B and then I could write that my genotype my genotype I have the allele I have one Big B from my mom and I have one small B for my dad each of these instances or ways that this gene is expressed is an allele so this is so these are two different alleles let me write that two different alleles or versions of the same gene and when I have two different versions like this one version from my mom one version from my dad I'm called a heterozygote or sometimes it's called a heterozygous genotype heterozygous zygous genotype and the genotype is the exact versions of the alleles I have if I had let's say in but I had the the lowercase B I had the blue-eyed gene from both parents so let's say that I was lowercase B lowercase B then I would have two identical alleles both of my parents gave me the same version of the gene and in this case I'm called this this genotype is homozygous zeig or this is a homozygous genotype or I'm a homozygote for this trait homozygotes now you might say Sal this is fine these are the traits that you have this case you know I have a brown from maybe my mom a and a blue from my dad in this case I have a blue from both my mom and dad how do we know whether my eyes are going to be brown or blue and the reality is is it's very complex that it's a whole mixture of things but Mendel he studied things that that X that that showed what we'll call dominance dominance dominance and this is the idea that one of these traits dominates the other so a lot of people originally thought that eye color especially blue eyes was a was always dominated by the other trait so we'll assume that here but that's a gross oversimplification so let's say that brown eyes are dominant brown eyes brown are dominant and blue are recessive blue I wanted to do that in blue and blue eyes blue eyes are recessive if this is the case and this is a as I've said repeatedly this is a gross oversimplification but if that is the case then if I were to inherit this genotype because brown eyes are dominant remember I said the big be the big B here represents brown eyes you won't and the lower case B is recessive all you're going to see all you're going to see for the person with this genotype is brown eyes so let me do this here let me write this here so genotype genotype and then I'll write phenotype and genotype is the actual versions of the genes you have and then the phenotypes are what's expressed or what do you see so phenotype so if I get a brown-eyed gene from my dad I was like doing this a big I wanted to do it in brown let me do it in brown so you don't get confused so if I have a brown-eyed gene for my dad and a blue-eyed gene from my mom my color transitions aren't there a blue-eyed gene from my mom because the brown eye is recessed the brown-eyed allele is recessive and I just said a brown-eyed gene but what I should say is the brown-eyed version of the gene which is the brown allele or the blue-eyed version of the gene from my mom which is the blue allele since the brown allele is dominant I wrote that up here what's going to be expressed are brown eyes brown eyes now if I have it let's say I have let's say if I had it the other way let's say I got a blue-eyed from my dad and I get a wide allele from my dad and I get a brown-eyed allele from my mom same thing the phenotype is going to be brown eyes now what if I get a brown-eyed allele from both my mom and my dad let me see I keep changing the shade of brown but they're all supposed to be the same so let's say I get two dominant brown-eyed alleles from my mom and my dad then what are you going to see well you could guess that I'm still going to see brown eyes brown eyes so there's only one last combination because these are the only two types of alleles we might see in our population although for most genes there's more than two types for example there's you know blood types there's a there's multiples four types of blood but let's say that I get two blue from each of a one blue allele from each of my parents one from my dad one from my mom then all of a sudden this is a recessive trait but there's nothing to dominate it so all of a sudden the phenotype will be blue eyes and I want to repeat again this isn't necessarily how the alleles for eye color work but it's a nice simplification to to maybe understand how heredity works and there are some traits that do that that can be studied in this way but what I wanted to do here is to show you that many different genotypes so these are all different genotypes they all coded for the same phenotype so just by looking at someone's eye color you didn't need you didn't know exactly whether they were homozygous dominant this would be homozygous dominant or were that whether they were are heterozygotes this is heterozygous right here these two right here are heterozygotes heterozygotes these are also sometimes called as hybrids but the word hybrid is kind of overloaded it's used a lot but in this context it means that you got different versions of the allele for that for that gene so let's think a little bit about what's actually happening when when when my mom and my dad reproduced so my dad well let's think of a couple of different scenarios let's say my dad was let's say that they're both hybrids my dad has has the brown eye dominant allele and he also has he also has the blue-eyed recessive allele and let's say my mom has the same thing so brown-eyed dominant and she also has the blue-eyed recessive allele now let's think about if these two people before you see what my eye color is if you said look I'm giving you what these two peoples genotypes are and let me label them this is the well I mean look if I can make you let me make this the mom I think this is the standard convention and let's make this right here this is the dad this is the dad what are the different genotypes that their children could have so let's say they reproduce and what I'm going to I'm going to draw a little grid here so let me draw a grid all right so we know from our study of meiosis that look my mom my mom has this gene on let me let me draw the genes again so there's a homologous pair right this is one gene right here one chromatid Zone right here that's another chromosome right there on this chromosome and the homologous pair there might be the at that at the eye color locus there's the brown-eyed gene and at this one at the eye color locus there's a blue-eyed gene and similarly for my dad when you look at that same chromosome in his cells let me do them like this so this is one chromosome there and this is the other chromosome here when you look at that locus on this chromosome or that location it has the brown-eyed allele for that gene and on this one it has the blue-eyed allele on this gene and we learn from meiosis when the chromosomes well they replicate first and so you have these you know two chromatids on a chromosome but they line up in meiosis one during the metaphase and we don't know which way they line up for example my dad might give me this chromosome or might give me that chromosome or my mom might give me that chromosome or might give me that chromosome so I could have any of these combinations so for example if I get this chromosome from my mom and this chromosome from my dad what is the genotype going to be for I color well it's going to be capital B and capital B what's if I get this this chromosome from my mom and this chromosome from my dad what's it going to be well I'm going to get the I'm going to get the the Big B from my dad and then I'm gonna get the lowercase B from my mom so this is another possibility now this is this is another possibility here where I get the with the brown-eyed allele from my mom from my mom and I get the blue I allele from my dad and then there is a possibility that I get this chromosome from my dad and this chromosome from my mom so it's this situation now what are the phenotypes going to be well we've already seen that this one right here is going to this one's going to be brown that one's going to be this one's going to be brown but this one is going to be blue I already showed you this but if I were to tell you ahead of time that look I have two people they're both hybrids or they're both heterozygotes for eye color and eye color is it has expressed as this recessive dominant situation and they're both heterozygotes where they have each have one brown allele and one blue allele and they're going to have a child what's the probability that the child has brown eyes brown brown eyes what's the probability well each of these scenarios are equally likely right there's four equal scenarios so let's put that in the denominator four equal scenarios and how many of those scenarios end up with brown eyes well it's one two three so the probability is three-fourths or it's a 75% probability 75% probability same logic what's the probability that these parents produce an offspring with the blue eyes well that's only one of the four equally likely possibilities so blue eyes blue eyes is is only 25% now what is the probability that they produce a heterozygote so what is the probability that they produce a heterozygous offspring heterozygous so now we're not looking at the phenotype anymore we're looking at the genotype so of these combinations which are heterozygous well this one is because it has a mix it's a hybrid it has a mix of the two alleles and so is this one so what's the probability well there's four different combinations all of those are equally likely and two of them result in a heterozygote so it's 2/4 or 1/2 or 50% so using this punnett square and of course we have to make a lot of assumptions about the genes and whether one's dominant or ones are recessive we can start to make predictions about the probabilities of different outcomes and as we'll see in future videos you can actually even go backwards you can say a given that this couple had five kids with brown eyes what's the probability that they're both as I goes or something like that so it's a really interesting area even though it's it is a bit of oversimplification but many traits especially some of the things that that Gregor Mendel studied can be studied in this way
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