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Introduction to heredity

Genetics is a fascinating world, focusing on the inheritance of traits from parents to offspring. Gregor Mendel, the father of classical genetics, made significant contributions to our understanding of this process. Dominant and recessive traits play a crucial role in determining an individual's characteristics. The difference between genotype and phenotype is also a key concept, illustrated here through the example of eye color. Created by Sal Khan.

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  • female robot grace style avatar for user Pinkcatcrystal
    Why is the darker eye color usually dominant?
    (141 votes)
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  • marcimus pink style avatar for user stevenlshi1337
    is it possible that one day people will all have the same eye color and skin color
    (76 votes)
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    • leafers sapling style avatar for user Jay
      If everyone has the dominant allele for eye and skin color by some chance, then yes I think it is POSSIBLE (trying to stress it), but highly improbable since there are now about 7 billion people in the world. By using punnet squares, even if everyone has the dominant allele from one parent, their children would have the possibility of having recessive alleles from both parents. It would all depend on the parent's genotypes. So in practical terms, it is not possible for everyone to have the same
      (86 votes)
  • male robot hal style avatar for user FengYiHung
    What eye colors are the most dominant in descending order?
    (22 votes)
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    • blobby green style avatar for user M H White
      You must remember that dominance of an allele and how common that allele is in the population (its frequency) are not the same thing. Most of the answers here are actually about frequency, not dominance. Also remember that eye color is not determined by a single gene. Any least 5 separate genes affect human eye color, which helps explain the many different colors possible. In general, alleles for darker eyes (more pigment) are dominant to those for lighter eyes (less pigment), but more precision than that is way beyond the scope of these lessons.
      (18 votes)
  • blobby green style avatar for user Mairead Sullivan
    This may be a silly question, but are the 46 chromosones in one cell different to the 46 in another cell or are they the same in each cell??
    (20 votes)
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    • leaf green style avatar for user Brandon
      All of your cells have the same chromosomes, in which 23 came from your father and 23 from your mother. Just remember, only human diploid cells have 46 chromosomes. The human haploid gametes have 23 chromosomes. When 2 gametes (male and female) meet, they fuse creating a zygote, that contains 46 chromosomes.
      (21 votes)
  • primosaur ultimate style avatar for user kots.yash
    Why is it that darker colors are dominant over lighter colors?
    Is the probability actually correct, that if the punnet square goes as such in the video that 3/4 chance of brown eyes, and 1/4 chance of blue eyes?
    (11 votes)
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    • male robot johnny style avatar for user N8-0
      If Brown is dominant and Blue is recessive: B=brown and b=blue, the chances of brown are dependent on the parental traits. If the parents had Bb and Bb, the chance of brown and blue would be 75% brown and 25% blue. But if the parents were Bb and bb, it's 50% for both. If it's BB and BB, you get 100% brown, and if you get bb and bb, you get 100% blue. Hope this helped!
      (22 votes)
  • blobby green style avatar for user chris438
    At does the order in which you write the heterozygous trait matter? Like does the dominant allele always appear first?
    (10 votes)
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    • piceratops seed style avatar for user Nanditha
      It doesn't matter in a Punnett square, you can put whichever genotype you prefer first. Personally, I prefer to put the dominant allele on the top. Either way, you will end up with the same genotypes for possible offspring. Hope this helped!
      (7 votes)
  • winston default style avatar for user ✯✔☠✨☢✦🔥⫸Kᴡɪs⫷🔥✦ᖭ👻ツ✘✯
    How are there so many eye colors if we all came from 2 people. If the two people had different colored eyes, like blue and brown, than they will most likely become brown. How can so many people have different colored eyes?
    (7 votes)
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    • mr pants teal style avatar for user Matthew Belliveau
      You pose a good question: how could all of this variation derive from just two people? We have people with red hair and black hair, brown, and blonde. Eyes of a huge variety, as well as differences in facial structure and skin color. And those are just the external differences.

      Scientists see this, and many other factors (such as fossils), as evidence that our human history is much older than we originally thought (~1 million years) and that there's no way that it started with two people (commonly called Adam and Eve).
      (15 votes)
  • aqualine ultimate style avatar for user athangyende459
    can a hetrozygote have more than 2 alleles for eye or skin color?
    (5 votes)
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    • female robot grace style avatar for user tyersome
      Note: My original answer was a bit sloppy — this is an edited version.

      First, many genes may influence a single phenotype. Eye color is an excellent example of this with 16 known genes that can have some influence on this trait§. My answer below assumes that we are talking about a single gene.


      Yes, but only under circumstances that are not addressed at this level of study.

      For example in organisms with more than two sets of chromosomes (e.g. a tetraploid organism has four copies of each chromosome and thus could have four different alleles for each gene).

      Another exception occurs when organisms are genetic mosaics aka. chimeras — i.e they contain cells that are not genetically identical! This is actually much more common than you might expect — for example any female animal that has been pregnant is likely to have at least a few cells from each fetus she carried growing somewhere in her body!

      So, the answer is "no", but only if you rephrase the question as "Can a diploid heterozygous cell contain more than two alleles for a single gene?"

      Does that help?

      §Note: Of the 16 genes involved in eye color two have major effects, and four more show relatively significant effects.

      This section of the wikipedia article on eye color is worth checking out:
      (14 votes)
  • winston default style avatar for user ph257747
    how does dna go in to a baby when born
    (5 votes)
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    • leaf blue style avatar for user Delta Whiskey
      So DNA goes into a baby by way of meiosis, which is where cells divide to reduce their chromosomal number to the haploid state, creating gametes, which are sperm in males and eggs in females.

      These then need to join together to make a zygote, which has DNA in it and that turns into a baby.

      Let me know if you have any other questions
      (7 votes)
  • blobby green style avatar for user SmittysHydro
    why are some eye colars rarer than others
    (5 votes)
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Video transcript

Well, before we even knew what DNA was, much less how it was structured or it was replicated or even before we could look in and see meiosis happening in cells, we had the general sense that offspring were the products of some traits that their parents had. That 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 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 this so they have a little brown-eyed baby here. 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. One example of that tends to be a darker pigmentation in maybe the hair or the eyes. Even 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 or became a big deal in the middle of the 20th century. This was studied a long time. And kind of the father of classical genetics and heredity is Gregor Mendel. He was actually a monk, and he would mess around with plants and cross them and see which traits got passed and which traits didn't get passed and tried to get an understanding of how traits are passed from one generation to another. So when we do this, when we study this classical genetics, I'm going to make a bunch of simplifying assumptions because we know that most of these don't hold for most of our genes, but it'll give us a little bit of sense of how to predict what might happen in future generations. So the first simplifying assumption I'll make is that some traits have kind of this all or nothing property. And we know that a lot of traits don't. Let's say that there are in the world-- and this is a gross oversimplification --let's say for 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 you could have blue eye color or you could have brown eye 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 for 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 and there's another allele for small teeth. And I want to make very clear this distinction between a gene and an allele. I talked about Gregor Mendel, and he was doing this in the 1850s well before we knew what DNA was or what even chromosomes were and how DNA was passed on, et cetera, but let's go into the microbiology of it to understand the difference. So I have a chromosome. Let's say on some chromosome-- let me pick some chromosome here. Let's 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 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. We know that when they're normally in the cell, they aren't nice and neatly organized like this in the chromosome, but this is just to kind of show you the idea. 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's also the eye color gene. Now, I might have 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. 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. 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-- I have the allele, I have one big B from my mom and I have one small b from my dad. Each of these instances, or ways that this gene is expressed, is an allele. So these are two different alleles-- let me write that --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. And the genotype is the exact version of the alleles I have. Let's say I had 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 this case, this genotype is homozygous, or this is a homozygous genotype, or I'm a homozygote for this trait. Now, you might say, Sal, this is fine. These are the traits that you have. I have a brown from maybe my mom 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 it's very complex. It's a whole mixture of things. But Mendel, he studied things that showed what we'll call 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 always dominated by the other traits. We'll assume that here, but that's a gross oversimplification. So let's say that brown eyes are dominant and blue are recessive. I wanted to do that in blue. 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 B here represents brown eye and the lowercase b is recessive --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, and then I'll write phenotype. Genotype is the actual versions of the gene you have and then the phenotypes are what's expressed or what do you see. So if I get a brown-eyed gene from my dad-- And I want to do it in a big-- I want to do it in brown. Let me do it in brown so you don't get confused. So if I've have a brown-eyed gene from my dad and a blue-eyed gene from my mom, because the brown eye is recessive, 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. Now, let's say I had it the other way. Let's say I got a blue-eyed allele from my dad and I get a brown-eyed allele for 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. 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 blood types. There's four types of blood. But let's say that I get two blue, 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 maybe understand how heredity works. There are some traits that can be studied in this simple 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 know exactly whether they were homozygous dominant-- this would be homozygous dominant --or whether they were heterozygotes. This is heterozygous right here. These two right here are heterozygotes. These are also sometimes called 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 gene. So let's think a little bit about what's actually happening when my mom and my dad reproduced. Well, let's think of a couple of different scenarios. Let's say that they're both hybrids. My dad has the brown-eyed dominant allele and he also has the blue-eyed recessive allele. 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 people's genotypes are. Let me label them. Let me make this the mom. I think this is the standard convention. And let's make this right here, this is the dad. What are the different genotypes that their children could have? So let's say they reproduce. I'm going to draw a little grid here. So let me draw a grid. So we know from our study of meiosis that, look, my mom has this gene on-- Let me draw the genes again. So there's a homologous pair, right? This is one chromosome right here. That's another chromosome right there. On this chromosome in the homologous pair, there might be-- 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 from 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 two chromatids on a chromosome. But they line up in meiosis I 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 eye color? Well, it's going to be capital B and capital B. If I get this chromosome from my mom and this chromosome from my dad, what's it going to be? Well, I'm going to get the big B from my dad and then I'm going to get the lowercase b from my mom. So this is another possibility. Now, this is another possibility here where I get the brown-eyed allele from my mom and I get the blue eye allele from my dad. And then there's 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 be brown, that one's going to be brown, 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 has this recessive dominant situation. And they're both heterozygotes where they 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? 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 3/4, or it's a 75% probability. Same logic, what's the probability that these parents produce an offspring with blue eyes? Well, that's only one of the four equally likely possibilities, so blue eyes is only 25%. Now, what is the probability that they produce a heterozygote? So what is the probability that they produce a heterozygous offspring? 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 had to make a lot of assumptions about the genes and whether one's dominant or one's a 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, hey, given that this couple had five kids with brown eyes, what's the probability that they're both heterozygotes, or something like that. So it's a really interesting area, even though it is a bit of oversimplification. But many traits, especially some of the things that Gregor Mendel studied, can be studied in this way.