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Boveri-Sutton chromosome theory

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Overview of how chromosomes were first proposed as the carriers of Mendel's "heritable factors" (what we now call "genes").

Video transcript

- [Voiceover] Let's give ourselves a reminder of how important Gregor Mendel's work was in 1866, that he published in 1866. And it's important to realize it wasn't like immediately in 1866 or 1867 the whole world changed, and everyone said, "Oh, Gregor Mendel figured it all out." Like a lot of times in science the big discoveries, the ones that really change people's thinkings, aren't really taken that seriously at first. And actually, Mendel's work, a lot of people either didn't take it seriously or kind of ignored it in 1866. And it wasn't until the early 1900's that people rediscovered his work and realized, "Wait. Wait. There is something very, very powerful here, "and we might be able to connect it to things "that we are actually observing inside of cells." But let's just remind ourselves about Mendel's work. So, for most of human history, we've recognized probably that, okay. It looks like animals, or not just animals, any type of living creature seems to pass on traits to their offspring. I could look at you and I'd say, "Oh, you know, your hair is kind of like your dad's, "and your eyes are kind of like your mom's." Maybe your nose looks like something in between. You walk a little bit like your uncle. So, we've always recognized that we pass on traits to our offspring, but we didn't really have a rigorous way of thinking about it. And we definitely didn't have a way to make predictions that were testable based on those traits, and that's what Mendel gave us. He said, "Well, look. I'm observing," and he did this with pea plants, "I observed these heritable factors, "and there might be heritable factors on, "let's say, height." If we're talking about plant, it would be the height of a plant. There might be heritable factors on, let's say, flower color. So, flower color. And he recognized that there were different versions of those factors, and, so, a given plant might have one of the tall versions. So, they might have a tall version for the height factor, and they might have a short version. Or they might have two talls, or they might have two shorts. Or they might have a red factor, and they have a pink factor. Or they could have two reds, two reds. It would look like that. Or two pinks would look like that, but the important realization was that there were these versions of the factors. Today, we call these factors, we say, "Hey, there's a gene for height." If there is one. Or that there is a gene for flower colors, and those variations of the genes, today, we call these alleles. So, we'd say, "Hey, you have the variation. "You have one copy of the tall allele "and one copy of the short." Let me just write it this way. Let me just say these are all alleles right here. So, you have one tall allele, one short allele. And what Mendel did is he realized, "Well, look. These things are," he didn't know how, "but these things are the things that get passed on "from a parent to their offspring." And he started to describe about how they got passed on. He observed that, even if you have two of these, that they tend to segregate when you go to the next generation, and what do we mean by segregation, or I guess we can say the law of segregation? Law of segregation. Well, that means if this was, if I'm a pea plant and these are the version that I have, to my offspring I might pass on an A, a capital A, the tall one, or I could pass on the lowercase A. I might pass on the tall, or I might pass on the red version of the flower color factor, or I might pass on the pink one. And he also realized that whether or not I pass on the capital A or the lowercase A, it's independent of whether I pass on the capital B or the lowercase B. So, they independently assort. How this one assorts is independent of how this one assorts. So, independent assortment. Independent. Independent Assortment. Assortment. Law. Law of independent assortment. And he also observed that some of these versions dominate the other one. So, if an offspring has a tall version and a short one, if the tall one is dominant, the observed trait will still look tall, and the only way they'd look short is if they have two versions of the short one. And, so, that one he described as his law of dominance. Law of dominance. And if all of this is completely new to you, I encourage you to watch the videos on Mendelian genetics on Khan Academy, but this is just gonna appreciate a little bit of a historical appreciation. But as big of a deal as Mendel's work was, it's also important to realize what he didn't know. He had no idea of how this was actually happening at a molecular level or at a cellular level, and it wasn't until the early 1900's that people started to have fairly robust theories of how this happens. And, so, in 1902 and 1903. So, 1902-1903 these two gentlemen independently start coming up with the chromosome theory of inheritance, and it's called the Boveri-Sutton Chromosome Theory of Inheritance, because right around the same time, they both started to realize that maybe chromosomes were the actual molecular mechanism, the cellular mechanism, by which these factors segregate and independently assort. And, so, let me write this down. This is the Boveri-Sutton, it's sometimes called the Sutton-Boveri. Boveri-Sutton Chromosome Theory. Chromosome Theory. And even though they're starting to say, "Maybe chromosomes have something to do with it." They still don't know exactly what is inside the chromosome that are allowing somehow this information to be encoded, and we'll get to that. And we will get to that in a little bit, but let me just underline this. Boveri-Sutton Chromosome Theory. And, so, what was their key insight? Well, they started to look inside of cells, meiosis was observed actually after Mendel published his laws of inheritance, and then chromosomes, or how chromosomes behave in meiosis, was discovered after that. And then these guys, they independently studied different organisms. Walter Sutton, he studied grasshoppers. Theodor Boveri, he studied sea urchins, but they looked at meiosis, and they looked at the reproduction and the fertilization during these processes. And they saw that the chromosomes seemed to do things that were very similar to these laws of segregation, laws of independent assortment, laws of dominance. And actually, the law of dominance we'll talk more about in future videos. But he saw that, let's say that you had a organism here, and in this particular organism, I just did it for simplification, it has two pairs of homologous chromosomes. So, what does homologous chromosomes means? Well, these two are different chromosomes, but they seem to be very similar. It seems like they're kind of the same length, same size, same shape. So, that's one pair of homologous chromosomes. That's another pair of homologous chromosomes. So, notice. Homologous chromosomes, two things that are kind of looking the same, but maybe they're a little bit different. We're not sure. Well, maybe, this is what fits what's going on right over here with these factors. Maybe, just maybe, one of these chromosomes somehow has on it some place what encodes for the capital A. Capital A. And maybe, the other chromosome in a similar part of the chromosome. In a similar part of the chromosome, has what encodes a similar part of a chromosome, has what encodes for a lower case A. Now, this is starting to make sense because they would be homologous chromosomes similar the chromosomes look like they code for the same thing, for the same factors, for the same genes, but there might be some variation between these chromosomes. And these guys weren't able to somehow sequence the chromosomes. So, they didn't know. They didn't even know that the DNA was what was important in the chromosomes, but they said, "Well, looks like these two things, "if we look through the process of meiosis, "it seems like they segregate from each other." For example, this capital A one, it will replicate. So, you have capital A, and then this is the lowercase A one right over here. You might have some cross over, and we'll talk about that when you can review meiosis, if it looks unfamiliar. But then they segregate. You could have you capital A ones right over here, and then these sister chromatids split apart. So, capital A capital A, and then you have lowercase A. These lower case A ones, they segregate. And they independently assort from the other chromosomes. So, this one right over here might be the capital B, this might be the lowercase B. And whether or not this gets a, whether or not this gets a capital B or a lowercase B, is independent of whether it got a capital A or a lowercase A. So, it seems like these chromosomes independently assort. And, so, they came up with this chromosomal theory that it looks like maybe chromosomes are what contain these heritable factors that Mendel was talking about, because it seems like chromosomes behave very similar to those heritable factors. Maybe chromosomes code for multiple of these heritable factors that segregate and independently assort. And as we know now, they were right. So, this is a very, very big deal, but it's important to realize that they weren't sure. They established a theory, they were able to make some observations with the grasshoppers and the sea urchins, and they saw the patterns between what Mendel was describing, and the way chromosomes behave during meiosis. And then they know that each of these products of meiosis, each of these gametes will then go and form with other gametes to form the next organisms who say, "Oh, look. Parents will contribute either capital A "or lowercase A. Either a capital B or a lowercase B." So, this is very similar to what Mendel was describing. So, they laid the groundwork for the theory, but they still weren't sure. They didn't definitively prove it, and it will take another decade or so until it's definitively proven. And even then, no one was really sure exactly how these different traits were encoded, and for that we would have to wait a little bit longer.