Boveri-Sutton Chromosome Theory

Let's give ourselves a reminder of how important Gregor Mendel's work was. In 1866, he published his findings, 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.

Actually, Mendel's work was either ignored or not taken seriously by a lot of people in 1866, and it wasn't until the early 1900s that people rediscovered his work and realized, "Wait, wait, there's 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. For most of human history, we've probably recognized that animals, or not just animals, any type of living creature, seems to pass on traits to their offspring. I could look at you and 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 have a rigorous way of thinking about it, and we definitely didn't have any way to make predictions that were testable based on those traits. That's what Mendel gave us. He said, "Well, look, I'm observing..." and he did this with pea plants. "I observe these heritable factors, and there might be heritable factors on, let's say, height. If we're talking about plants, it would be the height of a plant. There might be heritable factors on, let's say, flower color."

So, flower color—he recognized that there were different versions of those factors. A given plant might have one of the tall versions; 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 or two pinks.

It would look like that. But the important realization was that there were these versions of the factor. Today, we call these factors genes. We say, “Hey, there’s a gene for height if there is one, or there’s a gene for flower colors.”

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: these are all alleles right here.

So, you have one tall allele, one short allele. What Mendel did is he realized, "Well, look, these things—he didn't know how—but these things are what get passed on from a parent to their offspring." He's trying to describe how they got passed on.

He observed that even if you have two of these, they tend to segregate when you go to the next generation. What do we mean by segregation? Or, I guess we could say the law of segregation. Well, that means if I’m a pea plant and these are the versions that I have, I might pass on 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. He also realized that whether or not I pass on the capital A or the lowercase a is 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. This is independent assortment—the law of independent assortment. 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.

The only way they look short is if they have two versions of the short one. He described that as his law of dominance. 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 going to appreciate a little bit of historical appreciation. 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 1900s that people started to have fairly robust theories of how this happens.

In 1902 and 1903, these two gentlemen independently started coming up with the chromosome theory of inheritance. 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.

Let me write this down: this is the Boveri-Sutton chromosome theory. Even though they were starting to say, "Maybe chromosomes have something to do with it," they still didn't know exactly what was inside the chromosomes that were allowing this information to be encoded. We'll get to that in a little bit, but let me just underline this:

The Boveri-Sutton chromosome theory. 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 how chromosomes behave in meiosis was discovered after that.

These guys independently studied different organisms: Walter Sutton studied grasshoppers, and Theodor Boveri studied sea urchins. They looked at meiosis and they looked at the reproduction and the fertilization during these processes. They saw that the chromosomes seemed to do things that were very similar to these laws of segregation, laws of independent assortment, and laws of dominance.

Actually, the law of dominance—we'll talk more about in future videos—but he saw that, let's say, you had an organism here. In this particular organism, I just did it for simplification; it has two pairs of homologous chromosomes.

What does homologous chromosomes mean? 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.

Notice homologous chromosomes are two things that are looking the same, but maybe they’re a little bit different. We’re not sure. Well, maybe, just maybe, one of these chromosomes somehow has on it, someplace, what encodes for the capital A; and maybe the other chromosome, in a similar part of the chromosome, has what encodes for the lowercase a.

Now, this is starting to make sense because these 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.

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, it looks like these two things, as we look through the process of meiosis, they seem to segregate from each other."

For example, this capital A one will replicate. So you have capital A, but then this is the lowercase a one right over here. You might have some crossover (and we'll talk about that when you review meiosis, if that looks unfamiliar).

Then they segregate. You could have your 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 lowercase a ones segregate, and they independently sort from the other chromosomes.

So this one right over here might be the capital B; this might be the lowercase b. 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. It seems like these chromosomes independently assort.

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 similarly to those heritable factors. Maybe chromosomes code for multiple of these heritable factors that segregate and independently assort.

As we know now, they were right! This was a very, very big deal. But it's important to realize that they weren't sure; they established the 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.

Then they knew that each of these products of meiosis—each of these gametes—would then go and form with other gametes to form the next organism. You say, "Oh, look, parents will contribute either a capital A or a lowercase a; either a capital B or a lowercase b."

So, this is very similar to what Mendel was describing. They laid the groundwork for the theory, but they still weren't sure; they didn't definitively prove it. It will take a few more decades until it's definitively proven, and even then, no one was really sure exactly how these different traits were encoded. For that, we would have to wait a little bit longer.