Gregor Mendel's groundbreaking 1866 work on heritable traits laid the foundation for modern genetics. His laws of segregation, independent assortment, and dominance provided a framework for understanding how traits pass from parents to offspring. However, it wasn't until the early 1900s that the Boveri-Sutton Chromosome Theory linked Mendel's laws to the behavior of chromosomes during meiosis, solidifying the molecular basis of inheritance. Created by Sal Khan.
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- At7:40/10:58you said two pairs of homologous chromosomes .These are present during mieosis or in normal state?(7 votes)
- Two pairs of homologous chromosomes (tetrads) are present during meiosis, being genetic material is being duplicated (first meiotic division).
If we speak of the second meiotic division than only one pair of homologous chromosomes is present.(8 votes)
- Could there be a rapid breeding process as accurate as this with any other fast growing plant? Like Bamboo?(5 votes)
- The thing is, Mendel was extremely lucky with his pea plants- the outside phenotypes (such as the petal color, or the height of the plant) were controlled by minimal characteristics. If the plant you're trying to use is controlled by more, it'll be hard to breed them to the point of easy sorting (like tall, short plants). Think about human height -- controlled by like, 400+ different genes.
It's probably possible to find a plant anyways, though.(9 votes)
- at7:38why is it homologous? I thought AA or aa is homo and Aa is hetero...(4 votes)
- You mixed up homozygous with homologous.
Homologous chromosomes come from both parents, but represent the same chromosome. For example, chromosome 1 from mom and chrsomosome 1 from dad.They cross breed and have crossing over.
Homozygous chromosomes are same type of alleles for the one chromosome (coming from the same person). For example both dominant or bpth recessive alleles.(11 votes)
- What is the defect in the independently assortment in chromosomal theory(5 votes)
- Not sure if this is what you're asking but: some genes are not independently assorted. For example, if two alleles (for different traits) are on the same chromosome, and are close enough in distance, they'll be passed on together. [possibly view the Genetic Linking explanation on KA](5 votes)
- I was under the impression that Homologous Chromosomes had to do with synapsis during Prophase I of meiosis. At roughly time8:00Sal says that the chromosomes he was pointing to were "homologous." Wouldn't those two chromosomes be "sister chromatids"?(4 votes)
- Actually, he is pointing to two different chromosomes (at7:30ish).Homologous chromosomes are two different versions (as in A vs. a) of a chromosome with the same function. These chromosomes that he is pointing to just haven't replicated themselves into sister chromatids yet. Sister chromatids are two identical copies of the same chromosome, linked at the centromere (middle portion) immediately after replication. They're shown as the "X" shapes in the next cell of the flowchart.(4 votes)
- Why is the chromosome theory important(4 votes)
- Because it tied together Mendel's theories with genetics. This may seem trivial, but back then, people didn't realize exactly what Mendel's theories were describing. Boveri and Sutton showed that alleles were determined by chromosomes and occurred in pairs, which separated during meiosis and "may constitute the basis of the Mendelian laws of heredity." They pretty much explained Mendel's laws with definitive science and definitively proved them, using already known laws of genetics.(3 votes)
- If the alleles that determine flower color (B & b) and alleles that determine height (A & a) were found on the same chromosome, could we still say that they assort independently?(3 votes)
- It depends on how close together they are. If they're pretty much next-door neighbors, then probably not. If they're basically down the street from each other, then probably.(5 votes)
- so what exaclty is boveri-sutton chromosome theory? can anyone help me out with giving a proper definition of it plz...?(3 votes)
- Sutton and Boveri found a close similarity between the transmission of hereditary traits and behavior of chromosomes while passing from one generation to the next through the agency of gametes. They united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.
It isn't about definition of the theory, it is all about the facts they decoded and realised its close proximity to what Mendel said long back.
Features of chromosomal theory,
- Like the hereditary traits the chromosomes retain their number, structure and individuality throughout the life of an organism and from generation to generation. The two neither get lost nor mixed up. They behave as units.
- Both chromosomes as well as genes occur in pairs in the somatic or diploid cells. The two alleles of a gene pair are located on homologous sites on homologous chromosomes. Both chromosomes, as well as genes, segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete.
-A gamete contains only one chromosome of a type and only one of the two alleles of a trait.
-The paired condition of both chromosomes, as well as Mendelian factors, is restored during fertilization.(5 votes)
- [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.