 Hello everyone and welcome to Unit 2. So far we've covered some of the most basic and important principles of genetics and this has laid the foundation for what is to come. We've discussed Mendel's work, the Punnett Square, a brief bit on the concepts of probability and human pedigrees. We're going to expand upon this and begin to get more in-depth on the whole concept of chromosomes. We'll discuss locus and alleles and the difference between them. We'll talk about chromosomal theory, mitosis, meiosis, and then a comparison of mitosis and meiosis. We'll also have a few mini lectures which will focus on chromosomal theory, which will also give us additional information on mitosis, meiosis, in fact two on meiosis, and finally we'll have an additional lecture which covers the comparison of mitosis and meiosis. We hope you enjoy this unit. There's a lot of interesting information to come. Hello everyone. Today's lecture will focus more on chromosomal theory. If you'll recall, we mentioned earlier that it was Gregor Mendel who laid the foundation for the entire field of genetics. But remember, he never actually knew the concept of a chromosome. Chromosomes were not discovered as the main unit of heredity until later, when Walter Fleming would first describe chromosomes. It would then be Theodore Boveri, who would link chromosomes to the concept of heredity, and then Walter Sutton, who would find evidence to link chromosomes to Mendel's principles. And finally, it would be Thomas Hunt Morgan, who would actually put this concept to use when he tracked the white amutation in fruit flies. The concept of a chromosome wasn't instantly discovered by one scientist. In fact, it was discovered by many scientists who built upon each other's work. Morgan, however, was credited with actually giving evidence to linking chromosomes to heredity. Thomas Hunt Morgan worked on Drosophila melanogaster, otherwise known as the common fruit fly. Fruit flies are great lab specimens for several reasons. First of all, they reproduce quickly. Secondly, they're easy to care for. And third, they have many, many offspring. If you've ever left a banana out for too long, you've been evidence to this, I'm sure. Fruit flies look like this, a particular drawing here. Morgan's group used fruit flies and first described the chromosomes that were found in fruit flies. They found four pairs of chromosomes. They also discovered a mutation in fruit flies that causes white eyes. It was this mutation that would lead them to link chromosomes with heredity. Through thousands of breeding experiments, Morgan found that this mutation was recessive. He also found that it was on the sex chromosome, otherwise known as the X chromosome. Now, just like humans, male fruit flies have one copy of the X chromosome. And then they also have a copy of a Y chromosome. Females, however, just like humans, have two copies of the X chromosome. In order for a male to have a white eye mutation, he only needs one copy of the mutated gene. However, in order for females to actually show white eyes, they have to have two copies. Because remember, we said that this is a recessive mutation. In Morgan's experiments, he would take a white-eyed male fruit fly, as you can see here, and cross it with a red-eyed female. Now, the red eye is the normal trait that fruit flies usually have. And again, if you ever have your piece of fruit, take a good close look at those fruit flies and their eye colors, because usually you'll find that they're red. Now, when he did this, he found that the offspring were all red eyes. And this actually makes sense. Because the female, if in fact she's homozygous for the red eye, as you can see here, she will give one copy of her X to any males, and then another copy of her X to any females. The males, however, will give their X to any females, okay? But if they have any male offspring, they give the male offspring their Y. So in this case, we have all males with red eyes, and we have all females with red eyes. But the females actually are carriers for this white eye mutation. Let's do a second round of crosses. Morgan's group then took those heterozygous females and crossed them with red-eyed males. Now, again, in any male offspring, the male parent will give his Y chromosome. But in any female offspring, he will give the X. So this normal X chromosome is going to go to all the female offspring, and then his Y is going to go to all of the males. The female, however, will give one copy of her X to any females, and she'll also give one copy of her X to any males. And in this case, all of the females, again, are going to have red eyes. They'll either be homozygous for the red eye mutation, or they'll be a carrier and be heterozygous. The males, however, will be half with the red eyes, normal red eye gene, but they'll also have half of them should be white eyes, because they're going to receive half the number of chromosomes from the female in her case, which carry the mutation. So not only did Morgan give the needed experimental evidence for chromosomal theory, but he was one of the first to actually find a sex-linked trait. Sex-linked traits are those that are located on these sex chromosomes, which can either be the X or the Y. Morgan did thousands and thousands of breeding experiments, and this let him follow the inheritance of these mutations through many generations. And this gave the scientific proof that chromosomes carry the genetic material that in fact control what an organism looks like.