 Our first definition is just to deal with the word heredity. And all that is is passing traits from parents to offspring. That's super straightforward. What is not straightforward is what is a trait? Oh, guess what our second vocabulary word is? A trait is an observable characteristic. Dude, can we get any more vague? Heredity is passing observable characteristics from parent to offspring. Seems pretty straightforward. Has a lot to do with DNA. We have more words. Traits can come in flavors. If we have like a trait is a big picture, like broad generalized character, like eye color, height, hair curl, melanin production. These are all things that are traits. They're broad, they're general, but there are specific forms of the traits. And so your phenotype is the actual expression of a characteristic. So for example, the trait, the eye color trait has multiple possible phenotypes based on the genes that the person has. So one possible phenotype is blue eyes. Another possible phenotype is green eyes. Another possible phenotype is brown eyes. The phenotype can have much variation, but it's the actual physical expression of the trait in an individual. We can describe the phenotype of critters. Well, I hope that it isn't a huge leap to say, well, what causes a phenotype? There's got to be some link to DNA here. And in fact, the genotype is the alleles that code for a person or a critter's physical traits. And when I say the alleles that code for this, we talked about alleles are just forms of a gene. Probably the one word that we're going to use more than any other in the course of the next three lectures is allele. Because the different forms of the gene are the things that we're keeping track of when we make heritable crosses. When Homeboy Mendel was doing his baby pee making crosses, he would look at the traits, the phenotypes of his parent pee plants. And then he would predict, if I cross these two parents and they have these phenotypes, what are their babies going to look like? And then he used that to try to figure out, well, how is that happening? What is the thing that these parents are passing along the alleles that they have that code for these forms of genes? Genotypes. We're going to have so many opportunities to mess around with this. So be really patient if you're feeling like, ah, this is confusing. Be patient. Genotypes are always two alleles. Okay. In diploid critters, genotypes are always two alleles. And we're only going to deal with heredity in diploid critters. And that's because, dude, we do not want to try to keep track of more than two alleles. The alleles can be the same allele. Why do we have two? One came from the sperm parent. One came from the egg parent. When we make sperm and eggs of our own, they get one of our alleles. All of my eggs have one copy of alleles that are in my diploid cells. That's meiosis. We understand the process of meiosis. Genotypes can be homozygous, is two copies of the same allele, or heterozygous, two different alleles. All genes have two alleles in their genotype. Because they're diploid critters. Because you have two copies of every gene in your genome. With the exception of Xs and Ys, which we'll talk about in the next lecture. Okay. I, oh, one more thing. Ah, okay. We can have different alleles. And I need to give you a couple of words. These are familiar words, I think. Alleles can be dominant, or, oh, I'm sorry, I have to take it over here, recessive. I ran out of room. A dominant allele will be expressed, even in a heterozygous genotype. So a dominant allele, if that dominant allele is present, it will be expressed, even if there's only one copy of it. Recessive allele will only be expressed if there are two copies of it. Okay. How do you feel about that? Do you feel like, what is she talking about? If you do, take a deep breath, because we're about to go in and look at Mendel's peas. He found seven characteristics, describing seven traits, describing his peas. We're going to look at each one of those seven traits. Each trait had two alleles. We're going to look at what those two alleles are. And one allele was dominant, and one allele was recessive. So we're going to look at the dominant and recessive alleles, which I only tell you that to say we're going to have lots of practice with this vocabulary and something actual physical to talk about. Okay. Onward to pea plants.