Gregor Mendel

Between 1856 and 1863, Gregor Mendel, an Austrian monk, grew and studied peas. That probably doesn't sound that exciting, but what he learned about genetics changed everything. Let's look at what he did. Mendel bred pea plants, looking for many different traits, like color, wrinkly or round seeds, or tall or short plants, among other things. Once he had a purebred trait, he would run an experiment. Mendel crossed two purebred traits. We'll look at green versus yellow colored peas. When the offspring grew, he found that all of the peas were yellow. He couldn't understand why the green trait disappeared. Why didn't the peas have a 50-50 chance of being green or yellow? To explore this further, he took two peas from the first filial, or F1, generation that was just made, and he bred them together. Yes, this is inbreeding, but they're peas, so no one seems to mind. When the second filial generation arrived, F2, 75% of the peas were yellow, but 25% were green. The green peas seem to disappear in the F1, but magically reappear in the F2 generation. What could be happening? Mendel decided that there were factors, which we now call genes. The genes could be either dominant or recessive, and dominant alleles, or traits, mask recessive alleles. In this case, the yellow is dominant, and the green is recessive. So the purebred parents made offspring that had green and yellow alleles, but only the yellow showed because it was dominant. And when the F1 generation bred, there was a chance that they could both give the recessive allele to the offspring, so the green peas showed up again. If you could look at the chromosomes in the pea plants, you would find the gene for seed color. A gene is a section of DNA that controls a trait. The different seed colors have different alleles, which are variations of a gene, like green or yellow. Allele is a fun word to say, and it's actually even more fun to write in cursive, but you can try that next time you're bored. Now you get two alleles for every gene, because you have a set of chromosomes from mom, and a set of chromosomes from dad, and the same is true for plants. When you write alleles, dominant alleles get capitalized letters, and recessive alleles are lowercase. If a dominant and recessive allele are together, it's called heterozygous. Hetero means different, and the alleles are different, so that makes sense. If the alleles are the same, and dominant, we call that homozygous dominant. Homo means same. If the alleles are the same and recessive, we call them homozygous recessive. The letters we choose here are arbitrary, but it's good to use letters that are very different when you view capital and lowercase versions, otherwise it can be confusing. So letters like O aren't good, but letters like Y, T, or R are great. In peapods, green is dominant over yellow. If these are the genotypes, or allele combinations, what are the phenotypes? Having just one dominant allele of the gene will make it green in this case. So there are two green pods and one yellow pod. Now this is pretty simple, but let's try thinking about this in terms of making gametes. Assuming tallness is dominant, what would the genotype look like for a man who's homozygous for tall? And what are the possible gamete combinations? Now because tall is dominant, this would be a homozygous dominant genotype, two capital letters. Again, you can choose any two letters you like, but I'm going to use T for tall. When gametes form, only half of the DNA is in each cell, so only one allele can go to each of the sperm that are formed. There's only one possible gamete combination in this case, capital T, the tallness gene. But let's try another case. What would the genotype look like for a woman who's heterozygous for tall? And what are the possible gamete combinations? Now heterozygous means different alleles, so this will be written with a capital and a lowercase T. This also means that there are two possible gamete combinations, an egg with a big T for the tallness gene, or an egg with a little t for a short gene. Heterozygous traits will have more variation in their offspring. Thanks for watching this episode of Teacher's Pet. Don't forget to like and subscribe, and follow me on Twitter at SciencePet.