Monohybrids and the Punnett Square Guinea Pigs

A dominant allele is represented by a capital letter. And it's an allele that will show up. Think of it as. Dominating. So a hairless Guinea pig does not have a dominant allele for hair. And that's why the recessive trait not having hair shows up. A hairless Guinea pig's genotype, that's the genetic makeup of an organism, can be represented as a lower case little H little H a Guinea pig that does have hair can be represented as a big H big H or a big H little H see it only takes one dominant allele, a capital H for a trait to show up, that dominating allele means that the recessive allele is hidden. Now, we're not quite done with the vocab here. A genotype of big H big H or little H little H is considered homozygous. The root in this word means same and they are the same case. Big H big H, those are both capital letters and little H little H, those are both lower case letters. So big H big H is called homozygous dominant because of the capitals, and little H little H is homozygous recessive because of the lower case. A genotype of big H little H is considered heterozygous. The root in this word means different, because their letters are different cases. 

One is a capital, and one is a lower case. If I were to show you a Guinea pig with hair, would you know its genotype? Well, you would know it's not little H little H because if it was little H little H, then it would be hairless. But remember, a Guinea pig with hair, it could be big H big H or big H little H we don't really know. When we could do a test cross to eventually determine this, but you can't tell just by looking at the Guinea pig. Now, if we had a hairless Guinea pig, we do know for sure that it's little H little H because if it did have a capital letter, even one, that dominating allele would take over and it would have hair. So let's try a monohybrid cross. The root mono means one, because it focuses on one trait. In this case, hair. And that means a punnett square would need to be created like this with four squares. In our cross, we are going to cross two heterozygous Guinea pigs. So let's look at this in steps. Step one first, figure out the genotypes of the parents. Heterozygous means big H, little H step two, place one parent along the top of the punnett square, outside of the boxes like this. And place the other parent along the left side of the square. Outside of the boxes, like this. Step three cross them. 

For formatting purposes, we always put the capital letter first. The results that you get in the squares would be the offspring. The babies. Now, if I were to ask you the genotypes of the babies, you could list them out. One big age big H, two big H little H and one, little age little H I could even turn that into a genotype ratio of one homozygous dominant to two heterozygous to one homozygous recessive. Or percentage. 25% are homozygous dominant. 50% are heterozygous, and 25% are homozygous recessive. Well, what if I asked about the phenotypes? What does that mean? I like to think that Pheno sounds like physical. So the phenotype would be the physical traits of that organism. And in this example, it's whether they have hair or not. Now remember any babies that have a capital H, have a dominating allele, and they will have hair. So the babies that are big H big H or big H little H, they have hair. So three of them here, the one big H big H baby, and the two big H little H babies. They all have hair. 

The homozygous recessive baby, which was little H little H, that baby has no dominant allele present, so this little guy would be hairless. So you could say the phenotype ratio is three have hair to one that's hairless, or a phenotype percentage could be 75% hair, 25% hairless. One thing to emphasize about punnett squares is that they are predictions. These are probabilities. This means it's not necessarily exactly what you're going to get. For example, it's a probability that a child has a 50 50 chance of being born a boy or a girl. But we all know families that only have children that are girls or families that only have children that are boys. Probabilities are predictions. Another reason to love biology, it's exciting. Well, that's it for the amoeba sisters and we remind you to stay curious.