Worked examples: Punnett squares | Inheritance and variation | Middle school biology | Khan Academy

We're told that in a population of pea plants, some plants have round seeds and others have wrinkled seeds. The gene for seed shape in this population has two possible alleles. Remember, alleles are just versions of the gene. One allele, or one version, is for round seeds, which we're denoting as capital R, and the other allele is for wrinkled seeds, which we're denoting as lowercase r. They're telling us that the round seed allele, the capital R, is dominant.

For a cross between two pea plants that are heterozygous for the seed shape gene, determine the expected ratio of offspring with round seeds to offspring with wrinkled seeds. All right, like always, pause this video and see if you can have a go at this by yourself before we do this together.

All right, now let's think about the genotype for the two pea plants that are heterozygous. Heterozygous means they have two different versions of the gene or two different alleles. So these are going to be two pea plants that are capital R lowercase r, they have one of each. To see what happens in a cross between them, I will draw a Punnett square, so just like this, and I will explain what I am doing in a second here.

So I'll draw it like this. What you need to think about is you have your two parents, and so each parent is capital R lowercase r. Now that means that, let's say, parent one can contribute the capital R to the offspring, or it could contribute the lowercase r allele to the offspring. The same thing could happen for the other parent; it could contribute the capital R, or it could contribute the lowercase r.

So what are all of the equally likely possibilities for the offspring? Well, the offspring might get a capital R, the dominant allele, the round seed allele from both parents, capital R capital R. All I'm doing is I'm taking whatever letters in the column and whatever's in the row. It could get a capital R from this parent and a lowercase r from that parent. It could get a lowercase r from this parent and a capital R from that parent, or in this situation, it's getting a lowercase r from either one.

So these are the four equally likely genotypes. Now, they're not asking us about genotypes; they're saying determine the expected ratio of offspring with round seeds to offspring with wrinkled seeds. So what does the genotype have to look like in order to have round seeds? Remember, the round seed allele is dominant. Well, this would be a situation where you could be capital R capital R, but it's dominant. You could also be capital R lowercase r and still have round seeds.

The only way that you're going to have wrinkled seeds, right over here, is if you are homozygous for the wrinkled seed allele, so this is the only wrinkled seed scenario. If you look here, you see out of the four equally likely scenarios, three of them end up with a phenotype of round seeds, and only one of them ends up with a phenotype. Remember, the phenotype is what's being expressed. Only one of these four equally likely scenarios has a phenotype of wrinkled seeds.

So the expected ratio is going to be, you're going to expect to see three offspring with round seeds for every one with wrinkled seeds, or you would expect a 3 to 1 ratio. Now, I have another part of this question that is asking us what about a cross between a pea plant that is homozygous for wrinkled seeds and one that is heterozygous. Pause the video again and see if you can have a go at this one.

All right, so the first thing that I like to do is just think about what are going to be the genotypes for each of these parents. So if we're talking about homozygous for wrinkled seeds, remember the wrinkled seed allele we’re denoting with the lowercase r. So that's going to have a genotype of lowercase r lowercase r, and then they say one that is heterozygous. Well, we already saw that before; that's going to be capital R and lowercase r.

So let's draw a Punnett square again to be able to do this. So I'll do it like this, like that, and then make sure I have two columns and two rows. Let's put the homozygous parent up here. They could either contribute a lowercase r, or they could contribute a lowercase r. They're going to contribute one of these two.

Then let's think about the heterozygous parent. Well, they can contribute a capital R, the dominant allele, or they could contribute a lowercase r. So let's think about the equally likely scenarios here, at least for genotype. Well, these are all scenarios in which we get a capital R from this parent over here, the heterozygous parent. I guess I could say, so let me write capital R capital R.

In both of these scenarios, we're getting the lowercase r from the homozygous parent, and in both of these situations, we are getting the wrinkled seed allele from both parents. You're getting the lowercase r from the heterozygous parent, and in either scenario, you're getting a lowercase r from the homozygous parent. So let me write that right over here.

So which of these genotypes would have a phenotype of round seeds? Well, round seeds are going to be scenarios where you have at least one of the round seed alleles. So these are all going to be round seeds right over here. They're going to have a phenotype, I should say, of round seeds, because remember the round allele is dominant. Even though you have one of each, the dominant allele is what you're actually going to see in the phenotype here.

Then we see these two are going to be wrinkled. So what's the ratio of round to wrinkled going to be? Well, it's going to be one to one. For every two round, you're going to have two wrinkled, or for every one round, you're going to have one wrinkled. Two to two is the same thing as one to one when we're talking about ratios, and we're done.