In zebras, purple eyes are dominant to yellow ones. (I don’t know if this is true, but just go with it.)  A purple-eyed zebra-man has little zebralets with yellow-eyed zebra babe: 487 purple-eyed 520 yellow-eyed. What are the genotypes of the man and his kids? What are the probabilities of their possible children?

We’ll call the purple allele (P) because it’s dominant, and we’ll call the yellow allele (p) because it’s the recessive version of the gene. Now that we know how we’re gonna write the genotype of the parents, let’s figure those genotypes out.

We know that the female genotype is (pp) because we were told that, in this example, yellow is both recessive and the color of the female’s eyes.

Dad’s genotype is going to be a little harder to decide – but not much. Remember that with dominant alleles, both homozygotes and heterozygotes will show the dominant phenotype. If you’re not really good with the genetic jargan, that means that Dad will have purple eyes whether his genotype is (PP) or (Pp). That’s where we come in; we have to figure out which one he is.

This is usually where most people have problems. “Uhhhh… what now?” What a great question. Let’s figure out the children’s genotypes – since we already figured out everybody else’s. There are both purple and yellow eyed kids. That means that there are either (Pp) or (PP) AND there are (pp) represented.

REMEMBER that the children can only get alleles from mom and dad. SO if mom and dad don’t have them, the children can’t. Since Mom’s genotype is (pp), then she can only give (p) alleles to her kids. Since we know that there is going to be at least one (p) in all the kids, we can deduce that NONE of the kids will be (PP) and that the purple-eyed kids will have (Pp) as their genotype.

HEY! Wait a sec, we missed something. OH YEAH, we can also figure out Dad’s genotype now. Remember what I said earlier about kids only being able to get alleles from the parents? Let’s look at those yellow eyed kids again. They have a genotype of (pp) and we already know that one of those (p)’s came from Mom… so what about the OTHER one? It had to come from Dad, who has purple eyes. THEREFORE, his genotype must be (Pp).

FINALLY, we did ALL THAT WORK. Phew! But let’s check the question to make sure that we answered the question. I made that mistake a lot when I took genetics- so check to make sure that you’re done. There were two parts: genotype and probability. We know that Dad’s genotype is (Pp) and that his children are (Pp) and (pp). BUT we haven’t figured out the probabilities yet. Sorry.

It’s not that bad. Quit whining. Let’s just do this. It’s all down hill from here. Watch.

Mom: (pp)

Dad: (Pp)

If you didn’t get it that easily, check out my article on understanding Punnett Squares. But as you can see on the graph, 2/4 of the children are (Pp) and the other 2/4 are (pp). THEREFORE, we know that 0.5 of the children are (Pp) and the other 0.5 are (pp). And that makes sense seeing as 487+520 are 1007, and both numbers are just about half. NOW, we’re done.

Let’s recap the rules that we learned.

1. Name the alleles with the letter of the dominant. That way, it’s easier to remember which one is dominant.
2. The DOMINANT is capitalized and the recessive is not.
3. Use the genotypes of what’s given to find out the genotypes of what’s not.
4. If statistics are given, check the given data with the stats to make sure it makes sense.

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Best of Luck,

Grey