Genetics Info & Tutorial: More Punnetts

Let’s run through one more Punnett square to make sure you’re comfortable with them. It may be helpful to follow along by making your own Punnett square on a piece of paper. A couple pages ago, we mated two hets together. Let’s do that one as a Punnett square now.

First, we need to determine which genes can be passed down by each parent. What are the letters for our two het parents? If you said Aa for both parents, you are correct.

Now, if a parent has two different genes it can pass down, (A and a) then those are the two for that parent, so we will put one across and one down, like so:

Now we just fill in the offspring by going across and down from each square:

Now that we’ve determined what genes the offspring will have, it’s helpful to figure out what that means they will look like, too.

Our 4 outcomes, which are equally likely, are AA, Aa, aA, and aa. So what will those different offspring look like? (the way an animal looks is called the “phenotype.”) You should be able to figure it out, but here’s the answer:

AA - These are carrying both copies of the dominant wild-type gene. They will look normal.

Aa and aA - These are each carrying one copy of the wild-type and one copy of the mutation and will be “het for amel.” Remember that the mutation is recessive (it can still make melanin because it has the good copy) so these will also look normal. Note that the order (Aa versus aA) doesn’t make a difference.

aa - These are carrying both copies of the mutant gene. Since neither gene can help to produce melanin, the animal will lack melanin. In other words, its phenotype will be “amelanistic.”

Notice that one out of 4 (25%) will be mutants. The other 3 of 4 (75%) will all look normal. But the 3 normals will be AA Aa and aA. Since you can’t tell the difference right away, and 2 out of those 3 possibilities are het (Aa) then you can call all of the normals “66% possible het.”

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