4: Reviewing the Dreaded Double Hybrid

Mr and Mrs Mouse both have one copy of each allele for teeth (one normal copy, T; one vampire fangs copy, t). In that sense, they are hybrids for the vampire allele. They are also hybrids for the fur allele. Let’s assume that flat fur (F) is the normal trait and fluffy fur (f) is a recessive trait. That means that they will have one copy of the Flat fur allele (F), and one copy of the fluffy fur allele (f).

Now there are two pairs of alleles in the mother’s genotype. One allele pair has the vampire/non-vampire teeth. The other pair has flat/fluffy fur. But each egg gets only one allele from each of the pairs. So across the top of the Punnett Square, where we list all possible kinds of eggs, we have to list all possible combinations of one-allele-from-each-pair. We call this a dihybrid cross and it’s just like the possible combinations of heads and tails on two coin flips.

The father has the same two allele pairs. To make a baby, Mr Mouse flips his two coins, and Mrs Mouse flips her two. We can make a Punnett Square to help us figure out what their babies could look like.

Once again, I know you’ve seen this kind of Punnett Square in Introductory Biology. But if you are like most people (including me), you don’t quite remember where everything goes. However, please do not “look it up”. I will ask you some leading questions, but the idea is for you to figure it out, then you won’t need to memorise it or look it up.

1. Put Mum’s egg choices along the top. Put Dad’s sperm choices along the side. (Put the teeth gene first and the fur gene second). Each egg or sperm could have the allele for fangs or normal teeth, AND the allele for fluffy or normal fur. Click the
buttons to see if you filled it out correctly.
2. That was the hard part. Now you get to click the button to fill out the kids’ genotypes!
3. By looking at each box, you can figure out what each baby could look like (i.e. what’s its phenotype). So, fill in the questions below:

If you followed these steps faithfully, you should end up with 9 normal babies, 3 with fluff, 3 with fangs, and 1 fluffy vampire.

This is the famous 9:3:3:1 ratio.