MathBench > Probability

More Mice with Fangs: Intermediate Punnett Squares

Not all double crosses are dihybrids !

You could make a cross involving 2 gene pairs (2 coins for each parent) but the parents are not dihybrids. Let's try one of those:

Going back to vampire teeth and fluffy fur, let's abbreviate with T for wild-type teeth, t for vampire fangs, F for wild-type fur, and f for fluffy. And say the

So mom is wild-type and dad has fangs but wild-type fur.

Before you fill out the Punnett square, think for a moment about what to expect. For example, mom has only dominant fur alleles to contribute, so ALL babies must have wild-type fur. So instead of 4 possible phenotypes, there are only 2 (wild-type and fanged). What about teeth? Dad will always contribute the recessive allele, but half of the time it will be cancelled out by mom's dominant. So you might expect and half-and-half sort of situation. Now try the Punnett Square:

This time the ratio of wild-type:vampire:fluffy:both comes out to 8:8:0:0, or half wild-type and half vampire. It's important to remember that just because you're trying to figure out the proportions of phenotypes and there are 2 genes involved, it does NOT NECESSARILY mean that there will be a 9:3:3:1 ratio!!