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, lets abbreviate with T for normal teeth, t for vampire fangs, F for normal fur, and f for fluffy. And say the
- mom is TtFF
- dad is ttFf
So mom is complete normal looking and dad has fangs but normal 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 normal fur. So instead of 4 possible phenotypes, there are only 2 (normal 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 normal:vampire:fluffy:both comes out to 8:8:0:0, or half normal 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!!
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