MathBench > Probability

Mice with Fangs: Intro to Punnett Squares

Can this marriage be saved?

Meet Mrs. and Mr. Mouse.

They are hard-working, sober, and not given to being imaginative. Both completely normal in the dentistry department. Mrs. Mouse gives birth to her first litter, paints the nursery, picks out nicely rhyming names, and … but what's this? Two of the nine micekins have what can only be described as … fangs. Mr. Mouse peers suspiciously at Mrs. Mouse. Mrs. Mouse murmurs weakly that it must be from Mr. Mouse's side of the family. Mr. Mouse is thinking more likely from the milkman. Accusations fly. Fur flies. Baby mice wail. Another broken marriage, derailed by a lack of basic genetic understanding.

What went wrong? How could two fangless mice end up with vampire babies?

Remember that recessive alleles don't disappear, they just, well, bide their time. They can hang out for generations, on seemingly perpetual recess, until they meet another recessive allele. The key is that Mr. Mouse and Mrs. Mouse both have recessive alleles vam37. Neither of them knows, because those alleles are just lurking in the genotype, not showing their fangs. On the outside, nothing is different about these 2 mice. When searching for a mate, neither Mr. nor Mrs. Mouse could tell that anything was amiss. But when sperm meets egg, those two recessive alleles meet. And suddenly they don't have to recede anymore. Without the dominant ‘normal teeth' genes, vam37 can show its true colors.

And how often does that happen? Let's go back to the coin analogy. Each genotype is like a coin with 2 sides, so when Mr. and Mrs. Mouse get together to make a baby, its like flipping one coin for each of their genotypes. Mr. Mouse's coin might come up normal or vampire. Mrs. Mouse's coin might come up normal or vampire. If one or either or both are normal, so is the kid. But if both come up vampire, then that kid is … vampiric.