Meet Mrs and Mr Mouse.
They are hard-working, sober, and not given to being imaginative. Both have completely normal teeth. 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 Mouseboth have the recessive allele vam37. Neither of them knows, because the allele is just lurking in the genotype, not showing its 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 (T), vam37 (t) can show its true colors.
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 mouse. But if both come up vampire, then that kid mouse is… vampiric.