Genetics is complicated. I have taken courses to this effect; I have taught the concept in Introductory Biology. Mendel’s peas with their neat logical Punnett squares were a lucky rarity—each trait governed by just one gene, each of those genes on a separate chromosome. The genetic basis of the vast majority of traits is far more complex. If the genes involved aren’t physically linked (called “linkage disequilibrium”) then they are pleiotropic (influencing many different traits at once), or epistatic (modified by other genes), or simply so subtle that their effects disappear in the noise of environmentally-caused trait variation. Relating traits to genes is hard.
I know this; I understand it; but until recently, I had never actually seen it. Then my pet mice decided to give me an object lesson in genetics.
What happens when you think you have all female mice, but you actually have mostly females and one male?