William Harvey,  better known for describing the circulation of the blood, wondered in the 17th century what could explain why children’s skin colour was often a blend of their parents’, whereas they share a sex with only one, and can have an eye colour different from either. for describing.

The central problems of human genetics.From a book review in the Economist. It has always fascinated my why Mendel modelled on the basis of discrete states, rather than thinking of traits as ‘blended’. The guess, is that for this monk, sex was the answer: male or female, but not a little bit of either. [direct link to this aside]