Matthew Cobb describes it:

[Heredity] seems completely obvious to us today. But less than 200 years ago, the concept of “heredity” in biological terms simply did not exist. In the 17th century, the great physician William Harvey tried to work out why some characters seemed to reappear in different generations, others seemed to “skip” a generation, some (like skin colour) appeared to be a mixture of  the parents, while others again (like sex) were either one or the other but never (or very rarely) a mixture.

As he put it: “why should the offspring at one time bear a stronger resemblance to the father, at another to the mother, and, at a third, to progenitors both maternal and paternal, farther removed?” In the end, Harvey simply gave up – it was too complicated for him.

In a way, this isn’t surprising. Genetics is complicated – there is not, at first sight, a common explanation for skin colour (blending inheritance), eye colour (dominance in most cases) and sex (chromosomal determination – in humans at least). Harvey couldn’t see a pattern because, on the surface, there isn’t one.

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