Is no man shorter than his mother?

Jenny Chisholm doesn’t mention whether she has a particular man and his mother in mind but she wonders about the truth of this. We’re well conditioned to expect a boy to outgrow his mother, and it’s probably even the usual result, but it isn’t a foregone conclusion.

The genetics of height are not simple. Old Gregor “fiddle-the-numbers” Mendel got a lot of press with his peas and their recessive and dominant genes. Some human characteristics like eye colour work this way, where a single gene, or at most a handful of genes, control a feature in a simple and predictable way. But there is no single gene controlling skin colour or height, instead a great number work together to produce the fine gradation in colour and height that we see around us.

It may help to imagine that there is one gene controlling the length of every little part of the body, each saying long or short. Your height then depends on the combination of all of the genes controlling the length of your spine and legs because they all contribute to a little bit of height.

If you let a mathematician get hold of that idea then they can tell you some interesting things. One is that a better prediction of your height comes not from the heights of your mother and father but from the average of their heights. It follows (you are following, aren’t you?) that a tall mother and a short father can expect to have children shorter than the mother but taller than the father. If only Tom Cruise and Nicole Kidman hadn’t split before having any children of their own. That would have been a nice demonstration — what we in the science biz call a natural experiment.

A son can generally expect to be taller than his mother if his father is taller than his mother and our mores ensure this is generally the case. But this is a a cultural accident, not a biological law. The long and the short of it is that sons can be shorter than their mothers.

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