Back in 2015 I did a post on my own personal twin study, describing the interesting phenomenon of how one of my identical twin daughters, who learned to walk via the minority (5%) technique of bottom-shuffling, rather than the usual (95%) crawling, gave birth to a baby daughter who did the same thing. I wondered then about what would happen to the newborn daughter of the other twin, who crawled in the usual way, and what would be the genetic, or epigenetic, explanation of whatever resulted. I may even have updated the story since, but if so I can’t find it.
But now the full research programme is complete, for each of my daughters has two of her own (and are probably unlikely to repeat the exercise), and the youngest of them is now at the crawling stage. Only she’s not, because like her sister and her mother, she’s chosen to bottom shuffle her way to mobility, and is getting pretty handy at it. It looks like there’s a reasonably strong inherited pattern there – it’s vanishingly unlikely to be mere coincidence.
But, I hear you ask, what of the children of the twin who didn’t bottom shuffle, but crawled? Well, the latter’s older daughter, who is now pushing three, did the conventional thing of crawling, thus apparently confirming the epigenetic nature of the inheritance. I remind you that nobody else in the family tree is recorded to have shuffled rather than crawled, including my son and his daughter, so a simple genetic cause is unlikely anyway, quite apart from the identical twin element.
But then, one might call either mode of locomotion a “complex behavioural trait”, and those aren’t usually associated with simple genetic explanations anyway. But there ought to be some expectation other than “coincidence”, for we don’t normally treat mere 5% probabilities occurring in a parent and both offspring “chance.”
Lastly, then, the younger of Crawler’s two children. What was her introduction to mobility? The answer, ladies and gentlemen, is that she was another bottom crawler, like her aunt and two cousins. She breaks the neat pattern, but only by breaking the 95% habit of the rest of the human race.
So how do you geneticists explain this? A complex developmental behaviour occurs in one identical twin and her children, but not the other or one of her children, the latter’s second child apparently inheriting the trait (but from whom?). As I said, complex behavioural traits have not been found, in most cases, to have a genetic explanation, so why would this one? Epigenetics? Then why the partial difference between the two families?
Or are there things about inheritance that are not dreamed of in our philosophy? If genes are not to blame, then what is? And what about the consequences of such cases? Supposing bottom shuffling had some selective advantage, or disadvantage, over crawling, could not natural selection differentiate between identical twins, despite their identical genomes? How might that effect evolution?
I’ll let you know what happens if I’m still around for the next generation… but I might have shuffled off somewhere myself by then.