Two misinterpreted insights in the history of heredity

I am currently writing a book called THE IDEA OF HEREDITY, in which I explore why it took so long for humanity to realise that there was such a thing as heredity and what happened next.

Basically, ‘heredity’ was only used to describe similarities between parents and offspring in the early nineteenth century. The realisation that there was such a thing as heredity came about primarily through the work of English sheep breeders in the eighteenth century and French studies of polydactyly – six fingers on a hand.

I have just finished a chapter on the seventeenth century, which deals mainly with horse breeders, hereditary diseases and the discoveries of egg and sperm, although this last aspect had no consequence on the idea of heredity (they were only recognised as complementary male and female components in the mid-nineteenth century).

Here’s a section of the draft chapter, dealing with phenomena that were highlighted by two seventeenth century thinkers. These examples later helped reveal heredity, but at the time they were viewed very differently. Progress in understanding requires not only precise and telling observations but also the right theoretical framework within which to interpret those findings.

In 1644, the English philosopher, astrologer and pirate Sir Kenelm Digby published an exploration of the nature of matter, including problems associated with generation.^[i] In a brief aside, Digby described how, in his early twenties, after capturing a number of Spanish and Flemish ships he took some shore leave in Algiers. Hearing of a curious example of a local Muslim woman, he was given permission to talk to her. As he recalled:

And another particular that I saw when I was at Algiers, maketh to this purpose, which was, of a woman that having two thumbs upon the left hand; four daughters that she had, did all resemble her in the same accident, and so did a little child, a girl of her eldest daughters; but none of her sons.^[ii]

Keen to verify the claim, Digby inspected the woman’s daughters, sons and grand-daughter and confirmed her tale. Although he did not put it this way, Digby had observed the presence of an inherited condition^[1] over five generations. This was exactly the example that, a century later, would separately convince the French mathematician Pierre Louis Maupertuis and the naturalist René Antoine Ferchault de Réaumur that there was such a thing as heredity, and yet Digby could not see what was literally in front of him.

Digby’s understanding of generation was based on heat – he was not thinking in terms of characters produced by atoms – and although his complex, alchemically-tinged theory suggested that any defect in the seed of one parent should generally be corrected by the seed of the other parent, he recognised this was not always the case. This, presumably, was his explanation of this otherwise telling example.^[iii]

An even more striking observation was made in 1683 by one of the pioneering Dutch microscopists, Antoni Leeuwenhoek. In a letter to Sir Christopher Wren, President of the Royal Society, Leeuwenhoek described the Dutch habit of breeding rabbits; if a wild grey male mated with a pet female rabbit of any colour – white, piebald, ‘bleu’ or black (all of which were large, tame and had big ears) – all of the offspring were grey.^[iv] ‘Moreover,’ continued Leeuwenhoek, ‘they will never grow to the size of the mother, nor have long ears: also they will never be so tame as the mother, but will always remain rather wild.’

This example of the inheritance of coat colour is often seen as the first recorded example of a dominant character (in fact, characters, as ear size and tameness were also involved), but Leeuwenhoek drew a very different conclusion. He claimed that this observation was ‘a proof enabling me to maintain that the foetus proceeds only from the male semen and that the female only serves to feed and develop it.’ He saw what he wanted to see – he was already convinced that only male semen existed, and this observation confirmed that.

It is tempting to see this as one of the great ‘what if’ moments of the history of science. Had Leeuwenhoek asked the rabbit fanciers what happened if a female wild grey rabbit was mated with a long-eared, large, coloured male, or had he done the experiment himself, he would have again found that all the offspring were grey, which ought to have led him to recognise a role for the female.

Furthermore, had he tried crossing those grey offspring with each other, at least some of their babies would have reverted to the original coloured, big-eared and tame form, while others would probably show a mixture of characters. The breed, as the horse breeders knew well, would have degenerated.

But Leeuwenhoek did not take this significant step. And, even had he done so, it seems more probable that he would still have found a way to interpret his observations in the light of his spermatic theory – he was notoriously stubborn, and single results rarely change strongly-held views .

One and half centuries later, in the hands of Gregor Mendel, using huge data sets and consistent crossing of offspring, this kind of result would prove decisive. For Leeuwenhoek, trapped by his preconceptions and with the experiment incomplete, no such insight was possible.

As this example shows, in the seventeenth century, complex results, ideas and observations led to no great theoretical breakthroughs regarding heredity, despite serious attempts at interpreting those findings with logical rigour. Instead, as the philosopher Andrew Pyle put it, the outcome was ‘a veritable maze, a labyrinth of anecdotal traditions, obscure concepts, and ill-digested experimental results.’

^[1] This would now be called sex-linked pre-axial polydactyly.

^[i] Petersson, R. T. (1956), Kenelm Digby: The Ornament of England 1603–1665 (London: Cape). Digby, K. (1644), Two Treatises (Paris: Blaizot).

^[ii] Digby (1644), p. 214. The spellings have been modernised for ease of reading.

^[iii] Digby (1644), p. 222.

^[iv] van Leeuwenhoek, A. (1952), The Collected Letters of Antoni van Leeuwenhoek volume IV (Amsterdam: Swets & Zeitlinger), pp. 69–71.