Widespread inbreeding between the Darwin and Wedgwood families was probably to blame for Charles Darwin's ill health, and the childhood tragedies and infertility that blighted his family.
That's the conclusion of an analysis examining links between ill health over four generations of the Darwin-Wedgwood dynasty and the degree of inbreeding between the families.
The analysis supports Darwin's fears that inbreeding was damaging his health and that of his children, following his ground-breaking studies demonstrating that cross-bred plants are far fitter and more vigorous than self-fertilised plants. "This caused him to reflect on his own condition," says Tim Berra of Ohio State University in Mansfield.
After Darwin married his first cousin, Emma Wedgwood, they had 10 children, three of whom died as children. Three of the others married but remained childless, suggesting infertility problems. And Darwin himself, who suffered unremitting ill health following his epic trip on The Beagle, was the product of an "inter-Wedgwood" union, his maternal grandparents being third cousins to each another.
At least five of the 25 marriages in the Darwin-Wedgwood family tree that Berra analysed, including Darwin's own, were between close relatives, and these had knock-on effects for their descendants. The analysis included 176 children over four generations of ties between the Darwins and the Wedgwoods.
Working with geneticists Gonzalo Alvarez and Francisco Ceballos of the University of Santiago de Compostela in Spain, Berra revealed correlations between the likelihood of death and the amount of inbreeding in families.
From the family tree they worked out an "inbreeding coefficient" for each marriage – a figure reflecting the proportion of inherited genes in the children that would be identical from both parents. For Darwin's own children, the coefficient was 0.063, meaning that 6.3 per cent of his children's genes inherited from both father and mother were identical.
The researchers say that the higher the coefficient, the likelier that children would inherit pairs of faulty genes, with no prospect of a "good" gene from either parent to compensate.
Death and disease
They found that the families with the highest inbreeding coefficients had the highest mortality. In Darwin's own family, for example, 30 per cent of his children died, twice the norm for childhood mortality at the time.
In another marriage with a coefficient matching Charles Darwin's (between Josiah Wedgwood III and Caroline Darwin) the children's death rate was 25 per cent, and in a union with a coefficient twice that of Darwin's (between Henry and Jessie Wedgwood), 17 per cent of their children died.
Darwin's "favourite" child, Annie, died from tuberculosis and another, Charles, from scarlet fever. A third died in early infanthood from unknown causes. "Consanguinity is implicated in susceptibility to infectious disease," says Berra.
And three of Darwin's children died married but childless, possibly an effect of inbreeding. The researchers speculate that double inheritance of genes that interfere with production of sperm or ova may have been to blame.
"Putting together the mortality of his children and the unexplained fertility, I think Darwin was right to be concerned about these issues," says Berra.
Darwin was so concerned about inbreeding that he lobbied unsuccessfully in 1870 for questions about first-cousin marriages to be added to the following year's national census form.
At the time, "blood marriages" were common, unions with first or second cousins accounting for 10 per cent of all marriages, often to keep money or influence in the family. Today, around a fifth of all marriages in the world are consanguineous, although there is some dispute about how damaging it is to descendants, some arguing that the effects are inconsequential and no different to those affecting older parents.
Journal reference: Bioscience, DOI: 10.1525/bio.2010.60.5.7
This post originally appeared on New Scientist.