Darwin's Little Lesson on Inbreeding

From New Scientist:

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.

Nothing too new here. I told the tale, relating it to dogs of course, in an earlier post on this blog entitled Inbred Thinking. That said, it's nice to see the story getting out to a wider audience.


This is the pedigree of the Darwin/Wedgwood dynasty.

From Scientific American:

The analysis, led by Tim Berra, professor emeritus in the Department of Evolution, Ecology and Organismal Biology at the Ohio State University in Mansfield, found that Darwin's kids did have "a moderate level of inbreeding" and in the family's children, there was "a significant positive association between child mortality and inbreeding."

When two individuals mate, genetic material from both parents is passed on to the progeny. So even if one parent carries a harmful recessive trait, the other parent is likely to have a healthier version, which will manifest itself in the offspring. If both parents, however, carry a recessive allele—which is more likely to happen if they share much of their genetic material, as close relatives do—then they raise the chances that their child will have only the bad genes.

A first cousin marriage does not, of course, mean instant problems. In fact, across the world, first-cousin marriages represent about 10% of all marriages.

That said, first cousin marriages do increase the chance for genetic defect.

How much?

A first-cousin marriage raises the risk of birth defect and mortality about as much as giving birth at age 41 instead of age 30.

Of course, if a society has a history of marrying first-cousins, Coefficients of Inbreeding can rise rapidly and with it genetic defect

In Pakistan, where this kind of thing still occurs, one study estimated infant mortality at 12.7 percent for married double first cousins versus 5.1 percent for the progeny of unrelated parents.
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