Inbreeding in the Darwin dynasty?
May 8, 2010 Leave a comment
Darwin and his wife were first cousins
Charles Darwin married his first cousin, Emma Wedgwood, and his own mother was the product of a marriage between third cousins. Given his insights into the relationship among variation, nature’s choices, and adaptation and his observations of weakening in inbred plants, it is no surprise that Darwin worried about his own family’s consanguinity. Did the inbreeding in the Darwin/Wedgwood families show up in his children?
Is marrying your first cousin really so bad?
Had the Darwin/Wedgwoods only engaged in the first-cousin marriage between Charles and Emma, the outcome would likely not have been serious. A 2002 study reported by the National Society of Genetic Counselors found that having first cousins as parents raises the risk of having a significant genetic defect from 3-4% up to about 4-7%. The group concluded that first cousins planning to reproduce require no more intense genetic counseling than unrelated couples.
Consistent consanguinity, on the other hand
But that study didn’t address serial consanguinity of the kind seen in some European royal houses or in the Darwin/Wedgwood families. And a new analysis reported in BioScience avers that the Darwin offspring did experience the repercussions of such inbreeding. Applying an inbreeding coefficient to calculate whether childhood mortality in the Darwin/Wedgwood family across several generations was related to inbreeding, the authors indeed found an association.
Three of the Darwins’ ten children died at age 10 or younger, one of tuberculosis, one of scarlet fever, and one of an unidentified disease. Studies suggest an association between childhood mortality from bacterial infection and consanguinity, and the Darwin family seems to bear that out. In addition, three of the Darwin children who did live to adulthood experienced lengthy marriages without any children, and such infertility may be another manifestation of homozygous states that interfere with reproduction. A photograph of the youngest Darwin child, Charles, who died in toddlerhood, suggests that the baby had some congenital disorder, although the nature of it remains unclear. Emma Darwin was 48 years old when she gave birth to Charles, so Down Syndrome is one likely explanation.
In spite of some of the sad facts of the Darwin family story, a few of his children experienced successes of different kinds. Three of his sons were members of the Royal Society, a long-time Darwin family tradition that skipped over the most famous member of the tribe, Charles himself. And Darwin by any measure of fitness did pretty well: in spite of the loss of three children and the infertility of three children, he nevertheless had several grandchildren.
Did Darwin himself suffer from the effects of inbreeding?
Charles Darwin experienced a variety of chronic health conditions, but they do not necessarily seem to have been related to his family’s consanguineous status. Several theories abound to explain his symptoms, which included digestive and skin problems, but no one knows for certain what afflicted the great naturalist. One of the foremost hypotheses is that he had Chagas disease, occurring after a bug bite on one of his voyages transferred an infectious protozoan that may have permanently damaged the scientist’s gut. Stress seems to have exacerbated the problem, whatever its etiology.