One of these is Leber’s hereditary optic neuropathy, which causes vision loss, usually in young men. Recently, geneticists using French Canadian genealogy have reexamined the effects of Leber’s and found a striking pattern of inheritance: It seems to show a long-theorized but never-seen-in-humans pattern called the “mother’s curse.”
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Before all this, the most convincing evidence of the mother’s curse came from fruit flies.
The idea for the mother’s curse goes like this. Most human genes are on chromosomes, but a tiny number are in mitochondria, little power factories in human cells that for reasons of evolutionary history have their own loops of DNA. Sperm do not pass on any mitochondria, but eggs do. Therefore, all sons and daughters inherit their mitochondrial DNA from their mother. If a harmful mutation in mitochondrial DNA ends up in a woman, she will be less evolutionarily “fit” and thus less able to pass it along. But if the mutation ends up in a man, nothing happens. He never passes along mitochondrial DNA anyway.
The trouble comes when the mutation harms men but not women; then there is no way for natural selection to weed it out. The mother’s curse is passed on to her children. (There is no equivalent father’s curse because the Y chromosome is only passed to men and cannot affect women.) For example, Damian Dowling, a geneticist at Monash University, has found a gene in fruit flies that makes males completely sterile but somehow seems to be selected for in females.
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For many years, says Dowling, Leber’s hereditary optic neuropathy has been on the radar of evolutionary biologists interested in the mother’s curse. Doctors knew the mutation causing the disorder sits on mitochondrial DNA, and they have noticed that it seems to affect men eights times more than women.
In 2005, researchers at the Université de Montréal actually traced the ancestry of 11 modern-day patients with Leber’s back to a single fille du roi who came to New France at the age of 18. Her particular mutation is responsible for 89 percent of Leber’s cases in the Quebec population. By following her daughters, granddaughters, great-granddaughters and so on all the way to the present, it is possible to identify people who had lived with this particular Leber’s mutation over a 290-year period in the area. “It was the ideal setup to test the mother’s-curse hypothesis,” says Emmanuel Milot, a geneticist at Université du Québec à Trois-Rivières.
Milot and his colleagues ended up sorting people in the French Canadian genealogy databases by whether they had the Leber’s mutation and counting up their number of descendants. In straight evolutionary terms, the more descendants you have, the more “fit” you are. The men carrying the Leber’s mutation, it turned out, were indeed slightly less fit than men who did not, as well as women who did. Because of this pattern, the mutation harmful in males does not get weeded out at all over 290 years. It appear to be an example of the mother’s curse at work.