Selfish sperm cells
Usually, those alleles (versions of a gene) that become more common over generations are those that are most beneficial to the organisms in whose cells they live. But not always.
The latest issue of PLoS Biology has an open-access article on a particularly selfish gene responsible for Apert syndrome in humans.
The allele that causes this disease is much more common than expected, based on the assumption that it arises by mutation at the average rate for most DNA. Some regions in a DNA molecule have higher mutation rates, but could there be another explanation?
To find out, the authors of "The molecular anatomy of spontaneous germline mutations in human testes" cut human testes (from dead donors) into hundreds of pieces and analyzed the frequency of the mutant Apert allele in each piece, using quantitative PCR.
The mutant allele was up to 10,000 times more common in some pieces than in others, suggesting that cells with the mutant allele were reproducing rather than just replacing themselves, as spermatogonial cells normally do.
The bad news is that an allele whose affects favor its own reproduction will tend to spread, even if its effects on our bodies or our children are negative. Similar effects have been seen in plants, where genes transmitted only in the seeds block pollen production, freeing resources for seed production. Fortunately, natural selection will also favor any other genes, anywhere in the genome, that suppress recklessly selfish genes.