The slow death of inbreeding

As a caveat, I do not profess to be a geneticist, although this study is straightforward enough thatI feel like I understand it well enough to explain it. It's also nice to branch out and read a non-infectious disease based study.

This one, from this month's PLOS Genetics, is interesting, not necessarily for its methodology, but more for some of the conclusions that can be drawn from its results. What the researchers did is take two groups of heterogenous, American people, aged 19-99, performed genome-wide analyses, and quantified the degree of autozygosity within each. By autozygosity, they mean strings of homozygosity, which they took as a surrogate quantifier for degree of consanguinity, or inbreeding, in a population. They also controlled for linkage disequilibrium by getting rid of the SNPs that may have confounded the results. Their results are nicely summed up in the figure below (it's not as blurry if you click on it), which looks at birth date on the x-axis, and on the y, clockwise from upper left, percent of genome in these homozygous runs, number of runs, average length of the runs, and their inbreeding coefficient:



Essentially, the younger their subject, the lower percentage of their genome occupied by these runs of homozygosity, which has, in previous work, been linked to the degree of inbreeding in a population. They make the subsequent statement that the demographic shift over the past century, both from rural to urban and globally, has led to this admixing of gene pools, reducing the amount of inbreeding that's occurring. Since I can't think of any other plausible hypothesis to argue against this conclusion, I have to go along with it. Basically, your grandmother's generation was more inbred than yours, mostly because she didn't travel to the big city after college.

Since consanguinity increases the risk of rare genetic diseases significantly, we may be on the cusp of seeing these diseases disappear. From an evolutionary perspective, this makes perfect sense, as these diseases don't confer any advantage whatsoever. And beyond just reducing the incidence of otherwise rare diseases, this genetic admixing may provide all sorts of differential advantages that we've yet to determine. Another argument for leaving home and traveling. You're doing it for the human race.


Reference: Nalls, M., Simon-Sanchez, J., Gibbs, J., Paisan-Ruiz, C., Bras, J., Tanaka, T., Matarin, M., Scholz, S., Weitz, C., Harris, T., Ferrucci, L., Hardy, J., & Singleton, A. (2009). Measures of Autozygosity in Decline: Globalization, Urbanization, and Its Implications for Medical Genetics PLoS Genetics, 5 (3) DOI: 10.1371/journal.pgen.1000415