When it comes to the architecture of the human genome, it’s only a matter of time before harmful genes — genes that could compromise future generations — arise in a population. These mutations accumulate in the gene pool, primarily affected by a population’s size and practices like marrying within a small community. New research published in the Proceedings of the National Academy of Sciences (PNAS) journal provides rare direct evidence showing that increased homozygosity — meaning two identical alleles in a genome — leads to negative effects on fertility in a human population.