[Our hidden genetic defects]

A systematic study analysing the exomes of several thousand individuals indicates that each of them carries at least one strongly deleterious mutation that is innocuous in a heterozygote but results in a severe phenotype in the homozygous state. Most of these mutations are very rare, while a few are present in 1 or 2% of the population. The frequency of at-risk couples is approximately 1.5%, but increases dramatically to 25% if the partners of the couple are first cousins. This work has important implications for carrier screening and population genetics in general.