The aforementioned conversion of testosterone to dihydro-testosterone is done by an enzyme known as 5-alpha-reductase, which is absent among the guevedoces. The genetic mutation that causes the deficiency was passed down through generations and among themselves. 12 families out of 13 have at least one or more male who carries the mutation. However, being carriers doesn’t necessarily mean all of them are affected. On an average, 1 out of 90 males is an affected carrier, and the rest are either not carriers or not affected.
When puberty sets in, the children get a second surge of testosterone to which the body responds and it changes their external anatomy into male.
The high prevalence of 5-alpha-reductase deficiency in Las Salinas is thought to be because of “founder effect” or “bottleneck effect”, with the villagers being interrelated because they are geographically isolated and descending from an individual who carried the genetic mutation.