What are some interesting genetic anomalies in humans which could have benefits?

"Guevedoces", which translates in Spanish to "penis at twelve", is a genetic anomaly found only in a small community in the Dominican Republic… in which children who are born with what appear to be female genitals grow a penis when they hit puberty.

Why?

At conception, people typically have either XX (female) or XY (male) chromosomes… though for the first weeks of life, you are neither a boy nor girl. This is why men have nipples.

Then, around eight weeks after conception, the sex hormones kick in. If you're genetically male, the Y chromosome instructs your gonads to become testicles and sends testosterone to a structure called the tubercle, where it is converted into a more potent hormone called dihydro-testosterone. This transforms the tubercle into a penis. If you're female, you don't make dihydro-testosterone, and your tubercle becomes a clitoris.

Boys with guevedoces are genetically deficient in an enzyme called 5-alpha-reductase, which normally converts testosterone into dihydro-testosterone. So despite having XY chromosomes, they appear female at birth. At puberty, like other boys, they get a second surge of testosterone. This time the body does respond and they grow testes and a penis.


Though there’s no clear benefit to “pseudohermaphrodites,” there was a benefit to society. While studying this extremely rare (outside of one small village) condition, researchers found that men with guevedoces tend to have a small prostate. This observation led to the development of finasteride, a drug that mimicks the lack of dihydro-testosterone seen in the Guevedoces. It is an effective way to treat benign enlargement of the prostate, which plagues many older men. It’s also used to treat male pattern baldness.

Another observation was that guevedoces boys, despite being brought up as girls, were almost all heterosexual. This led Dr. Julianne Imperato-McGinley to conclude that prenatal hormones matter more than rearing when it comes to sexual orientation.

(Apparently this conclusion is controversial, but I’m not sure why.)

Another genetic anomaly that could have benefits… is really pretty much any genetic condition that is prevalent in some population groups and not others. In With advances in genome editing tech, how close are we to Gattaca -- and how will that affect your children?, I made some derisive remarks about people who think it’s somehow “unethical”, “bad” or “eugenics” to edit out genes that cause children to be born with genetic conditions.

There is no good reason NOT to help children be born healthy and pain-free…

Except for one.

In my opinion, there is exactly ONE good argument for letting kids be born with preventable genetic conditions. And that is that these genes could confer protection against something worse.

The classic example of this is sickle cell anemia. This condition is common in certain African population groups, who live in places where malaria is common. It turns out that having one copy of this recessive allele protects you from malaria… but having two gives you sickle cell.

For all we know, it’s possible that having one copy of the cystic fibrosis gene could protect us from some other disease or condition.

(EDIT: Apparently lots of people have studied the possible benefits of being a CF carrier: Has Tuberculosis Contributed to the Global Rates of Cystic Fibrosis?)

And, of course, we’re always learning new things about “bad” genes. For example, Study Finds Healthy Respiratory Disease Gene Carriers Have Greater Breathing Capacity.

Scientists from the University of Bristol report that rare respiratory disease gene carriers actually have increased lung function. Their research has revealed that healthy carriers of a gene that causes a rare respiratory disease are taller and larger than average, with greater respiratory capacity.

benefits?