5-alpha reductase deficiency is one of several conditions where girls are born with XY karyotype (the usual male pattern). It is characterized by an absence of the enzyme 5-alpha reductase, which converts testosterone change to dihydrotestosterone (DHT). Without DHT, prenatal genital development results in female appearance. Because masculine puberty depends more strongly on testosterone than on DHT, puberty will be virilizing (unless the gonads are removed or a blocking medication is used).

Genetic Information from: NIH, National Library of Medicine, Genetics Home Reference https://ghr.nlm.nih.gov/condition

https://ghr.nlm.nih.gov/condition/5-alpha-reductase-deficiency