5-alpha-Reductase 2 deficiency

5-alpha-reductase 2 deficiency is a rare autosomal recessive condition caused by mutations that impair the SRD5A2 gene, which sits on chromosome 2 and encodes the enzyme 5-alpha-reductase type 2. This enzyme is made in particular tissues and converts testosterone into dihydrotestosterone, a hormone that plays a key part in sexual differentiation. When the enzyme does not work, genetic males with a 46,XY karyotype develop atypically, and the effect is clearest in the genitalia. Presentation covers a broad spectrum: many affected people are assigned female at birth, some infants are assigned male on the basis of an unusually small penis and a urethral opening on the underside, and in others the genitalia look neither clearly male nor clearly female. At puberty rising androgen levels bring muscle growth, a deeper voice, pubic hair and a growth spurt, and the penis may enlarge, but facial and body hair stay sparse and male pattern hair loss does not develop.