Gonadal dysgenesis If a single or combination of these genes are mutated or deleted, downstream signalling is disrupted, leading to malformation of male external genitalia. SRY acts on gene SOX9 which drives Sertoli cell formation and testis differentiation. An absence in SRY causes SOX9 to not be expressed at the appropriate time or concentration, leading to a deficiency in testosterone and Anti-Müllerian hormone production. Inadequate levels of testosterone and Anti-Müllerian hormone disrupts the development of Wolffian ducts and internal genitalia that are key to male reproductive tract development. The lack of the male associated steroid hormones drives Müllerian duct development and perusal
Fraternal birth order and male sexual orientation not non-biological older brothers predicted sexual orientation in men. Further, if rearing or social factors underlie the fraternal birth order effect, then the amount of time reared with older brothers, either biological or non-biological, should predict sexual orientation because rearing time determines the relative opportunity that older brothers have to affect their younger sibling's (postnatal) sociosexual development. If a prenatal factor underlies the fraternal birth order effect, then a postnatal factor such as rearing time with older siblings (be they biological or non-biological) should have no impact on the sexual orientation of younger male siblings. The study found that the