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 studies have demonstrated that the fraternal birth order effect does not operate through social or postnatal (e.g., learning or rearing or environmental) mechanisms; rather it is biological in nature and operates prenatally. It is also known that the fraternal birth order effect on male sexual orientation is unaffected by family dynamics, living with older brothers or learning. Fraternal birth order and male sexual orientation A correlation between fraternal birth order and male sexual orientation has been suggested by research. Ray Blanchard identified the association and referred to it as the fraternal birth order effect. In several studies, the observation is