There is no consensus on what biological aspect or marker of the body determines sex (chromosomes, genitalia, etc.), but there are patterns in the ways legal sex is assigned at birth in the case of intersex infants which illuminate the prevailing frameworks in the social construction of sex.
With regards to infants with ambiguous genitalia, physicians make judgements based on presence or absence of an “adequate” penis and its perceived potential to penetrate a vagina in future. Modifications will be made to the genitals of the infant with an “inadequate” penis in order to label the infant female. In infants judged to have the potential to carry a pregnancy, the priority is typically preservation of reproductive capability, with appearance of external genitalia surgically altered to fit standards of the vulva, e.g. reduction of phallus to “acceptable” clitoral size.
Sexual determinations and subsequent surgical modifications of the intersex infant therefore are not made based on chromosomes but the enforcement of the male as capable of penetration leading to pregnancy and the female as capable of pregnancy.
references
- “The Road Less Traveled: The Problem with Binary Sex Categories” by Julie A Greenberg in Transgender Rights (2006)