Final answer:
Mutations that ablate the androgen receptor function in XY individuals would result in atypical sex development, often resembling female phenotypes, due to androgen insensitivity syndrome. Similarly, a nonfunctional SRY gene would prevent normal male sex development, while concurrent mutations in SRY and SF1 do not lead to normal male characteristic development. Epigenetic changes that inactivate one X chromosome in females adjust gene dosage levels between sexes.
Step-by-step explanation:
A mutation that completely ablates the function of the androgen receptor in humans with XY chromosomes would significantly impact their phenotypic development. These individuals would experience androgen insensitivity syndrome (AIS), a condition in which their bodies cannot properly respond to male sex hormones (androgens). As a result, despite having XY chromosomes, their external sex characteristics would develop along the typical female pattern. This would include the development of female genitalia, and during puberty, a lack of development of male secondary sexual characteristics such as deepening of the voice, facial and body hair, and increased muscle mass.
Mutations in the SRY gene, which typically triggers male sex differentiation, or mutations that impact factors like SF1 (steroidogenic factor 1), dramatically influence sexual development. A nonfunctional SRY gene results in abnormal or lack of development of male sex characteristics leading to a phenotype that can be more typically female due to failed testis development. On the other hand, a mutation in SF1 alone would not cancel out an SRY mutation. A concurrent mutation in both SRY and SF1 would not result in normal development of male sex characteristics. Indeed, an SF1 mutation might exacerbate the effects of a nonfunctional SRY gene, leading to an even more female-typical development.
In comparison with the XY chromosome scenario, in XX females, epigenetic changes lead to the inactivation of one X chromosome during embryonic development. These epigenetic changes result in more tightly packed chromatin, thereby reducing gene expression from the inactivated X chromosome. This process ensures that the gene dosage between males (XY) and females (XX) remains balanced.