Final answer:
The SRY gene on the Y chromosome initiates the development of male sex characteristics in embryos. The presence of this gene leads to the formation of testes; its absence results in the development of ovaries and a female reproductive system.
Step-by-step explanation:
The SRY gene is Y-linked and plays a major role in causing the undifferentiated gonad to develop into a testis. The correct answer to the question is: a) SRY. During embryonic development, the SRY gene on the Y chromosome is responsible for starting a chain of events that result in the birth of male sex characteristics. A deletion or mutation in the SRY gene can lead to Swyer syndrome, where the embryo does not develop into a male despite having an XY genotype. The SRY gene causes gonadal tissue to develop into testes, which then produce hormones like testosterone, prompting development of a male reproductive system. In the absence of the SRY gene, an embryo will instead develop ovaries and female reproductive organs.