Final answer:
The SRY gene on the Y chromosome initiates the development of male characteristics in mammals by producing a protein that stimulates testes formation and subsequent testosterone production.
Step-by-step explanation:
Development of male characteristics requires the action of the SRY gene on the mammalian Y chromosome. The SRY gene is responsible for initiating a series of events that lead to the development of the male reproductive system in an embryo with XY chromosomes. This gene triggers the production of testis-determining SRY protein, which directs the embryo's primordial gonads to develop into testes rather than ovaries. Testosterone, produced by Leydig cells in the embryonic testes, plays a crucial role in stimulating the development of male sexual organs. Without the presence of the SRY gene, as in individuals with XX chromosomes, the embryo will develop female sexual organs due to the absence of testosterone.