Final answer:
The SRY gene on the Y chromosome is crucial for the development of male characteristics in mammals, initiating the formation of testes and triggering the production of testosterone, which leads to male sexual organ development.
Step-by-step explanation:
Development of male characteristics requires the action of the SRY gene on the mammalian Y chromosome.
The reproductive systems of males and females begin to develop soon after conception, but it is the presence of the SRY gene that initiates male development. This gene is responsible for the cascade of genetic events that stimulate the development of testes and suppress the formation of female reproductive structures. As a result of testes formation, testosterone is produced which furthers the development of male sexual organs. Without the presence of testosterone, female sexual organs will develop by default.
In summary, the SRY gene on the Y chromosome in humans and other mammals is essential for the sexual differentiation that results in male characteristics. Its absence or malfunction, such as in Swyer syndrome, leads to the default development of female characteristics even if the individual possesses an XY genotype.