For mammals, what is the correct sex determination system?

In mammals, the correct sex determination system is based on the presence of sex chromosomes. The most common system is identified as XX/XY. In this system, individuals with two X chromosomes (XX) develop as female, while individuals with one X and one Y chromosome (XY) develop as male.

The presence of the Y chromosome carries the SRY gene (Sex-determining Region Y), which triggers male development. If the SRY gene is present, the developmental pathway will lead to the formation of male characteristics, while the absence of this gene in XX individuals allows for the default female development process. This binary system, essentially driven by the combination of X and Y chromosomes, is a hallmark of mammalian genetics and underpins the biological basis for sexual differentiation in mammals.

Other options represent different sex determination mechanisms found in other organisms or theoretical models but do not apply to mammals. For example, the ZZ/XY system typically describes birds, while XX/XO refers to some types of insects. The X/X2 system does not correspond to any well-characterized sex determination system in mammals.