MT readers who are interested in fundamental aspects of evolution, and human evolution in particular, might find the post on the Molecular Evolution blog interesting. There are a couple of posts on a kind of rat that has no Y chromosome, nor any SRY gene, both of which are assumed to be central to sex-determination in humans and mammals generally.
Sex determination is complex and variable among lineages of animals (and plants), but within major groups usually follows a particular pattern. In mammals, the Y chromosome has some genes, but not many and few seem to be necessary for sex-determination (for example, they also exist on the X chromosome, which is present in both males and females).
Insects, birds, and so on determine sex in different ways, each interesting and presenting challenges to explain how the transition of one mechanism evolved from another. I once wrote an article for Evolutionary Anthropology on this subject, in the context of the curious fact, at least as then known, that bdelloid rotifers had no sexual reproduction, which seemed quite strange.
Sex determination is to be distinguished from 'gender' (sex-associated behavior) or sexual preference, and there are many if not most sex-related traits not located on the X or Y chromosome. But to have no Y chromosome and yet have sex and sex-related traits like behavior, means that the functions must have been adopted by other genetic mechanisms. Apparently, judging from the ME post, these are not known for the spiny rats.
The specifics of rat genetics are of little direct relevance to humans, except to the extent that they raise a caution flag, warning us not to be too simplistic or categorical in our assertions about sex, even within our own species. We seem pretty much married, so to speak, to our XX/XY system, but the evolutionary fluidity of sex-related genetic mechanisms, and the high variability of our own behaviors, suggest that things are far more variable and less clearly or simply determined than we might think.