Sex determination
Humans and other mammals have an XY sex determination system: the Y chromosome carries factors responsible for male development. The default sex, in the absence of a Y chromosome, is female. XX mammals are female and XY are male. XY sex determination is also found in other organisms, including the common fruit fly and some plants. In some cases, including in the fruit fly, it is the number of X chromosomes that determines sex rather than the presence of a Y chromosome.