Which X-Y chromosomes have the developmental genes for mammals? I know that for a given mammal, half the chromosomes come from father and half from mother. This is typically denoted as x-y. I've recently read about "toolkit" genes that control how a cell develops into an embryo into an organism. **I'm interested which set of chromosomes(x,y or both) contains these toolkit genes for development of an organism from a fertilized cell?** In other words, does an X chromosome know how to build a female and Y chromosome how to build a male, or is it some "overriding" of developmental instructions that Takes place when Y is present?

First, while half the chromosomes come from each of the two parents, these two sets of chromosomes are _not_ termed X and Y (they would usually be called maternal and paternal). The terms X and Y refer to potential members of just one pair of the 23 pairs (in humans) of chromosomes, and X chromosomes can come from either the mother or the father. The presence or absence of a Y chromosome determines sex in mammals.

The Y chromosome contains one gene, SRY, which determines sex, and almost no nother genes. SRY simply switches on the "make a male" program in the cells.

Essentially all other genes, including any toolbox genes, come from both parents, one copy from each. (An exception, in males, are those genes on the X chromosome, which in males are present in only one copy and which came from the mother. Another exception is mitochondrial genes.)