That mammalian embryos start out essentially as females was rather startling news. But at first, the embryonic gonad (potential testis or ovary) is undifferentiated and if nothing were done, most mammals, including ourselves, would each develop into females (female elephants and, as we have seen, hyenas, by contrast, at first display male features, but they are exceptional). Usually, something is done, so that some originally undifferentiated embryos develop into males. This is because they become endowed with a testis. In other words, in these embryos, the gonad develops into a testis rather than an ovary. Thus it is that a testis normally casts the seal of masculinity on an individual, way back in its embryonic state, and if the gonads are experimentally prevented from developing, that is, an embryo is rendered agonadal (this can be done experimentally in rats by removal of the pituitary, a procedure known as hypophysectomy), it automatically proceeds to be female.

Precisely how the message is passed to make an embryo become a male is complex, but in mammals, it depends on the presence of a Y chromosome. Individuals that are destined to become females inherit two X chromosomes (XX), but males replace one X with a Y chromosome (XY). This depends on whether an X or a Y (female or male) sperm (with the haploid condition) penetrates and fertilizes the egg. X and Y chromosomes are called sex chromosomes to distinguish them from somatic chromosomes, which are associated with all other cells of the body. So as not to have too many X chromosomes (that is, one more than males), females eventually lose one so that each sex ends up with the same complement of X chromosomes. There is a bit of toing and froing between chromosomes too, whereby genes can occasionally stray from their original chromosome onto another one (translocation). This can even happen from a sex chromosome onto a somatic (body) chromosome, which itself has nothing to do with sex. As a result, so-called sex-linked genes occasionally arise on body chromosomes. Things are not as straight forward as they might at first seem to be.