WIn the earliest stages of development it is difficult to tell very much about an embryo, including its sex. Ultrasound scans, performed for medical reasons and for sex determination, are seldom done before the 18th week. In fact, at six or seven weeks, all embryos look almost exactly alike -- and not just on the outside. Inside, there are two gonads, which could become testicles or ovaries. And each embryo has two sets of reproductive tubes, one set in case it's a male, the other for a female.

One way to tell from the very beginning if an embryo is male or female is to look at the chromosomes inside one of its cells. A single pair among the 23 total pairs of chromosomes is responsible for determining sex. An embryo with two X chromosomes will develop into a female. An embryo with one X and one Y chromosome will develop into a male.

In recent years, geneticists have begun to understand what triggers the development of males as opposed to females. There are only about 30 genes on the Y chromosome. One particularly influential gene is called SRY, or sex-determining region Y gene. This gene seems to be active just once in a male's lifetime -- late in the sixth week of embryonic development -- and in only one place -- the gonad.

Despite being active for only a day or two, the SRY gene creates sweeping changes throughout the embryo. According to geneticists who study sex determination, SRY works by attaching itself to other portions of the DNA molecule. In doing so, it distorts the shape of the DNA and sets off a chemical chain reaction. These chemicals turn on other genes, which create scores of proteins and eventually cause the embryo's gonads to become testicles. Not long after, the testicles begin producing the hormone testosterone, which will flow through the embryo throughout development, and cause other changes that ultimately transform it into a male.

In the absence of a Y chromosome and its SRY gene, different genes become active. The gonads become ovaries, and without the flood of testosterone, the embryo (a bit less dramatically) becomes female.

Identify the effects of maternal consumption of alcohol at various stages of pregnancy in this NOVA classroom activity.