Many genes contribute to the development of internal sexual organs and external sexual characteristics. However, the SRY gene is particularly important to the Y chromosome. This is the gene which, in the seventh week of pregnancy, gives the starting signal for the male fetus to develop. To do this it calls in several other genes so that together they can gradually form all the male characteristics. If the SRY gene is missing, female organs develop, although, if a Y chromosome is present but the SRY gene is faulty, the result will be a sterile XY female.
The Y and X chromosomes-seen here through a electron microscope-determine a person's sex.