A baby’s sex is determined at conception (when the sperm fertilizes the ovum), but at first both female and male embryos develop in exactly the same way and they have identical genital parts until around the eighth week of gestation, when males and females start to develop in different ways.

There is an unexpectedly small number or genes that determine the differentiation male/female:  in the nucleus of every cell of our body there are 46 chromosomes, organized in pairs: 22 chromosome pairs that are identical in both males and females and 1 pair of sex chromosomes or gonosomes denoted as X and Y. The mother’s egg provides an X chromosome and the father’s sperm determines the baby’s sex by contributing either an X or Y sex chromosome. An XX embryo is female, while an XY embryo is male. When a Y chromosome is present, the development takes place in the direction of manhood; if it is missing, a feminine development occurs spontaneously in the direction of the feminine phenotype without particular hormonal influences. At this point, the foetus with an XX pair will start developing feminine-specific organs, such as ovaries, while in the foetus with an XY pair the action of all feminine hormones will be inhibited, and it will start develop masculine organs, such as testicles. However, the information encoded on the Y-chromosome is not enough to guide the formation of complicated male organs, but a localized gene on this chromosome, the SRY (sex determining region Y gene) operates very early in the development as a guide or “master gene“.