Production of Sex Cells

From the time of puberty on, men make sex cells (in the form of sperm cells) continuously. In contrast, by the time a female is born, she has made all of the eggs that she will ever have. As she reaches puberty, the eggs begin to develop and get released, and this process continues until menopause. In both males and females, the production of sex cells involves meiosis, a type of cell division whereby our two sets of genetic instructions are reduced to one set for the sex cell.

Photo courtesy U.S. DOE, Human Genome Project

Each cell in your body contains a set of chromosomes from your mother (her egg) and your father (his sperm). When you look at human chromosomes, they are X-shaped, except for the male Y chromosome. When your body produces sex cells (sperm or egg, depending on whether you are a male or female), your body must reduce the number of chromosomes by half to go into the sex cells. To do this, it randomly sorts chromosomes from both sets in one cell division and then reduces them by half in another. Therefore, each sperm or egg that your body produces is unique and different -- it contains a different mix of your mother's and father's genes. This is why two brothers in the same family can look and act totally different from one another even though they come from the same parents -- it all depends on which genes (chromosomes) were randomly chosen when producing the sex cells of the mother and father.

For details on meiosis, see The Biology Project: Meiosis Tutorial and this Question of the Day.