The differences between the two sexes depend on the presence of a single chromosome—the Y chromo-some—and two endocrine structures, the testis andthe ovaries. The hormonal secretions from the testis are responsible for the development of the male ex-ternal genitalia and male sexual behavior.
The sex of the individual is determined genetically by two chromosomes (X and Y), called the sex chro-mosomes. Each individual has two sex chromo-somes—XX (female) or XY (male)—and 22 other pairs of chromosomes, called autosomes. When the ovaries in the mother and the testis in the father manufacture reproductive cells (the ova and sperm, respectively), only one of the sex chromosomes is present in each of the cells. This is because cell divi-sion occurs by meiosis . Each ovum from the female contains one X chromosome (in ad-dition to 22 autosomes). Of the sperm, some contain one Y chromosome and others contain one X chro-mosome (in addition to 22 autosomes). Therefore, when the ova and sperm come together during fertil-ization, two types of combinations can occur—XX (genetic female) and XY (genetic male). The fertilized ovum has 23 pairs of chromosomes (22 autosomes plus two sex chromosomes).