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Whether a zygote will develop into a male or female is controlled by the sex chromosomes. As shown in Figure 2.1, 22 of the 23 pairs of chromosomes are matched; they contain similar genes in almost identical positions and sequence, reflecting the distinct genetic blueprint of the biological mother and father. The 23rd pair of chromosomes are sex chromosomes that specify the biological sex of the individual. In females, sex chromosomes consist of two large X-shaped chromosomes (XX). Males’ sex chromosomes consist of one large X-shaped chromosome and one much smaller Y-shaped chromosome (XY).

Figure 2.1 Chromosomes

Because females have two X sex chromosomes, all ova contain one X sex chromosome. A male’s sex chromosome pair includes both X and Y chromosomes; therefore, one-half of the sperm males produce contains an X chromosome and one-half contains a Y. The Y chromosome contains genetic instructions that will cause the fetus to develop male reproductive organs. Thus, whether the fetus develops into a boy or girl is determined by which sperm fertilizes the ovum. If the ovum is fertilized by a Y sperm, a male fetus will develop, and if the ovum is fertilized by an X sperm, a female fetus will form, as shown in Figure 2.2. (The introduction of sex selection methods has become more widely available, and some parents may seek to choose the sex of their child. For more on this topic, see the Applying Developmental Science feature.)

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Figure 2.2 Sex Determination