Читать книгу Lifespan Development - Tara L. Kuther - Страница 105

The sex chromosomes determine whether a zygote will develop into a male or female. As shown in Figure 2.2, 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 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).

Description

Figure 2.1 Meiosis and Mitosis

Figure 2.2 Chromosomes

Source: U.S. National Library of Medicine.

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 contain an X chromosome and one half contain 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 2.3. (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 accompanying feature, Applying Developmental Science: Prenatal Sex Selection.)