Chromosomes are small, threadlike structures within each cell that contain genes, which carry genetic instructions.

These genes control the physical and chemical traits inherited by children from their parents. The inherited traits are color of the eyes, sex, height, and skin color.

a. The female has 23 pairs of chromosomes. The pair of chromosomes that determined her sex are named “XX.” The ovum carries one chromosome from each of the female’s pairs (23 chromosomes). The ovum can only carry an “X” sex chromosome.

b. The male has 23 pairs of chromosomes. The pair of chromosomes that determined his sex are named “XY.” The sperm carries one chromosome from each of the male’s pairs (23 chromosomes). The sperm can carry either an “X” or a “Y” sex chromosome.

c. If the ovum is fertilized by a sperm carrying an “X” chromosome, the child is a girl.

d. If the ovum is fertilized by a sperm carrying a “Y” chromosome, the child is a boy.

e. The sperm of the father always determines the child’s sex (see figure 2-5).