How is gender determined?
X and Y: All human traits are determined by 23 pairs of chromosomes. Look at the 23
pairs of chromosomes shown in the karyotype (Figure 3-24.) In females, all pairs of
chromosomes look identical. In males, the chromosomes of the twenty-third pair are
not alike. These different chromosomes are called X and Y. Male cells have one X
chromosome and one Y chromosome. The Y chromosome is smaller than the X
chromosome. Female cells have two X chromosomes in the twenty-third pair. The X
and Y chromosomes determine the gender of an organism. Gender is the sex of an
organism.
Figure 3-24 Male Karyotypes
Gametes: Male gametes have an XY pair of chromosomes. During meiosis, each sperm
cell receives only one chromosome from each pair. Half of the sperm cells receive an X
chromosome. The other half receives a Y chromosome. Every sperm contains either an
X or a Y chromosome. Female cells have an XX pair of chromosomes. During meiosis,
each egg cell receives once chromosome from each pair of chromosomes. Therefore,
each egg cell receives one X chromosome. All egg cells contain X chromosome.
Gender of Offspring: The gender of offspring is controlled by chromosomes in the male
sperm cells. As a result, all children inherit their gender from their father. During
fertilization, if the egg cell is fertilized by a sperm carrying an X chromosome, the
fertilized egg will have two X chromosomes (XX). It will develop into a female. If the
sperm is carrying a Y chromosome, the egg will have one X and one Y chromosome (XY).
It will develop into a male.
Half the sperm cells carry the X chromosome. The other half carries the Y
chromosome. Therefore each new offspring has a 50 percent chance of being a girl and
a 50 percent chance of being a boy.