X and Y Chromosomes

Each cell’s nucleus contains chromosomes, thread-like structures that carry an organism’s genetic information. There are two copies of each chromosome. In humans, there are twenty-three pairs of chromosomes, twenty-two of which are called autosomes. The twenty-third pair contains the sex chromosomes. The sex chromosomes differ between females and males. Females have two X chromosomes, while males have one X chromosome and one Y chromosome. Unlike the body’s other cells, the cells of the egg and sperm carry only half the number of chromosomes. This means that an egg or sperm cell will only contain one sex chromosome. All eggs carry an X chromosome, while half of an organism’s sperm carries an X chromosome and the other half carries a Y chromosome. The sex of an offspring is determined during fertilization. For example, when a sperm carrying an X chromosome is fused with an egg, the zygote carries two X chromosomes along with its other autosomes. In this case, the offspring is female. Similarly, when a sperm carrying a Y chromosome is fused with an egg, the zygote contains an XY pair of sex chromosomes and gives rise to a male offspring. While a pair of autosomes will have the same size and shape, X and Y chromosomes differ in their structure. Notably, the X chromosome is bigger than the Y chromosome and contains more genes. Since the additional genes in the X chromosome don’t have any counterpart in the Y chromosome, these X genes are dominant. Any mutations in such X-linked genes are more deleterious in the male offspring due to the lack of a second X chromosome to compensate for the loss.  For example, mutations in the red-green light-sensitive genes present on the X chromosome can cause color blindness in humans. When a female with the mutation mates with a non-colorblind male, the resulting male offspring have a 50% chance of being colorblind, while female offspring have a 50% chance of being a carrier for the gene. Similarly, the genes present only on the Y chromosome are called Y-linked genes. They are expressed only in males. For example, the SRY gene, present only in the Y chromosome, is responsible for male sexual traits.