X and Y Chromosomes

Basics of X and Y Chromosomes

• The human genome consists of 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.
• The sex chromosomes are referred to as X and Y chromosomes.
• Females typically have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
• The Y chromosome is much smaller than the X chromosome and carries fewer genes.

Role of X and Y Chromosomes in Sex Determination

• Each human egg cell contains an X chromosome, while sperm cells can carry either an X or a Y chromosome.
• The type of sperm cell that fertilises the egg cell determines the baby’s sex.
• If an X carrying sperm cell fertilises the egg, the baby will be female (XX).
• If a Y carrying sperm cell fertilises the egg, the baby will be male (XY).

Genes on the X and Y Chromosomes

• The X chromosome carries around 800-900 genes, many of which are not related to sex determination.
• The Y chromosome carries around 50-60 genes, most of which are involved in male sexual development and reproduction.
• Sex-linked traits are traits whose genes are located on the sex chromosomes, most commonly on the X chromosome.

Disorders Linked to X and Y Chromosomes

• Disorders can occur when there is an additional or missing sex chromosome.
• Turner syndrome (X0) occurs in females when they have one X chromosome instead of two. Symptoms may include short stature and lack of sexual development at puberty.
• Klinefelter syndrome (XXY) occurs in males when they have an extra X chromosome. Symptoms may include small testes and reduced fertility.
• X-linked disorders, such as colour blindness and haemophilia, occur when there is a mutation in a gene on the X chromosome. As males only have one X chromosome, they are more likely to express X-linked disorders.