X and Y Chromosomes

The X chromosome is significantly larger than the Y chromosome and contains over 1,000 genes, including immune-related and housekeeping genes unrelated to sex determination. In contrast, the Y chromosome has only around 55 genes. This size and gene content difference means X-linked genes lack counterparts on the Y chromosome, making them dominant in males.

Sex is determined during fertilization based on which sex chromosome the sperm carries. All eggs contain an X chromosome. When a sperm carrying an X chromosome fuses with an egg, the zygote has two X chromosomes and develops as female. When a sperm carrying a Y chromosome fuses with an egg, the zygote has an XY pair and develops as male.

Males have only one X chromosome, so they lack a second copy to compensate for genetic defects in X-linked genes. Females have two X chromosomes, providing a backup copy that can mask recessive mutations. This makes X-linked disorders like color blindness and other X-linked genetic disorders more deleterious and often lethal in males.

Egg and sperm cells are haploid, containing only half the number of chromosomes found in body cells. All egg cells carry one X chromosome. Sperm cells carry either one X chromosome or one Y chromosome, with approximately half of an organism's sperm carrying each type.

Y-linked genes are genes present only on the Y chromosome and are expressed exclusively in males. The SRY gene, located on the Y chromosome, is responsible for male sexual trait development. These genes have no counterparts on the X chromosome and directly influence male-specific characteristics.

Turner syndrome occurs in females with only one X chromosome instead of two, causing infertility, broad chest, and webbed neck. Klinefelter syndrome (XXY) affects males, resulting in infertility, poor motor abilities, and weaker muscles. Both conditions demonstrate how incorrect sex chromosome numbers lead to significant genetic disorders in humans.

When a female carrier of an X-linked mutation mates with a non-affected male, male offspring have a 50% chance of expressing the disorder, while female offspring have a 50% chance of being carriers. This asymmetry occurs because males express all X-linked alleles, while females can be heterozygous carriers without displaying symptoms of recessive X-linked conditions.