12.13: X and Y Chromosomes

Among mammals, the gender of an organism is determined by the sex chromosomes. Humans have two sex chromosomes, X and Y. Every human diploid cell has 22 pairs of autosomes and one pair of sex chromosomes. A human female has two X chromosomes, while a male has one X chromosome and one Y chromosome.

The germline cells such as egg and sperm cells carry only half the number of chromosomes, i.e., 22 autosomes and one sex chromosome. All eggs have an X chromosome, while sperm cells can carry an X or Y chromosome. If a sperm carrying an X chromosome fuses with an egg, it produces a female embryo, whereas fusion of a sperm carrying a Y chromosome with an egg produces a male embryo.

The human genome sequencing project estimates that sex chromosomes have evolved from autosomes within the last 300 million years. Over time, the Y chromosome lost most autosomal sequences and currently has only around 55 genes. On the other hand, the X chromosome has over 1000 genes, most of which are unrelated to sex determination but include immune-related genes and several housekeeping genes. The mutations in the X- chromosome gene are often associated with X-linked disorders. These disorders are more prominent and lethal among males because they do not have another copy of the X chromosome to compensate for the genetic defects.

Additionally, an incorrect number of sex chromosomes can lead to genetic disorders. For example, females with Turner's syndrome have only one X chromosome instead of two. Such females are infertile with a broad chest and a wide, webbed neck. In contrast, Klinefelter syndrome (XXY) affects only males. Primary features include infertility, poor motor abilities, weaker muscle, among several other symptoms.

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.