Chromosome X

Chromosome X, one of the two sex chromosomes, plays a critical role in the determination of sex in mammals, including humans. It is part of the XY sex-determination system, where typically females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The X chromosome carries a larger number of genes compared to the Y chromosome, contributing not only to sexual differentiation but also to various other biological functions and traits.

The discovery of the X chromosome dates back to the late 19th century, with scientists such as Hermann Henking observing what would later be identified as the X chromosome during spermatogenesis in insects. This discovery laid the groundwork for the understanding of sex determination and the role of chromosomes in heredity.

The X chromosome contains approximately 155 million base pairs and houses over 1,100 genes. These genes are involved in a wide range of functions, including cognition, bone density, muscle strength, and the development of reproductive organs. Due to the presence of multiple copies of the X chromosome in females (XX), a mechanism known as X-inactivation occurs, where one of the X chromosomes is largely inactivated to prevent an overdose of the gene products that the X chromosome carries.

Genetic disorders linked to the X chromosome are termed X-linked disorders. These can be recessive or dominant, affecting males and females differently due to their different numbers of X chromosomes. Examples of X-linked disorders include hemophilia, Duchenne muscular dystrophy, and fragile X syndrome. Males are often more severely affected by recessive X-linked disorders because they have only one X chromosome, and thus one copy of the gene in question.

The study of the X chromosome has also led to the discovery of the XIST gene, which plays a crucial role in X-inactivation, and the concept of genomic imprinting, further expanding our understanding of genetics and heredity.