xchromosomal

Xchromosomal refers to anything related to the X chromosome, one of the two sex chromosomes in humans and many other organisms. The human X chromosome is large, about 155 million base pairs, and it carries roughly 800–900 protein-coding genes in addition to numerous non-coding sequences that regulate gene expression. It contains regions essential for development and various biological functions beyond sex determination.

In humans, females typically have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Because males have only a single copy of X-linked genes, a pathogenic variant on the X can more readily cause disease in men, while females may be carriers with a normal allele on the second X. Inheritance patterns are described as X-linked or X-chromosomal.

X-chromosome inactivation is a key mechanism in females to balance gene dosage with males. Early in embryonic development, one X chromosome in each cell is randomly inactivated, forming a Barr body. Most genes on the inactive X are silenced, but some escape inactivation, leading to mosaic expression of X-linked genes across tissues.

Clinically, many X-linked disorders are recessive and predominantly affect males, such as Duchenne muscular dystrophy, hemophilia A, and red-green color blindness. There are also X-linked dominant conditions and chromosomal variations involving the X, such as Turner syndrome (45, X) and Klinefelter syndrome (47, XXY), which illustrate how the dosage and behavior of the X chromosome influence phenotype.

The X chromosome also contains pseudoautosomal regions that recombine with the Y chromosome during male meiosis, contributing to genetic exchange between the sex chromosomes. Dosage compensation through X-inactivation ensures balanced expression of X-linked genes between the sexes.