Xchromosome

The X chromosome is one of the two sex chromosomes in humans and many other organisms. It is a large, gene-rich chromosome that carries roughly 800–900 protein-coding genes and spans about 156 million base pairs. In humans, females typically have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The X chromosome contains regions called pseudoautosomal regions PAR1 and PAR2 where it shares homology with the Y chromosome, allowing recombination during male meiosis.

In humans, dosage compensation balances X-linked gene expression between the sexes. Early in female embryonic development, one of the two X chromosomes is randomly inactivated in each cell, a process known as X-inactivation or lyonization. The inactive X becomes transcriptionally silent largely due to the XIST gene, which produces an RNA that coats the chromosome and facilitates silencing. Some genes on the X chromosome escape inactivation and are expressed from both copies, contributing to differences in gene expression between males and females. X-inactivation results in cellular mosaicism for X-linked traits in females.

X-linked inheritance describes traits associated with genes on the X chromosome. Males, with one X chromosome, can express recessive X-linked disorders with a single mutated allele, while females, with two X chromosomes, typically require mutations on both copies to show the condition, though carriers can pass affected genes to offspring and skewed X-inactivation can influence phenotype.

Clinical relevance includes X-linked recessive conditions such as hemophilia A and Duchenne muscular dystrophy, which are more common in males. Aneuploidies involving the X chromosome include Turner syndrome (monosomy X) and Klinefelter syndrome (XXY). The X chromosome’s evolution and regulation are central to sex development and genetic expression.