X-chromosome

Definition

One of the sex chromosomes that carries genes involved in sex determination and numerous other traits. In the XY and XO sex-determination systems common among insects and arachnids, individuals typically possess two X-chromosomes in the homogametic sex (usually female) and one X-chromosome in the heterogametic sex (usually male). The X-chromosome is generally larger and gene-rich compared to the Y-chromosome in XY systems, or unpaired in XO systems where males possess a single X and no Y.

Etymology

Named 'X' by early geneticists for its unknown nature at discovery; the Y-chromosome was named subsequently as the next letter in the alphabet.

Example

In Drosophila melanogaster, sex is determined by the ratio of X-chromosomes to autosomes: XX individuals develop as females, while XY individuals develop as males, with the Y-chromosome carrying fertility factors but not the primary sex-determining signal.

Synonyms

• X chromosome

Related Terms

• Y-chromosome
• Sex chromosome
• sex determination
• gynandromorph
• autosome
• heterogametic sex
• homogametic sex
• XO system
• XY system

Usage Notes

In entomology, the X-chromosome is particularly notable because many insects use XO sex determination (males have one X, no Y) rather than the XY system found in mammals. The term is sometimes written without hyphen as 'X chromosome' in modern genetic literature, though the hyphenated form persists in older texts and some glossaries. Contrast with Y-chromosome, which is typically smaller, gene-poor, and male-limited in XY systems. Gynandromorphs—mosaic individuals with both male and female tissues—often result from mitotic errors involving X-chromosome segregation.