Sex chromosome

Definition

A chromosome that carries genes determining the sex of an individual and differs from autosomes in form, size, gene content, or segregation behavior during meiosis. Sex chromosome systems establish the genetic basis of sex determination, with the specific mechanism (heterogametic males XY, heterogametic females ZW, or other variants) varying among taxa. In diploid species, sex chromosomes may be homomorphic (similar in appearance) or heteromorphic (visibly distinct, as in human X and Y).

Etymology

From Latin 'sexus' (sex) + Greek 'khroma' (color) + 'soma' (body), reflecting early staining techniques that revealed chromosome structure.

Example

In the fruit fly Drosophila melanogaster, females are XX and males are XY, with sex determined by the ratio of X chromosomes to autosomes rather than the presence of Y. In contrast, many Hymenoptera exhibit haplodiploidy: males develop from unfertilized (haploid) eggs and lack sex chromosomes entirely, while females are diploid. Some arachnids, such as certain spiders, employ XO or complex multiple-X systems.

Synonyms

• allosome
• heterosome
• gonosome

Related Terms

• autosome
• sex determination
• Haplodiploidy
• heterogametic sex
• homogametic sex
• karyotype
• Meiosis
• dosage compensation
• Sexual dimorphism

Usage Notes

The term 'allosome' is sometimes preferred in cytogenetics to emphasize structural distinction from autosomes, though 'sex chromosome' remains dominant in genetics and evolutionary biology. Specialists distinguish the heterogametic sex (producing two types of gametes with respect to sex chromosomes) from the homogametic sex. Note that not all species possess distinct sex chromosomes—environmental sex determination (temperature-dependent in some reptiles) and haplodiploidy represent alternative systems common in arthropods. The abbreviation 'X' and 'Y' (or 'Z' and 'W') are capitalized when referring to specific chromosomes.