X chromosome

American  

noun

Genetics.

1. a sex chromosome of humans and most mammals that determines femaleness when paired with another X chromosome and that occurs singly in males.

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X-chromosome British  

noun

1. the sex chromosome that occurs in pairs in the diploid cells of the females of many animals, including humans, and as one of a pair with the Y-chromosome in those of males Compare Y-chromosome

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X-chromosome Scientific  

/ ĕks′krō′mə-sōm′ /

1. The sex chromosome that in female mammals is paired with another X-chromosome and in males is paired with a Y-chromosome. Very few genes on the X-chromosome have counterparts on the Y-chromosome, and since males have only one X-chromosome, any gene present on it (even if the gene is recessive in females) is expressed in males. In females, one of the two X-chromosomes in each cell is deactivated.

2. See more at sex chromosome See note at sex

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Etymology

Origin of X chromosome

First recorded in 1910–15

Example Sentences

The team analyzed modern human DNA preserved in Neanderthals, and found an abundance on the X chromosome -- the mirror opposite of humans.

From Barron's • Feb. 26, 2026

Genomic sequencing also revealed that D. catalonica has a haploid chromosome number of four autosomes plus one X chromosome, while D. tilosensis has six autosomes and one X chromosome.

From Science Daily • Nov. 2, 2025

Researchers suspected that the mutation causing the condition was somewhere on the X chromosome, because females, which have two X chromosomes, are less affected.

From The Wall Street Journal • Oct. 6, 2025

"These ginger and black patches form because, early in development, one X chromosome in each cell is randomly switched off," explains Prof Hiroyuki Sasaki, geneticist at Kyushu University.

From BBC • May 15, 2025

The X sex-chromosome is not in itself either female or male, since, as we have seen, either ovum or spermatozoon may contain a single X chromosome.

From Hormones and Heredity by Cunningham, J. T.