Barr body

genetics

Also known as: sex chromatin

Written by

The Information Architects of Encyclopaedia Britannica

Last updated

June 22, 2026

Quick Summary

A Barr body is an inactivated, condensed X chromosome found in the somatic cells of most female mammals. It serves as a marker for genetic femaleness. In females, one of the two X chromosomes in each somatic cell undergoes a process called X inactivation, typically during early development. This process condenses one X chromosome into a Barr body, which is visible at the edge of the cell nucleus. X inactivation ensures that females do not express twice the amount of gene products from the X chromosome compared to males, who have only one X chromosome. This phenomenon is also known as "lyonization," named after British geneticist Mary Lyon.12

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

3 Britannica Sources

Ask Britannica AI about this topic

Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.

sex chromosome
…as a small, dark-staining structure—the Barr body—in the cell nucleus.
Read More
…as a small, dark-staining structure—the Barr body—in the cell nucleus.
Read More

human genetic disease
Abnormalities of the sex chromosomes
…physically condenses to form a Barr body, a small structure found at the rim of the nucleus in female somatic cells between divisions (see photograph). The discovery of X inactivation is generally attributed to British geneticist Mary Lyon, and it is therefore often called “lyonization.”
Read More
…physically condenses to form a Barr body, a small structure found at the rim of the nucleus in female somatic cells between divisions (see photograph). The discovery of X inactivation is generally attributed to British geneticist Mary Lyon, and it is therefore often called “lyonization.”
Read More