Sex Determination, X-Inactivation, and Barr Bodies Video

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  • 0:01 How Gender Is Determined
  • 1:48 X-Inactivation
  • 3:25 Barr Bodies
  • 4:15 Lesson Summary
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Lesson Transcript
Instructor: Sarah Lawson

Sarah has taught nursing courses and has a master's degree in nursing education.

Sex determination, X-inactivation and Barr bodies are complex chromosomal processes. Learn more about each of these processes and test your knowledge with a quiz.

How Gender Is Determined

Pregnancy is such an exciting time for many couples. They anxiously await the arrival of their precious bundle of joy. This anticipation only heightens as mom's baby bump continues to grow. Family members and friends congratulate the couple and often ask, 'Is it a boy or a girl?'

Determining whether a baby will become a boy or a girl is all about the sex chromosomes the baby inherits from mom and dad. In this lesson, we will discuss how sex is determined, what X-inactivation means and what Barr bodies are.

Sex determination in humans occurs at the time of conception. It is the process of the development of sex differences between males and females. This leads to the development of different internal and external genitalia, breasts, and hair, and plays a role in gender identification.

Sex determination is a result of the sex chromosomes a person has. Normally, each person has two sex chromosomes; one is passed down from mom and the other from dad. A woman has two X chromosomes and a male has an X and a Y chromosome. When a pregnancy occurs, the baby always inherits an X from mom, but it is up to dad whether the second chromosome will be another X or a Y. If dad gives an X, the baby will be a girl. If he gives a Y, the baby will be a boy. So, in humans, sex determination is determined by dad.


X chromosomes are gene-rich. Females receive two X chromosomes, whereas males receive only one chromosome. Y chromosomes have few genes. If the X-linked genes were expressed equally in both sexes, females would produce approximately twice the level of X-linked gene products as would males. This difference would disturb metabolic balance in one sex or the other. X-inactivation is the process of bringing equivalence in the expression of X-linked genes in females and males.

X-inactivation, or lyonization, occurs when one X chromosome (either from mom or from dad) is randomly inactivated in an early embryonic cell. In the early blastocyst, this initial, imprinted X-inactivation is reversed in the cells of the inner cell mass that give rise to the embryo. In these cells, both X chromosomes become active again. Each of these cells then independently and randomly inactivates one copy of the X chromosome. This inactivation event is irreversible during the lifetime of the cell, so all the descendants of a cell that inactivated a particular X chromosome will also inactivate that same chromosome.

X-inactivation can also occur in males with Klinefelter syndrome who have more than one X chromosome.

Barr Bodies

A Barr body is the inactive X chromosome in a cell. The process of X-inactivation leaves one X chromosome inactive - this is the Barr body - in order to create equilibrium between males and females in the expression of X-linked genes. In normal cases, women have one Barr body in each cell; men have none.

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