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Chapter 7: Problem 17

What is a Barr body, and where is it found in a cell?

Short Answer

Expert verified
Answer: A Barr body is an inactivated X chromosome present in the nucleus of most female mammalian cells, serving as a mechanism to balance the expression levels of X-linked genes between males and females. It is formed during early development and can be found within the nucleus, typically at the nuclear periphery.

Step by step solution

01

Definition of a Barr Body

A Barr body, also known as a sex chromatin, is an inactivated X chromosome present in the nucleus of most female mammalian cells. It represents a mechanism called X-chromosome inactivation used by female organisms to balance the expression levels of X-linked genes between males and females.
02

Formation of a Barr Body

In female mammals, one of the two X chromosomes is inactivated during early development to prevent the overexpression of X-linked genes. This inactivated X chromosome then condenses and becomes a transcriptionally silent structure called a Barr body. The process of X-chromosome inactivation is random, and each cell inactivates either the maternal or paternal X chromosome. As a result, females are considered "mosaic" because their cells may express genes from either the maternal or paternal X chromosome.
03

Location of a Barr Body in a Cell

In a cell, the Barr body is found within the nucleus, typically at the nuclear periphery. Being a condensed structure, it is tightly associated with the nuclear lamina and heterochromatin regions. This inactivated X chromosome remains in the Barr body state throughout the cell's life, and it is also replicated and passed to daughter cells during cell division.

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Most popular questions from this chapter

In the wasp Bracon hebetor, a form of parthenogenesis (the development of unfertilized eggs into progeny) resulting in haploid organisms is not uncommon. All haploids are males. When offspring arise from fertilization, females almost invariably result. P. W. Whiting has shown that an X-linked gene with nine multiple alleles \(\left(X_{a}, X_{b},\) etc.) controls sex determi- \right. nation. Any homozygous or hemizygous condition results in males, and any heterozygous condition results in females. If an \(X_{a} / X_{b}\) female mates with an \(X_{a}\) male and lays 50 percent fertilized and 50 percent unfertilized eggs, what proportion of male and female offspring will result?

Distinguish between the concepts of sexual differentiation and sex determination.

Describe how nondisjunction in human female gametes can give rise to Klinefelter and Turner syndrome offspring following fertilization by a normal male gamete.

An attached-X female fly, \(\overline{X X} Y\) (see the "Insights and Solutions" box \(),\) expresses the recessive X-linked white-eye mutation. It is crossed to a male fly that expresses the X-linked recessive miniature-wing mutation. Determine the outcome of this cross in terms of sex, eye color, and wing size of the offspring.

What type of evidence supports the conclusion that the primary sex ratio in humans is much higher than the secondary sex ratio?
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