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Chapter 5: Problem 7

How do mammals, including humans, solve the "dosage problem" caused by the presence of an \(\mathrm{X}\) and \(\mathrm{Y}\) chromosome in one sex and two X chromosomes in the other sex?

Short Answer

Expert verified
Answer: Mammals, including humans, solve the dosage problem through a mechanism called X-chromosome inactivation. In females, one of the two X chromosomes in each cell is randomly inactivated, ensuring both sexes have equal gene dosage from the X chromosome. This process involves the expression of long non-coding RNA called Xist, which leads to histone modifications and epigenetic silencing of gene expression on the inactive X chromosome. Some genes on the inactivated X chromosome escape inactivation and are expressed from both X chromosomes in females to ensure dosage compensation with their homologous counterparts on the Y chromosome in males.

Step by step solution

01

Introduction to the dosage problem

The dosage problem refers to the challenge of maintaining a balanced gene expression between males and females despite the different numbers of X chromosomes. Males (XY) have one X chromosome and one Y chromosome, while females (XX) have two X chromosomes. If both X chromosomes in females were active, it would lead to double the gene dosage compared to males, which could be detrimental.
02

X-chromosome inactivation

Mammals, including humans, have evolved a mechanism called X-chromosome inactivation in females to solve the dosage problem. In this process, one of the two X chromosomes in each cell of females gets silenced or inactivated, ensuring that both sexes have equal gene dosage from the X chromosome.
03

Random inactivation of X chromosomes

X-chromosome inactivation is a random process that occurs early in development. In each cell, either the maternal or paternal X chromosome is inactivated, resulting in a mosaic pattern of gene expression in tissues. One X-chromosome remains active in every cell while the other is tightly packed and inactive. The choice of which X chromosome to inactivate is random and is maintained in the descendants of that cell. This process ensures dosage compensation between males and females.
04

Mechanism of X-chromosome inactivation

X-chromosome inactivation is mediated through the expression of a long non-coding RNA called Xist, which is only transcribed from the inactive X chromosome. Xist coats the inactive X chromosome, leading to histone modifications and epigenetic silencing of gene expression. The inactivated X chromosome becomes a condensed structure called a Barr body located at the nuclear periphery.
05

Exceptions to X-chromosome inactivation

Some genes on the inactivated X chromosome escape inactivation and are expressed from both X chromosomes in females. These genes are thought to permit dosage compensation with their homologous counterparts on the Y chromosome in males, ensuring that males and females have a similar level of expression of these particular genes. In conclusion, mammals, including humans, solve the dosage problem caused by different sex chromosomes in males and females through X-chromosome inactivation. This mechanism ensures that gene dosage from the X chromosome remains balanced between sexes, maintaining proper gene expression and avoiding any detrimental effects.

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Most popular questions from this chapter

When cows have twin calves of unlike sex (fraternal twins), the female twin is usually sterile and has masculinized reproductive organs. This calf is referred to as a freemartin. In cows, twins may share a common placenta and thus fetal circulation. Predict why a freemartin develops.

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

Distinguish between the Protenor and Lygaeus modes of sex determination.

Distinguish between the concepts of sex determination and sexual differentiation.

The genes encoding the red-and green-color-detecting proteins of the human eye are located next to one another on the X chromosome and probably evolved from a common ancestral pigment gene. The two proteins demonstrate 76 percent homology in their amino acid sequences. A normal-visioned woman with both genes on each of her two X chromosomes has a redcolor-blind son who was shown to have one copy of the greendetecting gene and no copies of the red-detecting gene. Devise an explanation for these observations at the chromosomal level (involving meiosis).
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