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Chapter 5: Problem 10

With regard to the numbers of sex chromosomes, explain why dosage compensation is necessary.

Short Answer

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Dosage compensation is necessary due to the differing numbers of sex chromosomes in males and females. Females have two X chromosomes, whereas males have one X and one Y chromosome. Without dosage compensation, females would produce twice as much of any given X-linked protein than males, which could potentially lead to health issues. Dosage compensation in humans is largely achieved by X-inactivation, ensuring that females, like males, have one functional copy of the X chromosome in each cell.

Step by step solution

01

Understanding Sex Chromosomes

Sex chromosomes determine an individual's sex - females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). Hence, genes that are located on these chromosomes are called sex-linked genes.
02

The Requirement for Dosage Compensation

Dosage compensation is the mechanism by which the differences in the number of copies of sex chromosomes are normalized, to achieve a balance in gene expression. Without dosage compensation, females, having two X chromosomes, would express twice as many proteins - a condition that could lead to overproduction of certain proteins and challenge cellular function.
03

Mechanism of Dosage Compensation

In humans and other mammals, dosage compensation is achieved by inactivating one of the two X chromosomes in the cells of females randomly. This process, called X inactivation or lyonization, ensures that females, like males, have one functional copy of the X chromosome in every cell.

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Most popular questions from this chapter

On rare occasions, people are born with a condition known as uniparental disomy. It happens when an individual inherits both copies of a chromosome from one parent and no copies from the other parent. This occurs when two abnormal gametes happen to complement each other to produce a diploid zygote. For example, an abnormal sperm that lacks chromosome 15 could fertilize an egg that contains two copies of chromosome 15. In this situation, the individual has maternal uniparental disomy 15 because both copies of chromosome 15 were inherited from the mother. Alternatively, an abnormal sperm with two copies of chromosome 15 could fertilize an egg with no copies. This is known as paternal uniparental disomy 15 . If a female is born with paternal uniparental disomy 15, would you expect her to be phenotypically normal, have Angelman syndrome (AS), or have Prader-Willi syndrome (PWS)? Explain. Would you expect her to produce normal offspring or offspring affected with AS or PWS?

A black female cat \(\left(\mathrm{X}^{B} \mathrm{X}^{B}\right)\) and an orange male cat \(\left(\mathrm{X}^{0} \mathrm{Y}\right)\) were mated to each other and produced a male cat that was calico. Which sex chromosomes did this male offspring inherit from its mother and father? Remember that the presence of the \(Y\) chromosome determines maleness in mammals.

When does the erasure and reestablishment phase of genomic imprinting occur? Explain why it is necessary to erase an imprint and then reestablish it in order to always maintain imprinting from the same sex of parent.

Define the term epigenetic inheritance, and describe two examples.

What is a Barr body? How is its structure different from that of other chromosomes in the cell? How does the structure of a Barr body affect the level of \(\mathrm{X}\)-linked gene expression?
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