32. Dosage compensation in mammal females is achieved by
(1) Methylation of one X chromosome
(2) Hyper activation of one X chromosome
(3) Elimination of one X chromosome
(4) Hypoactivation of Both X chromosome

Dosage compensation in mammal females is primarily achieved by methylation and transcriptional silencing of one of the two X chromosomes, a process called X-chromosome inactivation. This equilibrates the gene expression between XX females and XY males.

Explanation of Options

1. Methylation of one X chromosome
   In female mammals, one X chromosome is randomly inactivated early in development through epigenetic modifications including DNA methylation and histone modifications. This silences most genes on that X chromosome, preventing overexpression of X-linked genes compared to males who have only one X chromosome.

2. Hyperactivation of one X chromosome
   This mechanism is typical of organisms like fruit flies (Drosophila), where the single male X chromosome is upregulated to match the output of two female X chromosomes. However, in mammals, this is not how dosage compensation is achieved.

3. Elimination of one X chromosome
   Elimination of an entire X chromosome does not happen in normal mammalian females; rather, the chromosome is retained but silenced. Deletion or loss would be lethal or cause severe genetic disorders.

4. Hypoactivation of both X chromosomes
   This happens in some organisms such as the nematode C. elegans, where both X chromosomes are downregulated. This is not the mechanism in mammals.

Correct answer: (1) Methylation of one X chromosome

This corresponds to X inactivation, where one X chromosome undergoes methylation and chromatin remodeling to become transcriptionally inactive, balancing gene dosage between males and females.

Introduction

Dosage compensation is a crucial biological process in mammal females to equalize the expression of X-linked genes between XX females and XY males. Unlike males, females have two X chromosomes, but only one is active for gene expression, achieved by X-chromosome inactivation involving methylation.

Detailed Explanation of Mechanisms

In mammalian females, dosage compensation occurs by methylation of one X chromosome to silence its gene expression. This epigenetic modification is essential to prevent an overdose of gene products from the two X chromosomes. The inactivated X forms a compact structure called the Barr body.

Other organisms use different dosage compensation strategies. For instance, fruit flies use hyperactivation of the male’s single X chromosome, while worms downregulate both X chromosomes in hermaphrodites. Mammals specifically use X inactivation for this purpose.

Summary of Options

• Methylation and inactivation of one X chromosome in females prevent excess X-linked gene expression.

• Hyperactivation is a male-specific mechanism in other species, not mammals.

• Elimination of an X chromosome is not viable in mammals.

• Hypoactivation of both X chromosomes occurs only in certain other species.

This explanation addresses the biology behind dosage compensation in mammals with a focus on the key mechanism — X-chromosome inactivation via methylation — making option (1) the correct answer.