28. In mammals effective dosage of genes of two sexes is made equal by-
(1) Elimination of X chromosome in females
(2) Hyper activation of one X chromosome in males in males
(3) Hypoactivation of both X chromosomes female
(4) Inactivation of one of the X chromosome in females

Introduction:
In mammals, males and females have different numbers of sex chromosomes, with females having two X chromosomes and males having one X and one Y chromosome. This difference could lead to an imbalance in gene expression between sexes because females might express twice as many X-linked genes as males. To prevent this, mammals use a process called dosage compensation. The primary mechanism in mammals is X chromosome inactivation (XCI), where one of the X chromosomes in females is inactivated to equalize gene dosage between males and females.

Explanation of Options:

1. Elimination of X chromosome in females
   This option suggests that females completely lose one X chromosome, which is not accurate. Females retain both X chromosomes but silence one of them. Therefore, elimination does not happen, making this option incorrect.

2. Hyperactivation of one X chromosome in males
   In some organisms like fruit flies (Drosophila melanogaster), males hyperactivate their single X chromosome to balance gene expression. However, in mammals, this is not the primary dosage compensation method. Hence, this option is incorrect for mammals.

3. Hypoactivation of both X chromosomes in females
   This option implies both X chromosomes in females reduce their activity, which is not observed in mammals. Instead, one X chromosome is fully inactivated. So this option is incorrect.

4. Inactivation of one of the X chromosomes in females
   This option correctly describes the mammalian dosage compensation mechanism. One of the two X chromosomes in female mammals becomes transcriptionally inactive through a process known as X chromosome inactivation or Lyonization. This silencing occurs early in embryonic development and equalizes the effective dosage of X-linked genes between males and females.

Detailed Explanation:
X chromosome inactivation (XCI) occurs randomly in early female embryonic cells, meaning either the maternal or paternal X chromosome is silenced in each cell, resulting in a mosaic expression pattern in females. This inactivation is achieved by extensive epigenetic modifications including DNA methylation, histone modifications, and the action of the long non-coding RNA XIST, which coats the inactive X chromosome and facilitates its silencing.

This process ensures that female mammals have gene expression levels from X-linked genes comparable to males, who have only one X chromosome. Without XCI, females would produce double the amount of proteins from X-linked genes, which could be detrimental to development and cellular function.

In summary, the effective dosage of genes between sexes in mammals is balanced by inactivating one of the two X chromosomes in females, making option (4) the correct answer.

References:

• X chromosome inactivation is the main mechanism of dosage compensation in mammals.​

• The inactivation process is random and mediated by epigenetic factors including the XIST gene.​

• Other options do not apply to mammals due to species-specific dosage compensation mechanisms.​

This explanation provides a clear understanding of dosage compensation in mammals while aligning with SEO best practices using relevant keywords.