As your text notes, in XX females the dosage of X-linked genes is twice what it is in XY males. However, the level of expression of X-linked genes is about the same in both males and females. In mammals this dosage compensation is the result of X-inactivation in females. a) trueb) false

a) true

Explanation:

It is absolutely right that in mammals dosage compensation occurs due to X-inactivation in females. It happens because in mammals like humans, females have XX and males have XY as allosomes. Because of the presence of two X chromosomes, each gene is present in 2 copies in females but in males only 1 copy is present as they have only one X chromosome. So in order to compensate for this kind of difference, one of the female X chromosomes undergoes inactivation and this phenomenon is known as dosage compensation.

The X chromosome undergoes inactivation because of its DNA modification and acts very different from other X chromosomes. Several methyl groups are added to the DNA of such X chromosome so as to inactivate it. In general such inactivated X chromosome is known as bar body which appears like a darkly stained structure.

Note: In other lower organisms like drosophila and C. elegans other mechanisms of dosage compensation exist.

In drosophila rather than inactivation of one chromosome in female, the transcription rate of one X chromosome of male is doubled so that it could produce as much transcripts as two X chromosomes of female are producing.

In C. elegans, both the X chromosomes are active but partially. The transcripts produced by two X chromosomes of female equalize the transcripts produced by one X chromosome of hermaphrodite counter part.