Sexually reproducing organisms use a variety of strategies to compensate for the gene dosage differences arising from different numbers of X-chromosomes present in males and females. Please describe how dosage compensation of X-chromosome is achieved in mice and Drosophila (fruit flies)?

A phenomenon by which organisms achieve equality in the expression of genes between the individuals of distinct biological sexes is termed as dosage compensation. In mice, dosage compensation can be achieved by the process of random inactivation of one X-chromosome in females. To attain the equivalent expression of the gene between the males and females that follow XX/XY sex pattern, random inactivation can eradicate the expression of one of the X-chromosome in females, which eventually results in the expression of only one X chromosome in both males and females.  

In the case of Drosophila, dosage compensation can be attained by a two-fold enhancement in the transcription of a single male X chromosome. This makes heterogametic male species with one X chromosome to equal the level of expression of the homogametic females comprising two active X-chromosomes.  

it provides an alternative energy pathway for the reaction to proceed.

a catalyst is a substance that speeds up the rate of a chemical reaction but is not consumed during the course of the reaction.

catalysts increase the rate of reaction without being used up. they do this by lowering the activation energy needed. with a catalyst, more collisions result in a reaction, so the rate of reaction increases. different reactions need different catalysts.

a catalyst works by providing an alternative reaction pathway to the reaction product. the rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst.