Dosage compensation
Two X chromosomes per cell are present in the mammalian females while in males it is in single dose per cell. Unlike the Y chromosome, the X chromosome contains thousands of genes that are essential for cell activity. Still, the intensity of phenotypes, the amount of enzymes produced, and the amount of RNA produced by the sex linked genes in both sexes is equal. This equalization is called dosage compensation. The intensity of eye colour in apr/apr females is same as in apr/Y males. Thr transcription rates of the X chromosomes are altered so that male and female cells transcribe the same amount of RNAs from their X chromosomes. In Drosophila, both X chromosomes in the female are active, but there is increased transcription from the male's X chromosome, so that the single X chromosome of male cells produce as much product as the two X chromosomes in female cells. This is accomplished by binding of particular transcription factors to hundreds of sites along the male X chromosome.
Two mechanisms may be involved 1) inactivation, by heterochromatization, of one of the two X chromosomes, 2) dosage compensation genes may proportionately reduce the transcription of both the X chromosomes in the homogametic sex. Thus, each mammalian somatic cell, whether male or female, has only one functioning X chromosome. The chromotin of the inactive chromosome is converted into heterochromatin chromatin that remains condensed through most of the cell cycle and replicates later than most of the other chromatin (the euchromatin) of the nucleus. The heterochromatin(inactive) X chromosome can often be seen on the nuclear envelope of female cells and is referred to as a Barr body. The presence of an extra segment of X chromosome lacking the apr locus reduces the intensity of eye colour in both the sexes. In Drosophila females, the partial suppression of the female X chromosomes is brought about by a set of dosage compensator genes present in the X chromosome. In mammals, dosage compensation appears to be due to the inactivation of one of the two X chromosomes in the female, generally clumped as a dense mass of heterochromatin near the nuclear membrane of the interphase nucleus; this heterochromatin is featureless and is known as sex chromatin or Barr body. However, all the X linked genes do not show dosage compensation, particularly in Drosophila.