Dosage Compensation in Organisms with Heterogametic Males

Earlier in this section, it has been shown that in majority of unisexual (dioecious) organisms, the sex is determined by chromosome constitution, although a number of individual genes also influence the sex of an individual. This led to the establishment of the concept of a heterogametic sex (XY) and a homogametic sex (XX). As shown earlier, the homogametic sex has two X-chromosomes, while heterogametic sex has only one X and a Y chromosome. In most species (animals or plants), homogametic sex (XX) is female and heterogametic sex (XY) is male. The exceptions to this rule are the lepidopterans and the birds,
where male is homogametic (ZZ) and female is heterogametic (ZW). It is thus obvious that in the homogametic sex, there will be two X or two Z-chromosomes carrying two sets of identical genes. In the heterogametic sex, there will be only one set of these genes. If both sets of genes are expressed in the homogametic sex, there will be twice as much X-coded gene products in the homogametic sex, as in the heterogametic sex. This would be abnormal situation. Therefore, a mechanism had to be evolved to bring about parity in the quantity of X-coded gene products in the two sexes. This phenomenon of bringing about equality in products synthesized under the control of genes carried on X-chromosomes,
was termed dosage compensation (Muller, 1932). This compensation in dosage of genes is achieved either by hypoproduction due to inactivation of one X-chromosome in homogametic sex, as observed in mammals, or by the hyperproduction due to hyperactivity of X-chromosome in the heterogametic sex as observed in Drosophila. These two entirely opposite mechanisms of dosage compensation in mammals and Drosophila will be discussed separately.