Dosage Compensation in Organisms with Heterogametic Males

Sex Determination, Sex Differentiation, Dosage Compensation and Genetic Imprinting
Chromosome Theory of Sex Determination 
Balance Theory of Sex Determination X/A ratio in Drosophila
Triploid intersexes in Drosophila and genie balance theory
X/A ratio and gynandromorphs in Drosophila
X/A ratio in Coenorhabditis elegans (a free living nematode)
Balance Between Male and Female Factors
- Diploid intersexes in gypsy moth (Lymantria)
- X/A ratio and multiple numerator elements (Drosophila and Coenorhabditis)
Sex Determination in Plants
Methods for determining heterogametic sex in plants
Sex determination in Coccinia and Melandrium
Sex determination in other dioecious plants
Sex Chromosomes in Mammals Including Humans (Homo sapiens)
TDF, ZFY and SRY genes in humans
H-Y antigen and male development in mammals
Single gene control of sex
Sex determination in Asparagus
Tassel seed (ts) and silkless (sk) genes in maize
Transformer gene (tra)in Drosophila
Haploid males in Hymenoptera
Hormonal control of sex
Environmental Sex Determination in Reptiles
Dosage Compensation in Organisms with Heterogametic Males
X-chromosome inactivation in mammals
Position effect variegation
Hyperactivity of X-chromosome in male Drosophila
Lack of Dosage Compensation in Organisms with Heterogametic Females
Genetic imprinting
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.