Males and females across many animal phyla are genetically separate in that they inherit different sets of sex chromosomes. As known, human females inherit two copies of X chromosomes while males inherit one X chromosome and one Y chromosome. The Y chromosome has very few genes but does carry the male determining gene that is expressed later in development. So do females have more genes expressed perhaps conveying their ability shop more and talk more? Not exactly. This genetic imbalance between the sexes is why dosage compensation evolved and different animal phyla have different ways of dealing with this imbalance issue.
Organisms originally had all homologous autosomes; there were no
sex-linked chromosomes. Reptiles for example have a temperature
dependent sex determination system. This strategy is beneficial
because different reptile sexes have differential success in different
temperatures. However, the major drawback is that during times of such
cases of constant temperature the entire species risks extinction.
Dosage compensation is a regulatory process that alters gene expression along entire X chromosomes resulting in equivalent levels of X-linked gene products in males and females.
Drosophila doubles their rate of transcription on the on the single male X which is the exactly oppisite of the mammalian gene dosage solution.
In C. elegans, there are two sexes males which only have one X chromosome, and hermaphordites while have two X chromosomes. The worms compensate by decreasing their transcription by 50%.
Mechanisms of dosage compensation seem to have evolved repidly, perhaps driven by the progressive degeneration of one chromosome in the heterogamtic sex.
