The Ratio of X Chromosome to Autosomes

In humans, the X and Y chromosomes play an important role in sex determination. Human males have one X and one Y chromosome, while females have two X chromosomes.

However, this sex-determination system is not present in all animals. For example, in insects such as Drosophila, sex is determined by balancing the female determinants on X chromosomes and male determinants on the autosomes.

Here, the chief regulator of sex determination is a switch gene called Sex-lethal, present on the X chromosome. In females, the gene is activated during early embryogenesis, while in males, the gene remains inactive throughout the early embryonic stages.

The difference in the activity of Sex-lethal genes in male and female flies is explained by the difference in the ratio of X chromosomes to the autosomes.

Generally, diploid cells of a normal female Drosophila have two X chromosomes and two sets of autosomes, whereas diploid cells in a normal male Drosophila have one X chromosome and two sets of autosomes.

Each of these X chromosomes synthesizes two types of proteins, Sisterless-a and Sisterless-b, while each set of autosomes synthesize a protein called Deadpan.

Every Deadpan protein binds to one Sisterless protein and inhibits its activity. Since females have two X chromosomes and two sets of autosomes, the amount of Sisterless proteins is double the Deadpan proteins.

Two of the Sisterless proteins are inactivated by the Deadpan proteins, while the free Sisterless proteins act as transcription factors and activate the Sex-lethal gene. The Sex-lethal proteins then trigger the synthesis of the Transformer protein, which in turn activates the doublesex gene. Under these conditions, the doublesex gene produces a female-specific Doublesex protein, and the embryo develops into a female offspring.

In male Drosophila, since the ratio of the X chromosome to autosome sets is one to two, both Sisterless and Deadpan proteins are formed in equal amounts. Hence, all the Sisterless proteins are inactivated by Deadpan proteins. 

In the absence of any free Sisterless proteins, both the Sex-lethal and the transformer genes are not activated.

This results in the production of a male-specific Doublesex protein and the development of the embryo into a male offspring.