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 Sis