The Ratio of X Chromosome to Autosomes

In Drosophila, sex is determined by the ratio of X chromosomes to autosome sets, not by Y chromosome presence. Females have two X chromosomes and two autosome sets (2:2 ratio), while males have one X chromosome and two autosome sets (1:2 ratio). This ratio controls activation of the Sex-lethal gene, which triggers a cascade of proteins that ultimately determines sexual development and phenotype.

Sisterless proteins are numerator proteins encoded by X chromosomes, while Deadpan proteins are denominator proteins encoded by autosomes. Each Deadpan protein binds to and inactivates one Sisterless protein. In females, excess free Sisterless proteins act as transcription factors to activate Sex-lethal. In males, all Sisterless proteins are inactivated, preventing Sex-lethal activation and leading to male development.

Sex-lethal activation depends on the X:A ratio. In females with a 2:2 ratio, free Sisterless proteins accumulate and activate Sex-lethal during early embryogenesis. In males with a 1:2 ratio, all Sisterless proteins are bound by Deadpan proteins, leaving none free to activate Sex-lethal. This differential activation of Sex-lethal determines whether the embryo develops male or female characteristics.

A gynandromorph is an organism with both male and female body parts. It forms when individual cells make independent sex-determination choices based on their X:A ratio. For example, if an embryonic nucleus loses an X chromosome, its progeny cells become XO and display male traits, while remaining XX cells display female characteristics, creating a mosaic phenotype.

When Sex-lethal is activated in females, it produces Sex-lethal proteins that trigger synthesis of the Transformer protein. Transformer then activates the doublesex gene, which produces a female-specific Doublesex protein. This protein cascade directs embryonic development toward female characteristics. In males, without Sex-lethal activation, doublesex produces a male-specific protein instead.

Variations in the X:A ratio produce different sexual phenotypes. A ratio greater than 1:2 produces metafemales, while a ratio less than 1:2 produces metamales. These variations alter the balance of numerator and denominator proteins, changing Sex-lethal activation levels and resulting in intersex flies or other abnormal phenotypes. Each cell responds independently to its own X:A ratio.

The Y chromosome in Drosophila does not determine sex; instead, it contains genes for sperm production in adult males. Sex determination relies entirely on the X:A ratio and the Sex-lethal regulatory cascade. This contrasts with mammals, where the Y chromosome carries male-determining factors. The structure and development of masculine features in Drosophila depends on the X:A ratio, not Y chromosome presence.