Genic Balance in Drosophila

Human and mammalian sex determination is dominated by a single master switch (SRY), but Drosophila proved that sex determination does not have to work that way. C.B. Bridges' classic experiment crossed a triploid (3n) female fly against a normal diploid male and examined the resulting mix of autosome and sex-chromosome combinations in the offspring (Table 4.2). From those results, Bridges proposed in 1921 that fly sex is set not by the presence of a Y chromosome (which in Drosophila is needed only for male fertility, not for maleness itself), but by a balance: femaleness genes located on the X chromosome are weighed against maleness genes located on the autosomes, and the ratio between the two decides the outcome.

That ratio — number of X chromosomes divided by number of autosome sets — is called the sex index, $\text{Sex index} = \dfrac{X}{A}$. Table 4.3 works through every dose combination Bridges observed: a normal diploid fly (2X : 2A) gives a sex index of $2/2 = 1.0$ and is a normal female; a normal male (1X : 2A) gives $1/2 = 0.50$; any fly at a sex index of $1.0$ or higher (diploid, triploid, tetraploid, haploid — the ratio, not the absolute chromosome count, is what matters) develops as female, and any fly at $0.50$ or below develops as male. Between those two clean outcomes sits a distinctive middle zone: at a sex index of exactly $0.67$ (2X : 3A) the fly is an intersex, showing a genuine mixture of male and female structures rather than being ambiguous between them. Push the ratio to the extremes and viability itself starts to fail — metamales at $X/A = 0.33$ and metafemales at $X/A = 1.5$ both develop but are usually very weak and sterile. …