Q.Discuss the genic balance mechanism of sex determination with reference to Drosophila.

Step 1. State the experiment: C.B. Bridges crossed a triploid (3n) female Drosophila with a normal diploid male and examined the wide range of autosome/sex-chromosome combinations that appeared among the offspring (Table 4.2), including triploid females, triploid intersexes, super males and super females alongside ordinary diploid males and females.

Step 2. State Bridges' 1921 conclusion: fly sex is fixed by a balance between femaleness genes located on the X chromosome and maleness genes located on the autosomes — not by whether a Y chromosome is present, since the Y in Drosophila is needed only for male fertility, not for maleness itself.

Step 3. Give the quantitative rule (the sex index) and its thresholds (Table 4.3): $\text{Sex index} = X/A$; a ratio of 1.0 or higher gives female (true for diploid, triploid, tetraploid or haploid flies alike, since it is the ratio that matters, not the absolute count), a ratio of 0.50 or below gives male, exactly 0.67 gives a true intersex (a genuine mixture of male and female structures), and the extremes — 0.33 (metamale) and 1.5 (metafemale) — are viable but usually weak and sterile. …

Trace Bridges' triploid-cross experiment to the sex-index formula and its threshold va …

• Assuming Drosophila sex determination works like the human XY system, with the Y chromosome itself determining mal …