Question

Describe the genic balance theory of sex determination in Drosophila.

Answer 1

• The theory of genic balance given by Calvin Bridges (1926) states that instead of XY chromo­somes, sex is determined by the genic balance or ratio between X-chromosomes and autosome genomes.
  
• The sex determination in Drosophila is quite different from humans. Drosophila has eight chromosomes (n = 4), three pairs of autosomes and one pair of sex chromosomes. Even though Drosophila possesses XX and XY sex chromosomal organization, unlike human beings, the Y chromosome does not have any role in determining the sex of individuals. The sex in Drosophila is determined by the ratio of the number of X chromosomes to that of the number of sets of autosomes. In simpler terms, the sex determination is achieved by a balance of female determinants on the X chromosome (X) and male determinants on the autosomes (A). This type of sex determination is called genic balance system.