Q.Define genetic balance.

Genetic balance (genic balance theory, proposed for Drosophila by Bridges) is the ratio between X-chromosome number and autosome sets that decides whether an individual develops as male or female.

In organisms such as Drosophila, sex is not decided simply by the presence of X or Y chromosomes but by the ratio of the number of X-chromosomes to the number of haploid autosome sets. A particular ratio value produces a female, another value produces a male, and intermediate/extreme ratios can produce intersexes, super-females or super-males. This dependence of sex on …

In Bridges' genic balance theory of sex determination in Drosophila, the sex index (X chromosomes / autosome sets) determines sex; an index of 1.0 corresponds to a normal female.

Calvin Bridges proposed the genic balance theory of sex determination in Drosophila melanogaster, based on the ratio of the number of X-chromosomes to the number of haploid sets of autosomes (A), called the sex index:

$\text{Sex index} = \dfrac{\text{Number of X-chromosomes}}{\text{Number of autosome sets (A)}}$

According to this theory:

• Sex index = 1.0 (e.g. 2X : 2A) -> normal female.
• Sex index = 0.5 (e.g. 1X : 2A) -> normal male.
• Sex index between 0.5 and 1.0 (e.g. 2X : 3A) -> intersex. …

With 2X chromosomes and 3 sets of autosomes, the X:A ratio is 0.67, which by Bridges' genic balance theory produces an intersex fly.

Calvin Bridges' genic balance theory states that sex in Drosophila depends on the ratio of the number of X chromosomes to the number of autosome sets (X:A ratio), not simply on the presence of a Y chromosome. A ratio of 1.0 (e.g. 2X:2A, normal diploid female) gives a normal female; a ratio of 0.5 (e.g. 1X:2A, normal diploid male) gives a normal male; a ratio greater than 1.0 gives a metafemale/superfemale (weak, often sterile); a ratio less than 0.5 gives a metamale/supermale (weak, often sterile); and a ratio between 0.5 and 1.0 (as here) gives an intersex individual with a mixture of male and female characteristics. Here the fly has …