Disccus the genic balance mechanism of sex determination in Drosophila ?

Explanation

Genic balance mechanisms of sex determination in Drosophila was first studied by C. B. Bridges.
(ii) In Drosophila, the presence of Y chromosome is essential for the fertility of male sex, but does not determine the male sex.
(iii) The gene for femaleness is located on the X chromosome and those for maleness are located on the automomes.
(iv) When geneticist C. B. Bridges, working with Drosophila, crossed a triploid (3n) female with a normal male, he observed many combinations of autosomes and sex chromosomes in the offspring.
(v) He suggested that sex in Drosophila is determined by the balance between the genes for femaleness located on the 'X' chromosomes and those for maleness located on the 'autosomes'. Hence the sex of an individual is determined by the ratio of its X chromosome to that of its autosome sets. This ratio is termed sex index and is expressed as:
`"Sex index"="Number if X Chromosomes"/"Number of Sets of Autosomes"(X/A)`
(vi) Change in this ratio leads to a changed sex phenotype. The results obtained from a cross between triploid female Drosophila (3A:3X) with a diploid male (2A:XY) is shown in below.
(vii) Bridges classical cross of a triploid (3A+XXX) female fly and a diploid (2A+XY) male fly

A sex-switch gene in Drosophila directs female development. This gene, sex-lethal (SxL) located on the X chromosome, has two states of activity. When it is 'on' it directs female development and when it is 'off' maleness ensures. Other genes located on the X chromosome and autosomes regulate this sex-switch gene. However, the Y-chromosome of Drosophila is required for male fertility.