. . Genic balance in Drosophila Genic balance mechanisms of sex determination in Drosophila was first studied by C.B. Bridges.
In Drosophila , the presence of Y chromosome is essential for the fertility of male sex, but does not determine the male sex. The gene for femaleness is located on the X chromosome and those for maleness are located on the autosomes. 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.
From his results Bridges in 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: Number of X Chromosomes Number of Sets of Autosomes Sex index = X A 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 tables .
. and . . XII Std Zoology Chapter XII Std Zoology Chapter Principles of Inheritance and Variation Table: .
Bridges classical cross of a triploid (3A+XXX) female fly and a diploid (2A+XY) male fly Triploid Diploid Parent 3A + XXX 2A + XY Gametes (2A + XX) (A + X) (A + X) (A + Y) (2A + X) (A + XX) A+X A + Y 2A+XX 3A + XXX Triploid Female 3A + XXY Triploid Intersex 2A+X 3A + XX Triploid Intersex 3A + XY Super Male A+XX 2A + XXX Super female 2A + XXY Diploid Female A+X 2A + XX Diploid Female 2A + XY Diploid Male • X-Chromosome was discovered by Henking ( ) • Y-Chromosome was discovered by Stevens ( ) When the X : A ratio is . as in a normal female, or greater than . , the organism is a female. When this ratio is .
as in a normal male or less than . the organism is a male. At . , the organism is an intersex.
metamales Phenotype Number of ‘X’ Chromosomes (X) Number of Autosome sets (A) Number of X chromosome Sex Index = Number of autosome sets Meta female / Super female / = . Normal Female Tetraploid / = . Triploid / = . Diploid / = .
Haploid / = . Inter sex / = . Normal male