Question

Describe the genic balance theory of sex determination in Drosophila.

Answer 1

Hint: The genic balance theory of sex determination in Drosophila was given by Calvin Bridges in 1926. He states that instead of XY chromosomes, sex determined by the genic balance or the ratio between the X-chromosomes and autosome genomes.

Complete step by step answer:
The genic balance theory is basically applicable to Drosophila. Calvin found that genic ratio X/A of 1 produces fertile females whether the flies have XX+2A or XXX+3A chromosomes.
A genic ratio of 0.5 produces male fruit fly. This takes place in XY+2A as well as X0+2A which means that the expression of maleness is not controlled by Y-chromosomes but instead gets localised on autosomes.
The X-chromosomes will carry female determining genes. Bridges further proposed that genetic ratio of less than 0.5 will produce meta-males. On the other hand, genic ratio between 0.5 to 1.0 will produce intersexes with a lot of sexual abnormalities and morphological abnormalities.
Super females or sterile meta-females are produced with a genic ratio of 1.5. the super females and meta-females have been called girls of fly and glamour boys by Dodson

Note:
Theory given by Bridges which states that the sex of a Drosophila is determined by the relative number of X chromosomes and autosomal sets. Genic ratio of 1 produces fertile females whereas genic ratio of 0.5 produces male fruit fly. Genic ratio less than 0.5 produces meta-males and between 0.5 to 1.0 produce intersex. Super females are produced with 1.5 genic ratio.