Question

Define genic balance theory of sex.

Answer 1

The theory of Bridges that the sex of a fruit fly is determined by the relative number of X chromosomes and autosomal sets.

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Answer 2

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 theory is basically applicable to Drosophila melanogaster over which Bridges worked. He found that the genic ratio X /А of 1.0 produces fertile females whether the flies have XX + 2A or XXX + ЗА chromosome complement. A genic ratio (X /А) of 0.5 forms a male fruitfully. This occurs in XY + 2A as well as X0 + 2A. It means that expression of maleness is not controlled by Y- chromosome but is instead localised on autosomes.

The X-chromosomes, however, carry female de­termining genes like Sxl. Bridges further proposed that a genic ratio of less than 0.5 (e.g., XY + ЗА or X/ЗА or 0.33) produced infertile meta-males (super males) while a genic ratio between 0.5 and 1.0 produces intersexes with a lot of morphological and sexual abnormali­ties.

Sterile meta-females (super females) are produced with the genic ratio of 1.5 (3X/2A). The sterile meta-males and meta-females have been called glamour boys and girls of fly world by Dodson.