GENIC BALANCE THEORY

Sex Determination in Drosophila

Calvin Bridges · X:A ratio

Bridges' genic balance theory explains that sex in Drosophila is determined by the balance between female-determining genes on the X chromosomes and male-determining genes on the autosomal sets, expressed as the X:A ratio.

Key idea: Sex is not set by "XX vs XY", but by the X/A ratio, where A represents a haploid set of autosomes. A ratio of 1.0 produces females, 0.5 produces males, intermediate ratios give intersex individuals, and extreme ratios yield metafemales or metamales.

Core concept X:A balance

  • The X chromosome carries female-determining genes, whereas autosomal sets contribute male-determining genes; sex is a result of their relative dosage.
  • The X:A ratio is defined as the number of X chromosomes divided by the number of haploid sets of autosomes (A), and this ratio, not Y presence, directs primary sexual fate.

Major X:A ratios

Normal female X/A = 1.0

Constitutions such as XX:2A (2 X chromosomes, 2 autosomal sets) or XXX:3A develop into normal, typically fertile females.

Normal male X/A = 0.5

Constitutions like XY:2A or XO:2A develop as males; Y is not required for maleness itself.

Intersex 0.5 < X/A < 1

Intermediate ratios (for example XX:3A, XXX:4A) yield flies with mixed male–female characteristics.

Meta types >1 or <0.5

High ratios (≈1.5) produce weak metafemales, while very low ratios generate abnormal metamales.

Quick note on Y fertility

  • XO and XY individuals with the same X:A ratio both develop as males, confirming that Y does not control primary sex determination.
  • However, the Y chromosome contains genes necessary for normal spermatogenesis, so XO males are typically sterile whereas XY males are fertile.