Sex Determination in Drosophila

The fruit fly Drosophila melanogaster, has eight chromosomes: three pairs of autosomes and one pair of sex chromosomes ( FIGURE 4.9). Normally, females have two X chromosomes and males have an X chromosome and a Y chromosome. However, the presence of the Y chromosome does not determine maleness in Drosophila; instead, each fly's sex is determined by a balance between genes on the autosomes and genes on the X chromosome. This type of sex determination is called the genic balance system. In this system, a number of genes seem to influence sexual development. The X chromosome contains genes with female-producing effects, whereas the autosomes contain genes with male-producing effects. Consequently, a fly's sex is determined by the X:A ratio, the number of X chromosomes divided by the number of haploid sets of autosomal chromosomes.

ffl A larva that settles on an unoccupied substrate develops into a female, which produces chemicals that attract other larvae.

ffl A larva that settles on an unoccupied substrate develops into a female, which produces chemicals that attract other larvae.

The larvae attracted by the female settle on top of her and develop into males, which become mates for the original female.

^ Eventually the males on top switch sex, developing into females

They then attract additional larvae, which settle on top of the stack and develop into males.

I 4.8 In Crepidula fornicata, the common slipper limpet, sex is determined by an environmental factor, the limpet's position in a stack of limpets.

II III II III

Jj^ IV Autosomes IV

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