Question

20. (6 points) You isolated a mutation on the Drosophila X chromosome with a recessivo white eye phenotype. You want to know whether it is mutant allele of the 'white' gene, which you have in your stock collection, or a mutation in another gene that happens to have the same phenotype. How would you figure it out? Show the cross you would perform, indicating the possible genotypes, phenotypes, and interpretations.

Answer 1

Fruit flies also have XX females and XY males. However, the mechanism of sex determination in Drosophila differs from that in mammals.

The wild type eye colour of drosophilla is dull red,but pure lines with white eyes are available. This phenotypic difference is determined by two allels of a gene located on differential regions of x chromosome.

When white eyed males crossed with red eyed females , Also the F1 progeny have red eyes, showing that the allele for white is recessive. Crossing the red eyed F1 males and females produces 3:1 F2 ratio of red eyed to white eyed flies, but all the white eyed flies are males. The genotypes are showed in following image.

117 First cross Second cross P Red White 8 White Red 8 xx gametes gametes gametes gametes Red Red 8 Red White & gametes & gametes w+ gametes Red O Red 8 gametes Red Red 8 Red White 8 White White

if we draw this in punnet square,

bwi bw? bw bwt F, from original cross Parental homozygous recessive gametes bw bw? bw bwt bw7 bw bw1

The reciprocal cross gives a different result.A reciprocal cross between white eyed females and red eyed males gives an F1 in which all the females are red eyed , but all the males are white eyed. The F2 consists of one half red eyed and one half white eyed flies of both sexes.

Sex-linked inheritance regularly shows different phenotypic ratios in the two sexes of progeny, as well as different ratios in reciprocal crosses.