Question

A virgin Drosophila female whose thorax bristles are very short is mated with a males having normal (long) bristles. The F1 progeny are 1/3 short-bristle females, 1/3 long bristle females, and 1/3 long bristle males. A cross of the F1 long bristle females with their brothers gives only long bristle F2 progeny. A cross of short bristle females with their brothers gave 1/3 long bristle females, 1/3 short bristle females, and 1/3 long bristle males. Explain what is going on genetically.

Answer 1

A virgin Drosophila female whose thorax bristles are very short is mated with a males having normal (long) bristles.The F1 progeny are 1/3 short-bristle females, 1/3 long bristle females, and 1/3 long bristle males. A cross of the F1 long bristle females with their brothers gives only long bristle F2 progeny.

The first cross was short bristled females with a normal male resulted in an abnormal ratio of 2 females :1 male. Again the ratio of females shows short and long bristled ones are equal in number.

This shows an X-linked /sex liked dominant gene played a role resulting in unequal ratios of males to females. All males showed normal phenotype and females showed 50% normal and 50% banormal phenotype. So the parental female might be heterozygous (A/a) in nature and allele might be lethal in males if they have A/y. Where as in parental males, the genotype might be ay. A cross in between these two.............A/a x a/y

	A	a
a	Aa (short bristled females)	aa (long bristles in females)
y	Ay (lethal if short bristles appear in males)	ay(long bristles in males)

1/4 of Aa.............short bristled females

1/4 of aa.............long bristled females

1/4 of Ay.............short bristled males (die immediately)

1/4 of ay.............long bristled males

A cross of short bristle females with their brothers gave 1/3 long bristle females, 1/3 short bristle females, and 1/3 long bristle males.

When long bristled female A/a of F1 progeny crossed with their brothers of long bristled males a/y, their progeny expected to be long bristled a/a or a/y..............

	a	a
a	aa (long bristled females)	aa(long bristled females)
y	ay(long bristled males)	ay(long bristled males)

The ratio of males to females is 1:1 and 100% long bristled.