3. A virgin Drosophila female whose thorax bristles are very short is mated with

Question

3. A virgin Drosophila female whose thorax bristles are very short is mated with a males having normal (long) bristles. The Fl progeny are 1/3 short-bristle females, 1/3 long bristle females, and 1/3 long bristle males. A cross of the FI 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. A) Explain what is going on genetically an why short bristled females can not be homozygous.

Explanation / Answer

a cross of the short-bristled female with a normal male results in two phenotypes with regard to bristles and an abnormal sex ratio of two females:one male. Furthermore, all the males are normal, while the females are normal and short in equal numbers. Whenever the sexes differ with respect to phenotype among the progeny, an X-linked gene is implicated. Because only the normal phenotype is observed in males, the short-bristled phenotype must be heterozygous, and the allele must be a recessive lethal. Thus, the first cross was A/a X a/Y.

Long-bristled females (a/a) were crossed with long-bristled males (a/Y). All their progeny would be expected to be long-bristled (a/a or a/Y). Short-bristled females (A/a) were crossed with long bristled males (a/Y). The progeny expected are

1/4 A/a   short-bristled females

1/4 a/a   long-bristled females

1/4 a/Y   long-bristled males

1/4 A/Y   nonviable

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