A virgin Drosophila female who
A virgin Drosophila female whose thorax bristles are very shortis mated with a males having normal (long) bristles. The F1 progenyare 1/3 short-bristle females, 1/3 long bristle females, and 1/3long bristle males. A cross of the F1 long bristle females withtheir brothers gives only long bristle F2 progeny. A cross of shortbristle females with their brothers gave 1/3 long bristle females,1/3 short bristle females, and 1/3 long bristle males. A) Explainwhat is going on genetically and B) why short bristled females cannot be homozygous.
Answer:
A cross of the short-bristled female with a normal male canproduce two phenotypes with regard to bristles and an abnormal sexratio of two females: one male. In addition, all males are normalwhile the females are normal and short in equal numbers. When thesexes differ with respect to phenotype among the progeny, anX-linked gene is implicated. The only normal phenotype is observedin males. Therefore, the short-bristled phenotype (in females) mustbe heterozygous, and the allele must be a recessive lethal. So, thefirst cross was A/a X a/Y.
When long-bristled females (a/a) were crossed with long-bristledmales (a/Y), all their progeny would be expected to belong-bristled (a/a or a/Y). Short-bristled females (A/a) werecrossed with long bristled males (a/Y). The progeny expectedare:
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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