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. A) Explain what is going on genetically and B) why short bristled females can not be homozygous.

Answer 1

A. Recessive cell lethal mutation is the condition, where homozygous recessive allele turns out to be lethal and eliminates the organism. Such a mutation can be attributed to P element mutagenesis in Drosophila.

B. In Drsosphila, P element codes for protein P transposase. These are common mutagenic agents in Drosophila. Wild type flies have express an inhibitor to the transposase activity of this protein. Since the disruption caused by P element to the genome is stopped, fertile progenies that are healthy can be born. However, in the absence of such an inhibitor, the P element gains access into the genome and to proliferate. in the process, the element disrupts many genes and kills the progeny. Hence, specifically a female Drosophila (homozygous recessive) may not be able to survive.