Assignment 3

Five of the mutations in FlyLab are lethal when homozygous. When you select a lethal mutation from the Design view, the fly is made heterozygous for the mutant allele. If you select two lethal mutations that are on the same chromosome (same linkage group, or the "cis" arrangement), then the mutant alleles will be placed on different homologous chromosomes (the "trans" arrangement). Crosses involving lethal mutations will not show a deficit in the number of offspring. FlyLab removes the lethal genotypes from among the offspring and "rescales" the probabilities among the surviving genotypes. Hence, the total number of offspring will be the same as for crosses involving only nonlethal mutations. Perform the following crosses to demonstrate how Mendelian ratios can be modified by lethal mutations.


1 .		Design a cross between two flies with aristapedia mutations for antennae shape. Mate these flies. What phenotypic ratio did you observe in the F₁ generation? What were the phenotypes? Perform an F₁ cross between two flies with the aristapedia phenotype. What phenotypic ratio did you observe in the F₂ generation? How do these ratios and phenotypes explain that the aristapedia mutation functions as a lethal mutation?

2 .		To convince yourself that the aristapedia allele is lethal in a homozygote compared with a heterozygote, perform a cross between a wild-type fly and a fly with the aristapedia mutation. What results did you obtain with this cross?

3 .		Design a cross between two flies with curly wing shape and stubble bristles. Develop a hypothesis to predict the phenotypic ratio for the F₁ generation. Mate these flies. What phenotypic ratio did you observe in the F₁ generation? Test your hypothesis by Chi-square analysis. Repeat this procedure for an F₁ cross between two flies that express the curly wing and stubble bristle phenotypes. Are the phenotypic ratios that you observed in the F₂ generation consistent with what you would expect for a lethal mutation? Why or why not? Explain your answers.