1 Two new hair dyes A and B were tested in the Ames test. Both were tested with and without liver homogenate. Two auxotrophic strains of bacteria were used, one in which the auxotrophy was caused by a nonsense mutation (mutant 1) and the other by a frameshift mutation (mutant 2). After treatment the cells were plated on minimal medium. The + signs show experiments in which prototrophic colonies were observed. Explain the mutagenic action of these chemicals, and say which would probably be safe for human use.
2. In a tetraploid plant the A and B loci are centromere-linked and on separate chromosomes. The following cross is made:
A/A/A/A ; b/b/b/b X a/a/a/a ; B/B/B/B
And an F1 is obtained that is selfed to give an F2. If only one dominant allele is needed to give the dominant phenotypes at each locus, what phenotypes are expected in the F2 and in what proportions?
3.. In Drosophila, a cross is made involving two closely linked genes on the very small chromosome 4. a and b are recessive alleles.
Female a b+ / a+ b X male a b / a b
In the progeny, the expected common types were found, but there was also one rare wild type female.
(a) Explain what the common progeny phenotypes are expected to be, and give their proportions.
(b) Could the rare wild type female have arisen by crossing over? By nondisjunction? Explain your answers.
(c) The rare wild type female was test-crossed to a male tester of genotype a b / a b, and the progeny of the test cross were
1/6 wild type
1/6 expressed both recessives a and b