Genetics in Auxotrophic Mutation Strains
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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
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Solution Summary
The expert examines genetics in auxotrophic mutation strains. The tetraploid plants for centromere-linked and separate chromosomes are examined.
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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.
Basically, if the chemical causes a reversion mutation, then it is a mutagen. In the body, the liver will process many chemicals. Usually, the liver detoxifies materials. However, sometimes the liver will take an otherwise benign material and make it a mutagen.
Historically, the Ames test was found to be fairly decent, say about 80% effective in identifying mutagens. However, animal studies showed that there was that 20% error. It was determined that much of this is due to the fact that bacteria don't have livers, so liver homogenate (extract) is added. There are still some discrepancies between what has an effect in the Ames test and what has an effect on animals. While it's not known for sure what these differences could be, I might suggest something like histone proteins protecting the DNA of eukaryotes (or alternately, providing a nearby source of protein for something to react with).
Anyhow, the question:
For dye A, we see that it reverses a nonsense mutation after being transformed by the liver. That is no good for human use, generally. Since it's a hair dye though, you may be able to get away with it, provided none of the dye seeped into the bloodstream through the scalp. But since there would be a chance that people would transfer the dye from their hair to their hands, and then their hands to their mouths, it should be banned.
For dye B, we see that it reverses a frameshift mutation without the presence of liver extract, but is ...
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