9.3: (a) One mutation that might cause the constitutive phenotype is a mutation in an operator region of the enzyme-coding gene that makes it insensitive to repression. The second is a mutation that impairs the structure of the repressor. Such mutations can range from deletion of the repressor gene to subtler mutations that impair binding of the repressor to arginine, to the DNA, or to both. (b) A mutation in an arginine biosynthetic enzyme, not sufficient to cause a requirement for arginine but enough to reduce the amount of arginine in the cell, could activate a regulatory response by a normal regulatory system and induce constitutive synthesis.
9.6: The mutant gene should bind more of the activator protein, or have more efficient binding of the activator. Thus, the mutant gene should be induced with lower levels of activator protein or expressed at higher levels, compared to wild-type.
9.10: (a) The phenotype of cells with mutant Gal4p would be non-inducible; the mutant gene would be recessive because the wild-type Gal4p would still function normally. (b) The phenotype of cells with mutant Gal80p would be constitutive; the mutant gene would be recessive because wildtype Gal80p would bind Gal4p in the normal way.
9.18: (a): Yes, the repressor is functional, and the presence of lactose activates transcription of the lac genes; (b) and (d): Yes, at 42C, the repressor cannot bind the operator, which means that the lac operon is transcribed whether or not the inducer is present. (c): No, at this temperature, the repressor functions normally – because lactose is absent, the lac operon is in a repressed state.
14.1. The frequency of A1 equals 0.35.
14.2. The expected frequency of A2 in the next generation is the same as its frequency in the current generation: 0.65.
14.4. (A) no; (B) yes.
14.8. The frequency of homozygous recessives is q2 = 0.16; this implies that q=0.4. The frequency of the dominant allele is therefore p=0.6.
14.9. Two issues need to be considered. First, recessive alleles are maintained in heterozygous individuals and so are not exposed to selection. Second, new mutations in each generation replenish the number eliminated by selection in the homozygous recessives.
14.16. The numbers of alleles are as follows: A, 8+10+2 = 20; B, 10+48+20=78; C, 20+20+2 = 42. The total number of alleles is 140, so the allele frequencies are as follows: A, 20/140 = 0.14; B, 78/140 = 0.56; C, 42/140 = 0.30. The expected numbers among the 70 plants are:
AA: 1.42, AB: 11.14, BB: 21.72, BC: 23.40, CC: 6.30, AC: 6.00.