Problem
Oil accumulates in corn kernels due to the production of triacylglycerol (Fig 3). In the absence of triacylgycerol kernels are starchy. You find a starchy mutant that differs from WT only by a homozygous recessive inactivating mutation in GPAT, and a second starchy mutant that differs from WT only by a homozgyous recessive inactivating mutations in PPH-1. Both mutants are homoygous at all loci and the loci are on separate chromosomes.
A. Will these loci exhibit epistasis? Why or why not?
B. You cross the two mutants and self-pollinate the F1. What proportion of the F2 offspring will be starchy?
C. I take a starchy plant from the F2. I cross it to the original GPAT mutant and all of the progeny are oily. I cross it to the original PPH-1 mutant and all of the progeny are starchy. What's the genotype of the starchy F2?
D. You isolate a third recessive mutant in the DGAT gene. After many generations of selfing, always observing starchy kernels, in generation 15 you suddenly observe segregation of 3:oily to 1:starchy in the selfed progeny. By crossing to the original homozygous recessive as a tester, you further discover that 1/3 of the new plants with oily kernels are homozygous. You sequence the DGAT gene in one of these homozygous oily plants and discover a 3bp deletion in the exon of the gene compared to the normal gene sequence. Explain what kind of mutation generated these oily plants, how it results in an oily phenotype, and its mode of action (recessive, codominant, etc.).