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Epistasis or Linkage?

Genetics as it applies to evolution, molecular biology, and medical aspects.

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Epistasis or Linkage?

Postby hartfan » Fri Jun 24, 2011 1:48 pm

Hi everyone, there is a multiple choice question which I cannot figure out the logic of the official answer. Thank you for your help.

Pure breeding tomato plants with green leaves and smooth fruit were crossed with pure breeding tomatoes with mottled leaves and rough fruit. The first generation (F1) all had green leaves and smooth fruit.

The F1 were test crossed with tomato plants with mottled leaves and rough fruit.

The phenotypes of 400 offspring were recorded. The results are shown.

green leaves and smooth fruit: 172
green leaves and rough fruit: 30
mottled leaves and smooth fruit: 34
mottled leaves and rough fruit: 164

Which of the following explains these results?

A. green allele for leaf color is dominant, rough allele for fruit texture is hypostatic, loci of genes are on different chromosomes
B. green allele for leaf color is epistatic, smooth allele for fruit texture is recessive, loci of genes are on different chromosomes
C. mottled allele for leaf color is hypostatic, smooth allele for fruit texture is dominant, loci of genes are on the same chromosome
D. mottled allele for leaf color is recessive, rough allele for fruit texture is recessive, loci of genes are on the same chromosome


The official answer is B, but the way I see it, the answer should be D. The loci should be on the same chromosome with some crossing over, that's why the parental phenotypes appear similar in ratio with a test cross. I cannot see why there should be epistasis at all; that would imply at least a third gene is involved. Please help. Thank you.
hartfan
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