In radishes, the gene that controls color exhibits incomplete dominance. Pure-br

Question

In radishes, the gene that controls color exhibits incomplete dominance. Pure-breeding red radishes crossed with pure-breeding white radishes make purple radishes. What are the genotypic and phenotypic ratios when you cross a purple radish with a white radish?

Explanation / Answer

Okay, so incomplete dominance means that the heterozygote will exhibit a phenotype distinguishable from homozygotes. In this case, the heterozygote is purple. Now, your question can go two ways depending on if white is homozygous dominant or if red is homozygous dominant 1. assume white is homozygous dominant: RR (white) * Rr (purple) -> RR,RR,Rr, and Rr (so, 2 whites and 2 purples) geno ratio: 2:2, pheno ratio: 2:2 2. assume that red is homozygous dominant (so white is recessive) rr (white) * Rr -> Rr, Rr, rr, and rr (so, 2 whites, 2 purples) geno ratio: 2:2, pheno ratio: 2:2 as you can see, the phenotypes will both produce the same results if white is dominant or recessive, only the phenotypes differ. You'd need to know whether red or white is dominant. Usually in textbooks and in classes, the red tends to be dominant and white tends to be recessive (think about blending. red + white = purple - it doesn't make any sense for whites + purples = reds.) So, if you have to assume, go with white being homozygous recessive.

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