Question

Below is a simplified version of the biochemical pathway responsible for fruit color in peppers. Suppose that gene A encodes enzyme 1, gene B encodes enzyme 2, gene C encodes enzyme 3, and that each gene only has two alleles: completely functioning (capital letters) and non functional (lower case letters). Note that red plus chlorophyll is brown, but yellow plus chlorophyll is green. A breeder crossed two pure lines of peppers and then intercrossed the resulting F1. The F2 was comprised of 351 plants with red peppers, 115 with yellow peppers and 154 with colorless (white) peppers. What must have been the genotypes and phenotypes of the two original pure lines

Answer 1

The original pure line peppers were probably AABB (red phenotype) and aabb (colorless phenotype). They were crossed to produce all AaBb (red phenotype) in the F1, which when intercrossed produce the F2 generation with red, yellow, and colorless phenotypes in roughly 9:3:4 ratio.

Step-by-step explanation:

This problem is related to Mendelian genetics and the interpretation of Punnett squares for dihybrid crosses. In brief, it seems that the trait for color in these pepper plants is governed by two genes, with three possible colors arising: red, colorless (white) and yellow. Given the F2 results, we're likely dealing with a crossing of two dihybrid heterozygotes (AaBb x AaBb).

Here's why: gene A is responsible for the transformation from colorless to yellow. If in lower case (aa), it will represent the colorless (white) phenotype. Gene B is responsible for the transformation from yellow to red. If in lower case (bb), it will represent the yellow phenotype. The dominant alleles (AA, BB) in concert, transform colorless to yellow, then yellow to red.

So, to get the ratios of red, yellow, and colorless peppers roughly in 9:3:4 (akin to Mendelian dihybrid cross ratios), the original pure line peppers were probably AABB (red phenotype) crossed with aabb (colorless phenotype). The resulting F1 generation would all be AaBb (red phenotype) due to dominance, which when intercrossed produce the F2 with red, yellow, and colorless phenotypes in the observed ratio.

Learn more about Mendelian genetics