Student Question

Dr. Baur crossed true breeding strains of Antirrhinum majus (Snapdragons). One strain had white, bilaterally symmetrical flowers (the normal form of the flower) and the other red, radially symmetrical (peloric) flowers. The F1 generation all had normal pink flowers. Crossing the F1 generation produced the following F2 generation of 234 individual plants: Normal pink = 94; Peloric pink = 28; Normal red = 39; Peloric red = 15; Normal white = 45; Peloric white =13. Use appropriate terminology to explain these results and draw a Punnett Square to compare the actual number of plants in the F2 generation with the predicted numbers. Briefly explain any differences.

Quick answer:

The results in this example, which left nearly twice the amount of pink flowers as white or red, happened due to incomplete dominance of color. This changes the phenotype ratio, which would create results that ultimately line up closely with this example. This is only a possible outcome calculated from probability, but differences are possible due to chance within genetics.

Expert Answers

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The first thing to do is not become intimidated by the technical terminology that this question is using. You have been given information that involves two traits, so what you are going to be doing is a dihybrid cross in the Punnet square. This will mean that you are working with four separate gametes, and sixteen boxes will be filled with genotypes in the Punnett square.

The information provided said that the initial plants were both "true breeding." This means that they are genotypically homozygous for their alleles, and you are told that they are different phenotypically. This means that one plant is homozygous dominant throughout (AABB), and the other plant is homozygous recessive throughout (aabb). The cross between these two parent plants will always result in plants that are heterozygous for both traits (AaBb). The phenotypes of the F1 generation let you know that something odd is happening with the dominance. White plants were crossed with red plants. In cases of regular dominance, one of those colors should have shown up in the F1 generation. Neither color showed up. Instead, a blend showed up, so the plants are showing incomplete dominance for color. That incomplete color dominance is going to change the phenotypic ratio in the F2.

Normally, using regular dominance, the phenotypic ratio in the F2 generation between two hybrid parents (AaBb x AaBb) would be a 9:3:3:1. Because of the incomplete dominance, you have to keep track of three flower colors rather than only two. The new phenotypic ratio with regular dominance for symmetry and incomplete dominance for color is 6:3:3:2:1:1. The data provided closely matches this predicted ratio. There should be double the number of pink normal flowers compared to white normal and red normal. It's not exactly doubled, but 94 is close enough to doubling 45 or 39. It is equally close with pink recessive flowers. Their numbers are just about a third of the pink normal.

The differences exist because a Punnett square only gives predicted outcomes. It is a tool that we use to calculate probabilities, and chance exists in the real world. Flipping a coin is a 50/50 chance heads or tails; however, it is possible to flip it 10 times in a row and land on tails every time. It isn't likely. Just like it isn't likely the coin will land on heads exactly five times and tails the other 5 times, but a 6 to 4 is close to a 50/50 split.

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