Week of May 13th Biology Problem: Analyzing a Dihybrid Cross When One Trait Exhibits Incomplete Dominance

Students who study genetics are familiar with monohybrid crosses, that is, crosses between two parents of (usually) known genotype to produce offspring of various genotype and phenotype. Typically, the genotypic and phenotypic ratios are easily analyzed:

In this cross of pea plants, a yellow seed parent (heterozygous, Yy) is crossed with a homozygous recessive (green, yy) parent to produce offspring pea plants of known phenotypic and genotypic ratios, as shown in the Punnett square above. 

How about a more difficult problem? 

Predict the percentage of pink-flowered tall plants (F1 generation) that result from the cross between a red-flowered tall (heterozygous) plant and a white-flowered short plant. Assume tall is dominant to short.

Since the offspring (F1 generation) have pink flowers, we note red and white flower color exhibit incomplete

dominance to give pink-flowered offspring. These must have a heterozygous genotype with respect to flower color. Therefore, the cross is TtRR x ttWW. This dihybrid cross is shown below:

The tall pink-flowered plants, all of which have the same genotype (TtRW), are highlighted in red. Statistically, 50% of the F1 generation is predicted to be tall and pink based on the parents of the offspring. The rest of the offspring are pink-flowered and short (ttRW).

For more on dihybrid crosses, see http://www.youtube.com/watch?v=FJKHC6wX1Hk.

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