Incomplete Dominance in the F1 Generation: Predicting Phenotype
Incomplete dominance results in a blending of traits in the F1 generation, where the heterozygous offspring exhibit a phenotype intermediate between the two homozygous parents. Instead of one allele completely masking the other (as in complete dominance), both alleles contribute to the phenotype.
Predicting Phenotypes:
To predict the phenotypes in the F1 generation for a specific cross involving incomplete dominance, we need information about the parental phenotypes and the mode of inheritance for the trait.
Steps
- Identify Parental Phenotypes: Determine the phenotypes of the two homozygous parents (P generation). Let’s call them Phenotype A and Phenotype B.
- Intermediate Phenotype: The F1 generation will exhibit a phenotype that is a blend of Phenotype A and Phenotype B. This intermediate phenotype may appear as a mix of the traits, a dilution of one trait, or a completely new phenotype.
- Example: Imagine a flower with red petals (Phenotype A) is crossed with a flower with white petals (Phenotype B). If petal color exhibits incomplete dominance, the F1 offspring would have pink petals, a blend of red and white.
Examples of Incomplete Dominance:
- Flower color in four o’clock plants: A red flower crossed with a white flower produces pink flowers.
- Feather color in chickens: A black feathered chicken crossed with a white feathered chicken produces chickens with “blue” (gray) feathers.
- Snapdragon flower color: A red flower crossed with a white flower produces pink flowers.
Examples of Incomplete Dominance in Action
Example 1: Flower Color in Four O’Clock Plants
- Parental Phenotypes:
- Homozygous red flower (RR)
- Homozygous white flower (WW)
- F1 Generation:
- All offspring will be heterozygous (RW) and exhibit pink flowers, a blend of red and white.
Example 2: Feather Color in Chickens
- Parental Phenotypes:
- Black-feathered chicken (BB)
- White-feathered chicken (WW)
- F1 Generation:
- All offspring will be heterozygous (BW) and have “blue” (gray) feathers, an intermediate color between black and white.
Example 3: Snapdragon Flower Color (Multiple Alleles)
Snapdragon flower color involves multiple alleles, leading to a wider range of phenotypes:
- Alleles:
- C^R (red)
- C^W (white)
- Parental Phenotypes:
- Homozygous red flower (C^R C^R)
- Homozygous white flower (C^W C^W)
- F1 Generation:
- All offspring will be heterozygous (C^R C^W) and exhibit pink flowers, a blend of red and white.
- Additional Crosses:
- Crossing pink (C^R C^W) with pink (C^R C^W) would result in a 1:2:1 ratio of red (C^R C^R): pink (C^R C^W): white (C^W C^W) flowers in the offspring.
Punnett squares
Using Punnett squares can help visualize the genotypes and phenotypes in the F1 generation. For example, the Punnett square for the four o’clock plant example would look like this:
| R | R | |
| W | RW | RW |
| W | RW | RW |
As you can see, all offspring (RW) have the genotype for pink flowers, demonstrating the intermediate phenotype characteristic of incomplete dominance.