Incomplete dominance is a type of genetic inheritance where neither allele is completely dominant over the other. As a result, the heterozygous offspring displays an intermediate phenotype, which is a blend or mixture of the two parental traits.
In incomplete dominance, the phenotype of the heterozygous individual is somewhere between the phenotypes of the two homozygous parents, rather than exhibiting one of the parents’ traits completely.
Incomplete Dominance:
- Definition: In incomplete dominance, the dominant allele does not fully mask the effect of the recessive allele. The heterozygous genotype results in an intermediate or blended phenotype.
- Outcome: The phenotype is a blend of both alleles’ traits.
Example: Flower Color in Snapdragons
One of the most well-known examples of incomplete dominance is the inheritance of flower color in snapdragon plants (Antirrhinum majus). In this case, the gene for flower color has two alleles:
- R (Red flower color) – The allele for red flowers.
- W (White flower color) – The allele for white flowers.
In snapdragons, the allele for red color (R) is incompletely dominant to the allele for white color (W). When these two alleles are inherited together, the resulting offspring has a pink flower color, which is a mixture of the red and white colors.
Genotypic and Phenotypic Ratios in Incomplete Dominance
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Parental Generation:
- RR (homozygous red) × WW (homozygous white)
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F1 Generation:
- All F1 plants are RW (heterozygous), and they have pink flowers, an intermediate color.
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F2 Generation (F1 × F1 cross):
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The F2 generation consists of the following genotypes:
- RR: Red flowers (homozygous red)
- RW: Pink flowers (heterozygous)
- WW: White flowers (homozygous white)
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The phenotypic ratio in the F2 generation is 1 red : 2 pink : 1 white.
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Key Points of Incomplete Dominance:
- Neither allele is fully dominant over the other.
- The heterozygote displays an intermediate phenotype, which is a blending of the traits from both parents.
- Both alleles contribute equally to the phenotype in the heterozygous condition.
- The phenotypic ratio in a monohybrid cross involving incomplete dominance is typically 1:2:1 (one homozygous dominant, two heterozygous, and one homozygous recessive).
Other Examples of Incomplete Dominance:
- Color in four o’clock plants: The flower color in four o’clock plants (Mirabilis jalapa) follows a pattern of incomplete dominance. The alleles for red and white flowers produce pink flowers in the heterozygous condition.
- Color in horses: In some horse breeds, coat color exhibits incomplete dominance. For example, crossing a red horse (RR) with a white horse (WW) may produce a roan horse (RW), which has a mixture of red and white hairs, resulting in a unique coat color.