The inheritance of flower color in Antirrhinum (snapdragon) is a classic example of incomplete dominance, a type of genetic inheritance where neither allele is completely dominant over the other. Instead of one allele fully masking the effects of the other, the heterozygous condition results in a blended phenotype.
In snapdragons, when a red-flowered (RR) plant is crossed with a white-flowered (WW) plant, the resulting offspring have pink flowers (RW). This intermediate color demonstrates that incomplete dominance leads to a mixture of parental traits rather than one trait overpowering the other.
This topic explores the genetics of flower color in Antirrhinum, how incomplete dominance works, why it differs from other inheritance patterns, and its significance in biology.
What Is Incomplete Dominance?
Definition of Incomplete Dominance
Incomplete dominance is a form of genetic inheritance where the heterozygous genotype produces a phenotype that is an intermediate between the two parental phenotypes. Unlike complete dominance, where one allele completely masks the other, both alleles contribute partially to the trait’s expression.
Key Features of Incomplete Dominance
- The dominant allele is not completely expressed in heterozygous individuals.
- The heterozygous phenotype is a blend of the two parental traits.
- The original parental traits can reappear in later generations when two heterozygous individuals cross.
Incomplete dominance is not limited to flowers; it occurs in various organisms, including animals and humans. However, Antirrhinum flowers are one of the most well-known examples of this phenomenon.
Genetics of Flower Colour in Antirrhinum
The Role of Alleles in Snapdragon Flower Colour
In snapdragons, flower color is controlled by a single gene with two alleles:
- R (Red allele) – Encodes the production of red pigment.
- W (White allele) – Produces no pigment, resulting in a white flower.
The inheritance pattern follows this scheme:
| Parental Genotype | Gametes Produced | Offspring Genotype | Offspring Phenotype |
|---|---|---|---|
| RR (Red) x WW (White) | R, W | RW | Pink |
| RW (Pink) x RW (Pink) | R, W, R, W | RR, RW, RW, WW | Red (25%), Pink (50%), White (25%) |
This 1:2:1 phenotypic ratio in the second generation (F2) is a hallmark of incomplete dominance.
How Incomplete Dominance Differs from Complete Dominance
- In complete dominance, one allele completely masks the other (e.g., Mendelian traits like pea plant color).
- In incomplete dominance, the heterozygous phenotype is a blend of both parental traits.
For example:
- Mendelian inheritance (Complete dominance): RR (Red) x WW (White) = RW (Red, since red dominates).
- Incomplete dominance (Snapdragons): RR (Red) x WW (White) = RW (Pink, a blend of both).
Difference Between Incomplete Dominance and Codominance
Incomplete dominance is often confused with codominance, but they are distinct:
- Incomplete dominance: The heterozygous phenotype is a blend of the two parental traits (Pink flowers in snapdragons).
- Codominance: Both alleles are fully expressed side by side in heterozygous individuals (e.g., AB blood type, where both A and B antigens are present).
Why Is Incomplete Dominance Important?
1. It Demonstrates Variability in Genetic Expression
Incomplete dominance proves that genetic inheritance is not always black and white. Instead, some genes exhibit gradual expression, leading to a variety of possible phenotypes.
2. It Helps in Plant Breeding
Understanding incomplete dominance can help scientists and farmers develop new plant varieties with intermediate traits, such as modified flower colors or improved fruit characteristics.
3. It Provides Insight into Genetic Disorders
Some human traits and genetic conditions, such as familial hypercholesterolemia (FH), exhibit incomplete dominance. In heterozygous individuals, the disorder is less severe than in homozygous individuals, proving that genetic conditions can vary in expression.
Examples of Incomplete Dominance Beyond Antirrhinum
While snapdragon flowers are one of the best-known examples, incomplete dominance also occurs in:
1. Mirabilis jalapa (Four O’Clock Plant)
Like snapdragons, Mirabilis jalapa also exhibits incomplete dominance. A cross between red-flowered and white-flowered plants results in pink offspring.
2. Andalusian Chickens (Feather Colour)
In Andalusian chickens, a cross between black-feathered chickens (BB) and white-feathered chickens (WW) results in blue-feathered (BW) offspring, an intermediate shade between the two.
3. Human Hair Texture
Hair texture in humans is an example of incomplete dominance.
- Straight hair (SS) x Curly hair (CC) = Wavy hair (SC).
- The heterozygous condition (SC) produces an intermediate wavy hair type, blending traits from both parents.
4. Coat Color in Horses
In some horse breeds, coat color is controlled by incomplete dominance. For instance, a chestnut (CC) horse crossed with a white (WW) horse produces a palomino (CW) horse, an intermediate golden color.
Genetic Variation and Evolutionary Significance
Incomplete dominance plays a crucial role in maintaining genetic diversity. Since heterozygous individuals display a different phenotype from both parents, they may survive better in certain environments, contributing to evolutionary adaptation.
Misconceptions About Incomplete Dominance
- Does incomplete dominance mean blending inheritance?
- No, the alleles remain separate at the genetic level. Only the phenotype appears blended, but the original parental traits can reappear in later generations.
- Is incomplete dominance rare?
- No, it is quite common in plants and animals. However, it is less frequently studied compared to Mendelian dominance.
- Do all flower colors follow incomplete dominance?
- No, some flower colors follow dominant-recessive inheritance or even codominance. The mode of inheritance depends on the specific genetic interactions involved.
The inheritance of flower color in Antirrhinum (snapdragon) is a perfect example of incomplete dominance, where the heterozygous condition results in a blended phenotype. Unlike complete dominance, where one allele completely masks the other, incomplete dominance creates a new intermediate trait—in this case, pink flowers from a cross between red and white-flowered plants.
Understanding incomplete dominance helps scientists better understand genetic inheritance, breed new plant varieties, and study human genetic traits. Beyond Antirrhinum, incomplete dominance is also seen in hair texture, feather color, and animal coat colors, proving that genetic traits are often more complex than simple dominant-recessive patterns.
By studying these inheritance patterns, geneticists continue to uncover the fascinating complexity of how traits are passed down through generations, shaping the diversity of life on Earth.