Mirabilis jalapa (Four O’Clock plant) Antirrhinum (snapdragon) and Snapdragon are well-known examples of incomplete dominance in genetics. Incomplete dominance occurs when neither allele is completely dominant over the other resulting in a blended phenotype in heterozygous offspring.
These plants have been widely studied in genetics due to their distinct flower color variations which demonstrate how dominant and recessive traits interact. Understanding incomplete dominance helps explain genetic inheritance plant breeding and even human traits.
This topic explores the genetic principles behind incomplete dominance using Mirabilis jalapa Antirrhinum and Snapdragon as prime examples.
1. What Is Incomplete Dominance?
A. Definition of Incomplete Dominance
Incomplete dominance is a type of genetic inheritance where:
- The dominant allele does not completely mask the recessive allele.
- The resulting phenotype is a blend of both parental traits.
- It is different from codominance where both alleles are fully expressed instead of blending.
B. How Incomplete Dominance Differs from Complete Dominance
| Feature | Incomplete Dominance | Complete Dominance |
|---|---|---|
| Offspring Phenotype | Blended (mix of both alleles) | Resembles one dominant parent |
| Example | Red × White flowers → Pink flowers | Red × White flowers → Only red flowers |
2. Examples of Incomplete Dominance in Plants
A. Mirabilis Jalapa (Four O’Clock Plant)
- Scientific Name: Mirabilis jalapa
- Genetic Example: When a red-flowered Mirabilis jalapa is crossed with a white-flowered Mirabilis jalapa the offspring produce pink flowers.
- Genotypic Ratio: 1 Red (RR) : 2 Pink (RW) : 1 White (WW)
- Why It Happens: The red pigment (R) is not strong enough to fully dominate the white (W) leading to an intermediate pink color.
B. Antirrhinum Majus (Snapdragon Plant)
- Scientific Name: Antirrhinum majus
- Genetic Example: Like Mirabilis jalapa when a red Antirrhinum is crossed with a white one the F1 generation produces pink flowers.
- Use in Plant Breeding: This concept is used in horticulture to create new flower color varieties.
C. Other Plants Showing Incomplete Dominance
- Carnations (Dianthus spp.) – Red and white flowers produce pink hybrids.
- Japanese Morning Glory (Ipomoea nil) – Color blending is observed in hybrid varieties.
3. The Genetic Basis of Incomplete Dominance
A. The Role of Alleles
- Each plant carries two alleles for flower color (one from each parent).
- In incomplete dominance the heterozygous allele pair results in a mixed phenotype instead of a fully dominant trait.
B. Punnett Square Representation
For a cross between a red-flowered (RR) and white-flowered (WW) plant the results are:
| Parent 1 (R) | Parent 2 (W) | Offspring Genotype | Offspring Phenotype |
|---|---|---|---|
| R | W | RW | Pink Flower |
| R | W | RW | Pink Flower |
| R | R | RR | Red Flower |
| W | W | WW | White Flower |
Genotypic Ratio: 1 RR : 2 RW : 1 WW
Phenotypic Ratio: 1 Red : 2 Pink : 1 White
4. Applications of Incomplete Dominance
A. Importance in Plant Breeding
- Incomplete dominance helps create new hybrid flower colors.
- Farmers and breeders use it to develop attractive ornamental plants.
B. Role in Genetic Studies
- Scientists use plants like Mirabilis jalapa to demonstrate non-Mendelian genetics.
- It provides insights into gene expression and mutation effects.
C. Incomplete Dominance in Humans
- Hair Type: A straight-haired parent crossed with a curly-haired parent often produces wavy-haired children.
- Skin Color: Mixing of dark and light skin tones leads to intermediate shades.
5. Differences Between Incomplete Dominance and Codominance
| Feature | Incomplete Dominance | Codominance |
|---|---|---|
| Phenotype | Blended (mixed) | Both traits fully expressed |
| Example | Red × White → Pink | Red × White → Red & White patches |
| Genetic Mechanism | Partial dominance of one allele | Equal expression of both alleles |
| Human Example | Wavy hair | Blood type AB (both A and B expressed) |
Mirabilis jalapa Antirrhinum and Snapdragon are classic examples of incomplete dominance in genetics. Their ability to produce blended flower colors provides a fundamental understanding of non-Mendelian inheritance.
By studying these plants scientists and breeders can create new plant varieties improve genetic research and better understand inheritance patterns in humans and animals.
