Incomplete Dominance Is When Neither Allele Is Completely Dominant
- Incomplete dominance and codominance are two distinct patterns that differ at the phenotypic level.
- In addition to the codominance seen in the ABO blood group system, there are other inheritance patterns where neither allele is completely dominant.
Incomplete Dominance: Heterozygotes Display Intermediate Traits
- In incomplete dominance, neither allele is entirely dominant.
- The heterozygous phenotype represents a blend or intermediate of the two parental traits.
Example
Pink Flowers in Mirabilis jalapa
- CR is the allele for red flowers, and CW is the allele for white flowers.
- Heterozygotes (CRCW) produce pink flowers because neither allele completely dominates.
- Predicting Flower Color
- If two pink-flowered plants (CRCW) are crossed:
- The offspring ratio is 1 red (CRCR), 2 pink (CRCW), and 1 white (CWCW).
- This demonstrates how incomplete dominance leads to a unique intermediate phenotype.
- If two pink-flowered plants (CRCW) are crossed:
| Gametes | $C^R$ | $C^W$ |
|---|---|---|
| $C^R$ | $C^RC^R$ (Red) | $C^RC^W$ (Pink) |
| $C^W$ | $C^RC^W$ (Pink) | $C^WC^W$ (White) |
Tip
When solving genetic problems, always define the alleles clearly. For example, use $C^R$ for red and $C^W$ for white to avoid confusion.
Codominance: Heterozygotes Display Both Traits
- In codominance, both alleles in a heterozygote are fully expressed, resulting in a dual phenotype.
- Unlike incomplete dominance, there is no blending of traits.
Example
Blood Type AB in Humans
- IA codes for A-type glycoprotein, and IB codes for B-type glycoprotein.
- Individuals with the genotype IAIB express both A and B glycoproteins equally, creating the AB blood type.
- This dual expression highlights how codominance differs from other inheritance patterns.
| Gametes | $I^A$ | $I^B$ |
|---|---|---|
| $i$ | $I^Ai$ (Blood Type A) | $I^Bi$ (Blood Type B) |
| $i$ | $I^Ai$ (Blood Type A) | $I^Bi$ (Blood Type B) |
Tip
- When working with codominance, remember that both alleles contribute equally to the phenotype.
- There is no dominant or recessive relationship between them.
Key Differences Between Incomplete Dominance and Codominance
| Feature | Incomplete Dominance | Codominance |
|---|---|---|
| Phenotype | Intermediate or blended | Both traits are visible |
| Expression | Partial expression of both alleles | Full expression of both alleles |
| Example | Pink flowers in Mirabilis jalapa | AB blood type in humans |
Tip
- In codominance (e.g., AB blood type), heterozygotes display a dual phenotype with both traits fully expressed.
- In incomplete dominance (e.g., pink flowers in Mirabilis jalapa), heterozygotes display an intermediate phenotype that blends the two parental traits.
Self review
Which form of dominance represents a heterozygous phenotype that represents a blend of two parental traits?
