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.
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 genotypeIAIBexpress 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?
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Note
Introduction to Non-Mendelian Inheritance
Non-Mendelian inheritance refers to genetic patterns that don't follow Mendel's classic dominant-recessive rules.
Two important types are incomplete dominance and codominance.
AnalogyThink of non-Mendelian inheritance like mixing paint colors. Incomplete dominance is like blending red and white to get pink, while codominance is like painting red and white stripes where both colors remain distinct.
DefinitionNon-Mendelian InheritanceGenetic inheritance patterns that do not follow the traditional dominant-recessive rules established by Mendel.
