1. Incomplete Dominance
Definition: Incomplete dominance occurs
when neither allele is completely dominant over the other in a heterozygous
pair. As a result, the phenotype of the heterozygote is an intermediate or
blending of the two parental traits.
Mechanism: In cases of incomplete
dominance, both alleles contribute partially to the organism's phenotype. This
blending effect results in a unique phenotype that appears as a mix between the
dominant and recessive alleles.
Example: One classic example of
incomplete dominance is the color of flowers in certain plants, such as
snapdragons. When a red-flowered plant (RR) is crossed with a white-flowered
plant (rr), the resulting offspring (Rr) exhibit pink flowers—a color that is
intermediate between red and white. Here, the pink flower phenotype represents
incomplete dominance.
Significance: Incomplete dominance shows that
not all traits follow the traditional dominant-recessive pattern. It
contributes to the diversity of traits seen within species and highlights that
gene interactions can produce unique phenotypic expressions.
2. Co-Dominance
Definition: Co-dominance occurs when both
alleles in a heterozygous pair are fully expressed, resulting in a phenotype
where both traits are visible simultaneously rather than blended.
Mechanism: In co-dominant inheritance,
neither allele masks the other. Instead, each allele’s effect is visible in the
organism, allowing both traits to be equally displayed in the phenotype of
heterozygotes.
Example: A well-known example of
co-dominance is the ABO blood group system in humans. Individuals with genotype
IAIB have both A and B antigens on their red blood cells, resulting in blood
type AB. Both alleles (IA and IB) are equally expressed, leading to the
co-dominant AB blood type, where neither antigen is dominant over the other.
Significance: Co-dominance adds another layer
to genetic variation, demonstrating how two alleles can work together without
one being recessive. It has implications for understanding how genetic traits
are expressed, particularly in complex traits that involve multiple alleles.
Comparison
of Incomplete Dominance and Co-Dominance
Aspect
Incomplete Dominance
Co-Dominance
Expression
Blending
of both traits
Both
traits are fully expressed
Phenotypic
Result
Intermediate
phenotype (e.g., pink flowers)
Dual
phenotype (e.g., AB blood type)
Example
Snapdragons
with pink flowers
Human
blood type AB
Allelic
Relationship
Partial
expression of each allele
Equal
and full expression of both alleles
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