Incomplete Dominance and Codominance in Genetics

Genetics: Incomplete Dominance vs. Codominance

Incomplete Dominance

Incomplete dominance is a form of inheritance in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele. This means neither allele is completely dominant over the other.

• Definition: In incomplete dominance, the heterozygote shows a blended phenotype that is an intermediate of the two homozygotes.

• Example: Crossing red-flowered (R1R1) and white-flowered (R2R2) snapdragons produces pink-flowered (R1R2) offspring.

Allele	Phenotype
R1R1	Red Flower
R2R2	White Flower
R1R2	Pink Flower (Intermediate)

• Punnett Square Example: Crossing R1R1 (red) with R2R2 (white) yields 100% R1R2 (pink).

• F2 Generation: Crossing two pinks (R1R2 × R1R2) yields a 1:2:1 ratio of red:pink:white.

Codominance

Codominance occurs when both alleles in a heterozygote are fully expressed, resulting in offspring with a phenotype that shows both parental traits simultaneously.

• Definition: In codominance, the heterozygote expresses both phenotypes from each allele as "patches" or distinct areas.

• Example: In some flowers, crossing a red-flowered plant with a white-flowered plant produces offspring with both red and white patches.

Allele	Phenotype
R1R1	Red Flower
R2R2	White Flower
R1R2	Red & White Flower (Both colors visible)

• Punnett Square Example: Crossing R1R1 (red) with R2R2 (white) yields 100% R1R2 (red and white patches).

Practice Question Example

• If you cross true-breeding red and white flowers and the F1 offspring are all pink, this is an example of incomplete dominance.

Codominance & Blood Type

ABO Blood Group System

The ABO blood group in humans is a classic example of codominance and multiple alleles. There are three possible alleles: IA, IB, and i.

• Genotypes: The combination of these alleles determines the blood type.

• Phenotypes: The presence of A and/or B antigens on the surface of red blood cells determines the blood group.

Genotype	Surface Molecules	Phenotype
IAIA or IAi	A Only	Type A blood
IBIB or IBi	B Only	Type B blood
IAIB	A and B	Type AB blood
ii	NONE	Type O blood

• Codominance: Both IA and IB are codominant; individuals with genotype IAIB express both A and B antigens.

• Multiple Alleles: The ABO system is an example of a gene with more than two alleles.

Practice Question Example

• If you test a blood sample from a person who is heterozygous at the IA and IB loci, you will see both round and sickle-shaped red blood cells. This is an example of codominance.

Codominance in the MN Blood Group

The MN blood group is determined by two codominant alleles, M and N. Individuals can be MM, MN, or NN.

• If both children in a family are of blood type M, each parent must be either M or MN.

Practice Question Example

• The number of different alleles for ABO blood types is 3, resulting in 4 different blood types (A, B, AB, O).

Summary Table: Incomplete Dominance vs. Codominance

Type	Heterozygote Phenotype	Example
Incomplete Dominance	Intermediate (blended)	Pink flowers from red × white
Codominance	Both traits fully expressed	AB blood type, red & white patches in flowers

Key Terms

• Allele: Different forms of a gene.

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable traits of an organism.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

Additional info: The notes also reference the concept of multiple alleles and the importance of understanding genotype-phenotype relationships in human blood groups, which are foundational for Mendelian genetics and human inheritance patterns.