Understanding Codominant Incomplete Dominance Genetics
In genetics, the relationship between different alleles of a gene can be described in terms of dominance and recessiveness. While many genes exhibit a simple dominant-recessive relationship, some genes exhibit more complex relationships, such as codominance and incomplete dominance. In this worksheet, we will explore the concepts of codominance and incomplete dominance, and how they affect the expression of traits in organisms.
Codominance
Codominance occurs when two different alleles of a gene have an equal effect on the expression of a trait. In other words, neither allele is recessive, and both alleles contribute to the phenotype. This means that the heterozygous genotype (having two different alleles) will express a combination of the two alleles.
🔍 Note: In codominance, the two alleles do not blend together, but rather, both alleles are expressed simultaneously.
For example, let’s consider the ABO blood type system in humans. The ABO gene has three alleles: A, B, and O. The A and B alleles are codominant, while the O allele is recessive. When an individual has the genotype AO or BO, they will express the A or B blood type, respectively. However, when an individual has the genotype AB, they will express both A and B blood types, resulting in the AB blood type.
Example Problems:
- What is the genotype and phenotype of an individual who has the AB blood type?
Answer: Genotype: AB, Phenotype: AB blood type
- What is the genotype and phenotype of an individual who has the AO blood type?
Answer: Genotype: AO, Phenotype: A blood type
Incomplete Dominance
Incomplete dominance occurs when one allele does not completely dominate the other allele. In other words, the dominant allele will not completely mask the effect of the recessive allele. This means that the heterozygous genotype will express a phenotype that is a blend of the two alleles.
🔍 Note: In incomplete dominance, the dominant allele does not completely dominate the recessive allele, resulting in a blended phenotype.
For example, let’s consider the flower color of snapdragons. The gene for flower color has two alleles: R (red) and r (white). The R allele is incompletely dominant over the r allele. When a plant has the genotype Rr, it will express a pink flower color, which is a blend of the red and white colors.
Example Problems:
- What is the genotype and phenotype of a snapdragon plant that has pink flowers?
Answer: Genotype: Rr, Phenotype: Pink flowers
- What is the genotype and phenotype of a snapdragon plant that has red flowers?
Answer: Genotype: RR or Rr, Phenotype: Red flowers
Comparison of Codominance and Incomplete Dominance
| Codominance | Incomplete Dominance | |
|---|---|---|
| Definition | Two alleles have an equal effect on the expression of a trait | One allele does not completely dominate the other allele |
| Effect on Phenotype | Both alleles are expressed simultaneously | The dominant allele does not completely mask the effect of the recessive allele |
| Example | ABO blood type system | Snapdragon flower color |
| Genotype | Phenotype (Codominance) | Phenotype (Incomplete Dominance) |
|---|---|---|
| AA or aa | A or a | A or a |
| Aa | Both A and a | Blend of A and a |
Practice Problems
- A certain gene has two alleles: T (tall) and t (short). The T allele is incompletely dominant over the t allele. What is the genotype and phenotype of a plant that is tall?
Answer: Genotype: TT or Tt, Phenotype: Tall
- A certain gene has two alleles: F (flowered) and f (non-flowered). The F allele is codominant with the f allele. What is the genotype and phenotype of a plant that has flowers?
Answer: Genotype: FF or Ff, Phenotype: Flowered
What is the main difference between codominance and incomplete dominance?
+The main difference between codominance and incomplete dominance is the way in which the alleles affect the expression of the trait. In codominance, both alleles have an equal effect on the expression of the trait, while in incomplete dominance, one allele does not completely dominate the other allele.
Can you provide an example of codominance in humans?
+Yes, the ABO blood type system in humans is an example of codominance. The A and B alleles are codominant, while the O allele is recessive.
What is the phenotype of an individual who has the genotype Rr for the snapdragon flower color gene?
+The phenotype of an individual who has the genotype Rr for the snapdragon flower color gene is pink flowers.
