Question

In the cross AaBb x aabb if incomplete dominance is exhibited at both the loci, what genotypic ratio is expected in the resulting offspring?

• A. 1:1:1:1
• B. 9:3:3:1
• C. 3:6:3:1:2:1
• D. 1:2:1:2:4:2:1:2:1

Hint:

Always distinguish between genotypic and phenotypic ratios in questions. Genotypic segregation is unaffected by dominance mechanisms.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The genotypic ratio of a genetic cross depends on how alleles segregate and assort during meiosis, regardless of how those alleles are expressed phenotypically.

Step 2: Detailed Explanation:
The given cross is between a dihybrid parent (\( \text{AaBb} \)) and a double homozygous recessive parent (\( \text{aabb} \)), which is a dihybrid testcross.
The heterozygous parent (\( \text{AaBb} \)) produces four distinct types of gametes due to independent assortment:
- \( \text{AB} \), \( \text{Ab} \), \( \text{aB} \), and \( \text{ab} \), each with a probability of \( 0.25 \).
The homozygous recessive parent (\( \text{aabb} \)) can only produce one type of gamete:
- \( \text{ab} \).
Combining these gametes yields the following offspring genotypes:
1. \( \text{AB} \times \text{ab} \rightarrow \text{AaBb} \)
2. \( \text{Ab} \times \text{ab} \rightarrow \text{Aabb} \)
3. \( \text{aB} \times \text{ab} \rightarrow \text{aaBb} \)
4. \( \text{ab} \times \text{ab} \rightarrow \text{aabb} \)
Because each combination has an equal probability of occurring, the expected genotypic ratio is exactly 1:1:1:1.
Dominance relationships (such as complete dominance, incomplete dominance, or codominance) only affect the phenotypes of the offspring, not their underlying genotypes.

Step 3: Final Answer:
The expected genotypic ratio is 1:1:1:1.