Monohybrid cross with incomplete dominance What phenotypic ratio is expected in the F2 generation when a trait shows incomplete dominance?
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A1:2:1
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B3:1:1
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C1:3:1
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D1:1:2
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E9:3:3:1
Answer
Correct Answer: 1:2:1
Explanation
Introduction / Context:Incomplete dominance occurs when the heterozygote phenotype is intermediate between the two homozygotes (e.g., red x white snapdragons producing pink F1). Understanding the resulting F2 ratios is foundational to classical genetics.
Given Data / Assumptions:
- Single gene with two alleles.
- Incomplete dominance: heterozygote has an intermediate phenotype.
- Random mating or selfing of F1 heterozygotes.
Concept / Approach:From two true-breeding parents (AA and aa), F1 are all Aa and intermediate. Selfing F1 (Aa x Aa) yields genotypes AA, Aa, aa in a 1:2:1 ratio. With incomplete dominance, phenotypes mirror genotypes: 1 homozygote phenotype : 2 intermediate : 1 other homozygote.
Step-by-Step Solution:
List F2 genotype ratios: AA:Aa:aa = 1:2:1.Map genotype to phenotype under incomplete dominance (three distinct phenotypes).Conclude F2 phenotypic ratio is also 1:2:1.Verification / Alternative check:Classic examples (flower color) reproduce the 1:2:1 phenotypic ratio in lab crosses.
Why Other Options Are Wrong:
- 3:1:1 and 1:3:1 are not standard monohybrid outcomes.
- 1:1:2 misorders counts and is nonstandard.
- 9:3:3:1 applies to a dihybrid cross with independent assortment, not monohybrid.
Common Pitfalls:Confusing incomplete dominance with codominance; both differ from strict dominance but have different phenotype interpretations.
Final Answer:1:2:1