O Levels Biology (5090)•5090/11/M/J/19
Explanation
Monohybrid Cross Genotypic Ratio
Steps:
- Cross two heterozygous plants for one trait: Aa × Aa.
- Each parent produces 50% A gametes and 50% a gametes.
- Construct Punnett square: possible offspring are AA, Aa, Aa, aa.
- Frequencies: 1/4 AA (homozygous dominant), 1/2 Aa (heterozygous), 1/4 aa (homozygous recessive).
Why A is correct:
- In the 1:2:1 genotypic ratio, homozygous dominant (AA) to heterozygous (Aa) is 1:2 by definition of Mendel's monohybrid cross.
Why the others are wrong:
- B. Duplicate of A but listed separately; does not apply uniquely.
- C. 3:1 is the phenotypic ratio of dominant to recessive traits, not genotypes.
- D. Duplicate of C; confuses phenotype with genotype ratio.
Final answer: A
Topic: Inheritance
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