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UPCAT Science practice question 5: In a cross between two heterozygous individuals (Aa × Aa), what is the expected genotypic ratio of the offspring?

Correct answer

C. 1 : 2 : 1

The Punnett square gives AA, Aa, Aa, and aa, which is 1 AA : 2 Aa : 1 aa. The 3 : 1 ratio is the phenotypic ratio for complete dominance.

In a cross between two heterozygous individuals (Aa × Aa), what is the expected genotypic ratio of the offspring?

  1. 1 : 1
  2. 3 : 1
  3. 1 : 2 : 1Correct
  4. 9 : 3 : 3 : 1

Why C is correct

The Punnett square gives AA, Aa, Aa, and aa, which is 1 AA : 2 Aa : 1 aa. The 3 : 1 ratio is the phenotypic ratio for complete dominance.

Why the other options are wrong

A 1 : 1
The ratio from a heterozygous crossed with a homozygous recessive, not from two heterozygotes.
B 3 : 1
The *phenotypic* ratio for complete dominance. The question asks for genotypes, and Aa must be counted separately from AA.
D 9 : 3 : 3 : 1
The phenotypic ratio for a dihybrid cross, which involves two traits rather than one.
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