What would be the phenotypic ratio of the offspring of a cross between a heterozygous dominant parent and a double recessive parent?

Analyze the question: We're crossing a heterozygous dominant parent (Aa) with a double recessive parent (aa). This is a test cross (back cross). We need the phenotypic ratio of the offspring.

Concept: A test cross is used to find out whether an organism showing the dominant trait is homozygous (AA) or heterozygous (Aa). The recessive parent (aa) can only give the gamete "a".

Solution:

Parents: Aa × aa

Gametes: Aa parent gives A or a; aa parent gives a only.

| | a | a | |---|---|---| | A | Aa | Aa | | a | aa | aa |

Offspring genotypes: 2 Aa : 2 aa, which is 1 Aa : 1 aa

Phenotypes:

• Aa → dominant phenotype
• aa → recessive phenotype

Phenotypic ratio = 1 : 1 (dominant : recessive)

Correct option: B

Why the others are wrong:

• A (1:2:1) is the genotypic ratio of Aa × Aa (incomplete dominance would also give this phenotypic ratio).
• C (3:1) is the phenotypic ratio of Aa × Aa.
• D (2:1) is typical of lethal-gene crosses.

Exam tip: Remember "Test cross with a heterozygote = 1 : 1". If a test cross gives all dominant offspring, the unknown parent was homozygous dominant (AA). If you get a 1:1 ratio, it was heterozygous.