Question

The phenotypic ratio of a Mendelian dihybrid cross is:

• A. 9 : 3 : 3 : 1
• B. 9 : 7
• C. 3 : 1
• D. 1 : 2 : 1

Hint:

A dihybrid cross between two heterozygous parents (AaBb × AaBb) always gives a phenotypic ratio of 9:3:3:1 in the F₂ generation.

Answer 1

The Correct Option is A

Step 1: Understanding Mendel's dihybrid cross.
In a dihybrid cross, Mendel studied the inheritance of two traits simultaneously. He crossed pea plants with two contrasting traits (e.g., round yellow seeds × wrinkled green seeds).

Step 2: Law of independent assortment.
According to Mendel's law of independent assortment, alleles of different genes assort independently during gamete formation, producing new combinations of traits.

Step 3: Phenotypic ratio.
When the F₁ heterozygotes are self-crossed, the F₂ generation shows four phenotypes in the ratio: - 9 with both dominant traits, - 3 with first dominant and second recessive, - 3 with first recessive and second dominant, - 1 with both recessive traits. Thus, the phenotypic ratio is \(9:3:3:1\).

Step 4: Conclusion.
The correct answer is (A) 9 : 3 : 3 : 1.