Question

What is the phenotypic ratio of a Mendelian dihybrid cross in F\(_2\)?

• A. \(3:1\)
• B. \(1:2:1\)
• C. \(9:3:3:1\)
• D. \(1:1:1:1\)

Hint:

The \(9:3:3:1\) ratio is observed only when genes assort independently and there is no linkage between them.

Answer 1

The Correct Option is C

Concept:
A dihybrid cross involves two pairs of contrasting traits. According to Mendel’s law of independent assortment, alleles of different genes assort independently during gamete formation.

Step 1: Consider the parental cross.
Example: \(YYRR \times yyrr\)

Step 2: F\(_1\) generation.
All offspring are heterozygous: \(YyRr\)

Step 3: F\(_2\) generation.
Self-cross: \(YyRr \times YyRr\) Gametes formed: \(YR, Yr, yR, yr\)

Step 4: Phenotypic ratio.
On combining these gametes, four phenotypes appear in the ratio: \[ 9:3:3:1 \]

Step 5: Interpretation.
• 9: Both dominant traits
• 3: First dominant, second recessive
• 3: First recessive, second dominant
• 1: Both recessive \[ \boxed{9:3:3:1} \]