Question

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

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

Hint:

In a dihybrid cross involving two heterozygous parents (\(AaBb \times AaBb\)), the F\(_2\) phenotypic ratio is always \(9:3:3:1\), provided the genes assort independently and there is complete dominance.

Answer 1

The Correct Option is C

Concept: A dihybrid cross involves the inheritance of two different traits simultaneously. According to Mendel’s Law of Independent Assortment, the alleles of different genes assort independently during gamete formation. Consider two traits:
• Seed shape: \(R\) (Round) dominant over \(r\) (Wrinkled)
• Seed color: \(Y\) (Yellow) dominant over \(y\) (Green) A typical dihybrid cross is: \[ RrYy \times RrYy \]

Step 1: Determine the possible gametes. Each parent can produce four types of gametes: \[ RY, \; Ry, \; rY, \; ry \]

Step 2: Form the Punnett square combinations. Combining the four gametes from each parent gives \(4 \times 4 = 16\) possible offspring combinations.

Step 3: Determine the phenotypes. The resulting phenotypes are:
• \(9\) Round Yellow
• \(3\) Round Green
• \(3\) Wrinkled Yellow
• \(1\) Wrinkled Green

Step 4: Write the phenotypic ratio. \[ 9 : 3 : 3 : 1 \]