Question

Mendel crossed peas having round green seeds with peas having wrinkled yellow seeds. All \(F_1\) plants had seeds that were round and yellow. What will be genotypic ratio of testcrossing these \(F_1\) plants?

• A. \(3:1\)
• B. \(9:3:3:1\)
• C. \(12:3:1\)
• D. \(1:1:1:1\)

Hint:

Remember: \[ \text{Dihybrid test cross: } RrYy \times rryy \] Produces: \[ 1:1:1:1 \]
• Round Yellow
• Round Green
• Wrinkled Yellow
• Wrinkled Green all in equal proportions.

Answer 1

The Correct Option is D

Concept: In peas:
• Round seed shape (\(R\)) is dominant over wrinkled (\(r\))
• Yellow seed color (\(Y\)) is dominant over green (\(y\)) A test cross involves crossing an individual with dominant phenotype to a homozygous recessive individual to determine its genotype. For a dihybrid heterozygote: \[ RrYy \times rryy \] the offspring are produced in equal proportions.

Step 1: Determining the parental genotypes. Given:
• Round green seeds \(\rightarrow RRyy\) (dominant round, recessive green)
• Wrinkled yellow seeds \(\rightarrow rrYY\) (recessive wrinkled, dominant yellow) Cross: \[ RRyy \times rrYY \] Gametes produced: \[ Ry \quad \text{and} \quad rY \] Thus, all \(F_1\) offspring are: \[ RrYy \] Phenotype: \[ \text{Round and Yellow} \]

Step 2: Performing the test cross. Test cross: \[ RrYy \times rryy \] Gametes from \(RrYy\): \[ RY,\ Ry,\ rY,\ ry \] Gamete from \(rryy\): \[ ry \] Offspring formed: \[ RrYy,\ Rryy,\ rrYy,\ rryy \] Each occurs in equal proportion. Therefore, the ratio is: \[ \boxed{1:1:1:1} \]

Step 3: Understanding the result. A dihybrid test cross always produces: \[ 1:1:1:1 \] when the genes assort independently. Hence, option (D) is correct.