Question

In a dihybrid cross between two heterozygous pea plants (RrYy × RrYy), what is the phenotypic ratio of the offspring for seed shape and seed color? (R = round, r = wrinkled; Y = yellow, y = green)

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

Hint:

In a dihybrid cross between two heterozygous parents, the phenotypic ratio is typically 9:3:3:1 if the traits are independently assorting. Memorize this ratio for quick recall, but understand it by considering the independent segregation of each trait.

Answer 1

The Correct Option is B

A dihybrid cross involves two traits, each controlled by a single gene with two alleles. Here, seed shape (R = round, dominant; r = wrinkled, recessive) and seed color (Y = yellow, dominant; y = green, recessive) are considered. Both parent plants are heterozygous (RrYy), meaning they have the genotype RrYy for both traits.
To determine the phenotypic ratio of the offspring, we analyze the inheritance of both traits using Mendel’s law of independent assortment, which states that alleles for different traits segregate independently during gamete formation. The possible gametes for each RrYy parent are RY, Ry, rY, and ry.
Using a Punnett square for a dihybrid cross (RrYy × RrYy), we can calculate the phenotypic outcomes. However, for simplicity, we can use the phenotypic ratios derived from Mendel’s experiments:
- For each trait individually, a cross between two heterozygotes (e.g., Rr × Rr) produces a 3:1 phenotypic ratio (3 dominant : 1 recessive).
- For seed shape: 3 round (RR or Rr) : 1 wrinkled (rr).
- For seed color: 3 yellow (YY or Yy) : 1 green (yy).
Since the traits assort independently, the combined phenotypic ratio for the dihybrid cross is calculated by multiplying the ratios of the individual traits:
- Round, yellow: \( \frac{3}{4} \text{(round)} \times \frac{3}{4} \text{(yellow)} = \frac{9}{16} \)
- Round, green: \( \frac{3}{4} \text{(round)} \times \frac{1}{4} \text{(green)} = \frac{3}{16} \)
- Wrinkled, yellow: \( \frac{1}{4} \text{(wrinkled)} \times \frac{3}{4} \text{(yellow)} = \frac{3}{16} \)
- Wrinkled, green: \( \frac{1}{4} \text{(wrinkled)} \times \frac{1}{4} \text{(green)} = \frac{1}{16} \)
Thus, the phenotypic ratio of the offspring is 9 round, yellow : 3 round, green : 3 wrinkled, yellow : 1 wrinkled, green, or 9:3:3:1.