Understanding the Dihybrid Cross
A dihybrid cross involves two traits, each governed by different alleles. In Mendel's experiments, he studied pea plants that had two distinct traits. For simplicity, let’s consider seed shape (round vs. wrinkled) and seed color (yellow vs. green).
Parental Generation (P Generation)
- The parental generation consists of true-breeding plants for both traits (e.g., Round Yellow x Wrinkled Green).
- Round (R) and Yellow (Y) are dominant traits, while wrinkled (r) and green (y) are recessive.
Gamete Formation
- Each parent produces gametes with combinations of alleles:
- Round Yellow (RY)
- Wrinkled Green (ry)
F1 Generation
- The offspring from this cross (F1 generation) will all be heterozygous (RrYy) and display the dominant traits (Round Yellow).
F2 Generation and Phenotypic Ratio
- When F1 plants are crossed (RrYy x RrYy), the F2 generation shows a variety of combinations.
- The resulting phenotypes can be broken down as follows:
- Round Yellow (RY) – 9
- Round Green (Rg) – 3
- Wrinkled Yellow (rY) – 3
- Wrinkled Green (rg) – 1
Final Phenotypic Ratio
- The total ratio of phenotypes is 9:3:3:1.
- This ratio indicates that there are 9 Round Yellow, 3 Round Green, 3 Wrinkled Yellow, and 1 Wrinkled Green.
Conclusion
- Thus, the correct answer to the phenotypic ratio of a dihybrid cross in Mendel’s experiments is indeed 9:3:3:1 (option B). This illustrates the principle of independent assortment, where different traits segregate independently during gamete formation.