How are the predicted genotypic and phenotypic ratios calculated in dihybrid crosses involving unlinked genes?

Sourav PanNovember 9, 2024

Answered

To predict the genotypic and phenotypic ratios in dihybrid crosses involving unlinked genes, the following steps are typically followed:

Understanding Dihybrid Crosses

A dihybrid cross examines the inheritance of two traits, each controlled by different genes. For example, consider two traits in pea plants: seed shape (Round, R, dominant over Wrinkled, r) and seed color (Yellow, Y, dominant over Green, y).

Steps to Calculate Genotypic and Phenotypic Ratios

1. Determine Parental Genotypes:
   • For a typical dihybrid cross, both parents are usually heterozygous for both traits. For example:
     • Parent 1: RrYy (Round Yellow)
     • Parent 2: RrYy (Round Yellow)
2. Generate Gametes:
   • Each parent can produce four types of gametes due to independent assortment. Use the FOIL method (First, Outside, Inside, Last) to determine the possible gametes:
     • Possible gametes from RrYy: RY, Ry, rY, ry.
3. Set Up a Punnett Square:
   • Create a 4×4 Punnett square to represent all combinations of the gametes from both parents.
   text

   | RY | Ry | rY | ry |
-----------------------------------------
RY | RRYY | RRYy | RrYY | RrYy |
-----------------------------------------
Ry | RRYy | RRyy | RrYy | Rryy |
-----------------------------------------
rY | RrYY | RrYy | rrYY | rrYy |
-----------------------------------------
ry | RrYy | Rryy | rrYy | rryy |

4. Count Genotypes:
   • After filling out the Punnett square, count the occurrences of each genotype:
     • 1 RRYY
     • 2 RRYy
     • 2 RrYY
     • 4 RrYy
     • 1 RRyy
     • 2 Rryy
     • 1 rryy
5. Calculate Genotypic Ratio:
   • The genotypic ratio can be summarized as:
     • 1:2:2:4:1:2:1 for the various genotypes.
6. Determine Phenotypes:
   • Assign phenotypes based on dominant traits:
     • Round Yellow (RRYY, RRYy, RrYY, RrYy): Dominant for both traits.
     • Round Green (RRyy, Rryy): Dominant for shape only.
     • Wrinkled Yellow (rrYY, rrYy): Dominant for color only.
     • Wrinkled Green (rryy): Recessive for both traits.
7. Calculate Phenotypic Ratio:
   • The expected phenotypic ratio from a dihybrid cross of two heterozygous parents (RrYy x RrYy) is typically:
     • 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green, or a ratio of 9:3:3:1.