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

Sourav PanNovember 9, 2024

Answered step-by-step

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.