How can Punnett squares be used to predict the genotypic outcomes of dihybrid crosses?

Sourav PanNovember 9, 2024

Answered

Punnett squares are a valuable tool in genetics for predicting the genotypic outcomes of dihybrid crosses, which involve two traits controlled by different genes. Here’s how they work and how they can be used effectively:

Understanding Dihybrid Crosses

A dihybrid cross examines the inheritance of two different traits simultaneously. For example, consider a cross between pea plants where one trait is seed shape (round vs. wrinkled) and the other is seed color (yellow vs. green). The dominant alleles are typically represented as follows:

• Round seeds: R (dominant)
• Wrinkled seeds: r (recessive)
• Yellow seeds: Y (dominant)
• Green seeds: y (recessive)

Steps to Use a Punnett Square for Dihybrid Crosses

1. Determine Parental Genotypes:
   • For a typical dihybrid cross, both parents are usually heterozygous for both traits. For example, crossing two heterozygous plants: RrYy x RrYy.
2. Generate Gametes:
   • Use the FOIL method (First, Outside, Inside, Last) to determine the possible gametes from each parent:
     • Parent 1 gametes: RY, Ry, rY, ry
     • Parent 2 gametes: RY, Ry, rY, ry
3. Set Up a 4×4 Punnett Square:
   • Create a square with four rows and four columns to represent all combinations of the gametes from both parents.
   • Place one parent’s gametes along the top and the other parent’s gametes along the side.

1. Fill in the Punnett Square:
   • Each cell in the square represents a possible genotype of the offspring resulting from the combination of gametes from both parents.
2. Analyze Genotypes and Phenotypes:
   • Count the occurrences of each genotype in the completed Punnett square to determine genotypic ratios.
   • From these genotypes, you can also derive phenotypic ratios based on dominant and recessive traits.

Expected Ratios

For a dihybrid cross between two heterozygous parents (RrYy x RrYy), you can expect:

• Phenotypic Ratio: The typical phenotypic ratio is 9:3:3:1:
  • 9 round yellow (dominant for both traits)
  • 3 round green (dominant for seed shape, recessive for color)
  • 3 wrinkled yellow (recessive for seed shape, dominant for color)
  • 1 wrinkled green (recessive for both traits)
• Genotypic Ratio: The genotypic outcomes will yield various combinations such as:
  • 1 RRYY
  • 2 RRYy
  • 2 RrYY
  • 4 RrYy
  • 1 RRyy
  • 2 Rryy
  • 1 rryy
    This results in a total of nine distinct genotypes that correspond to the phenotypic outcomes.