Dihybrid Crosses

Introduction

• Dihybrid crosses involve tracking two traits, unlike monohybrid crosses which involve one trait.
• Traits discussed:
  • Color: Yellow (dominant) vs. Green (recessive).
  • Texture: Round (dominant) vs. Wrinkled (recessive).
• In a dihybrid cross, individuals are heterozygous for both traits.

Punnett Squares

• Used to calculate genotypic proportions.
• Determine possible gametes through meiosis:
  • Big R Big Y
  • Little r Big Y
  • Big R Little y
  • Little r Little y
• Mendel’s Law of Independent Assortment applies, making each gamete equally likely (1/4 chance for each).
• Punnett squares for dihybrid crosses involve 16 squares, representing combinations of gametes.
• Example frequency: Big R Big R Big Y Big Y zygote = 1/16.
• With more traits, Punnett squares become complex (e.g., trihybrid crosses require 64 squares).

Complexities and Alternatives

• Punnett squares can be tedious and complex.
• Difficult to determine specific genotype frequencies.

Branching Diagrams

• Simplify computation of genotypic and phenotypic proportions.
• Scale better for more than two traits (e.g., 3, 4, 5 traits).

Mendel’s Law of Independent Assortment

• Allows separate consideration of each trait.
• A dihybrid cross is a combination of two monohybrid crosses.
  • Round vs. Wrinkled: 1/4 Big R Big R, 1/2 Big R Little r, 1/4 Little r Little r.
  • Yellow vs. Green: 1/4 Big Y Big Y, 1/2 Big Y Little y, 1/4 Little y Little y.

Genotype Frequency Calculation

• Use branching diagrams to represent possible combinations of genotypes.
• Use the product rule to calculate frequency:
  • Example: Big R Big R and Big Y Big Y = 1/16 frequency.
  • Example: Heterozygous texture (Big R Little r) and Homozygous dominant color (Big Y Big Y) = 1/8 frequency.

Conclusion

• Mendel's law and branching diagrams are effective tools for evaluating genotypes and their proportions.
• Will be used extensively in future calculations.