Genetics Lecture: Dihybrid Crosses and Punnett Squares

Introduction

• Punnett Squares
  • Typically involve one pair of alleles (Monohybrid)
  • Example: Hair trait in guinea pigs represented by alleles HH, Hh, hh

Dihybrid Cross

• Definition: Crossing with two pairs of alleles (Dihybrid)
  • "Di" means two, involving two traits
• Traits in Example:
  • Hair (H) vs Hairless (h)
  • Loving sinks (S) vs Not loving sinks (s)

Genetic Traits in Cats Example

• Characteristics:
  • Hair trait (Hair: H, Hairless: hh)
  • Sink-loving trait (Dominant: S, Recessive: s)
• Cat Example: Crossing heterozygous cat (HhSs) with hairless, non-sink-loving cat (hhss)

Punnett Square Setup

• Monohybrid Example:
  • Crossing Hh with hh
  • Gametes carry half genetic material (one allele for each gene)
  • Mendel’s Law of Segregation: Gametes carry one allele per gene
• Dihybrid Setup:
  • Two alleles per gamete
  • Mendel’s Law of Independent Assortment: Alleles are not linked

Steps for Dihybrid Cross

1. Parent Cross: Use a 16-square Punnett square
2. Gamete Combinations: Write along top and side
   • Use FOIL method: First, Outside, Inside, Last
   • Example: HhSs → HS, Hs, hS, hs; hhss → hs, hs, hs, hs
3. Combine Gametes: Determine offspring predictions
   • Format: Capital letters before lowercase

Results and Interpretation

• Genotype Ratio: 4/16 (25%) HhSs, 4/16 (25%) Hhss, 4/16 (25%) hhSs, 4/16 (25%) hhss
  • Ratio 1:1:1:1
• Phenotype Ratios:
  • 25% have hair/love sinks
  • 25% have hair/dislike sinks
  • 25% hairless/love sinks
  • 25% hairless/dislike sinks
  • Ratio 1:1:1:1

Important Considerations

• Genotype vs Phenotype ratios may differ
• Handout example will show when they are not the same
• Punnett Squares predict chances, not certainties

Conclusion

• Key takeaway: Follow steps to solve problems correctly
• Reminder to stay curious and continue exploring genetics.