Lecture Notes: Dihybrid Crosses and Punnett Squares

Introduction

• Focus on Punnett Squares, specifically dihybrid crosses.
• Monohybrid involves one pair of alleles (e.g., guinea pig hair trait).
• Dihybrid involves two pairs of alleles, addressing two traits simultaneously.

Example of Traits in Dihybrid Cross

• Traits: Hair presence (H) and sink-loving behavior (S) in cats.
• Alleles:
  • Hair trait: Dominant (H) for hair, Recessive (h) for hairless.
  • Sink-loving trait: Dominant (S) loves sinks, Recessive (s) dislikes sinks.

Cross Example

• Parent Genotypes:
  • Cat 1: Heterozygous for both traits (HhSs).
  • Cat 2: Hairless and dislikes sinks (hhss).

Steps to Work Out Dihybrid Cross

1. Write the Parent Cross:
   • Use a 16-square Punnett square.
2. Gamete Combinations:
   • Use the FOIL method to determine combinations.
   • For HhSs: HS, Hs, hS, hs.
   • For hhss: hs, hs, hs, hs.
3. Combine Gametes:
   • Fill in the Punnett square to see offspring prediction.
   • Genotype Ratio: 1:1:1:1 (HhSs : Hhss : hhSs : hhss).
   • Phenotype Ratio: 1:1:1:1 (hair/love sinks : hair/dislike sinks : hairless/love sinks : hairless/dislike sinks).

Important Concepts

• Mendel’s Law of Segregation: Gametes carry one allele per gene.
• Mendel’s Law of Independent Assortment: Alleles for different traits are segregated independently.

Observations and Predictions

• Phenotype and genotype ratios can vary.
• Punnett squares are predictive tools for genetic makeup in offspring.
• Ratios can differ in other examples (as given in handouts).

Conclusion

• Dihybrid crosses demonstrate genetic probability for two traits.
• Practice is key to mastering the setup and interpretation of dihybrids.
• Punnett squares are predictive, not deterministic.

Note: Always remember these are predictions and actual genetic outcomes may vary.

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