Lecture Notes on Dihybrid Crosses and Punnett Squares

Introduction

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• Introduction to the concept of dihybrid crosses as an extension of monohybrid crosses.

Monohybrid vs. Dihybrid Crosses

• Monohybrid Cross: Focuses on one pair of alleles (e.g., hair trait in guinea pigs: HH, Hh, hh).
• Dihybrid Cross: Involves two pairs of alleles (two traits).
  • Example: Guinea pigs have multiple traits.
  • "Di" means two, indicating two traits are considered.

Example Scenario

• Cats and Traits: Moo the Cat
  • Moo's Behavior: Loves sinks, no genetic basis found, but used as a hypothetical trait.
  • Hypothetical genetic trait: Loving sinks is dominant (S), not loving sinks is recessive (s).
  • Hair trait: Having hair (H), hairless (hh).

Setting Up a Dihybrid Cross

• Example Cross: HhSs x hhss
  • Heterozygous for hair and liking sinks: HhSs.
  • Cross with a hairless, non-sink-loving cat: hhss.

Understanding Gametes and Genetic Laws

• Gametes: Carry one allele per gene.
• Mendel’s Law of Segregation: Gametes carry one allele for a gene.
• Mendel’s Law of Independent Assortment: Alleles are not linked.

Creating a Dihybrid Punnett Square

• Steps Involved:
  1. Write the parent cross with a 16-square Punnett square.
  2. Determine gamete combinations (FOIL method preferred):
     • HhSs yields HS, Hs, hS, hs.
     • hhss yields hs, hs, hs, hs.
  3. Combine gametes for offspring prediction.

Analyzing Offspring Predictions

• Genotype Ratio: 1:1:1:1 ratio (HhSs, Hhss, hhSs, hhss).
  • Each genotype occurs 4/16 or 25% of the time.
• Phenotype Ratio:
  • Hair/Love sinks: 25%
  • Hair/Dislike sinks: 25%
  • Hairless/Love sinks: 25%
  • Hairless/Dislike sinks: 25%

Important Considerations

• Genotype vs. Phenotype Ratios: Often the same in examples but not always.
• Role of Punnett Squares: Predict chances of genotypes/phenotypes, not certainties.

Conclusion

• Emphasize on the predictive nature of Punnett squares.
• Reminder to stay curious and explore genetic concepts.