Punnett Squares and Genetic Traits

Introduction to Punnett Squares

• A Punnett square is a grid used to understand allele combinations from parents.
• Named after baskets called "punnetts" in some countries (like the UK) used for holding produce.

Application of Punnett Squares

• Useful for predicting genotypes in monohybrid crosses (one trait).
• Can be applied to various crosses, not limited to dominant/recessive traits.

Example: Eye Color

• Parents' Genotypes:
  • Mom: Homozygous dominant for brown eyes (BB).
  • Dad: Heterozygous (Bb - brown and blue).
• Punnett Square Setup:
  • Mom can contribute B.
  • Dad can contribute B or b.
• Genotype Combinations:
  • BB, Bb, Bb, Bb
  • Probability:
    • 0% for blue-eyed child (needs bb).
    • 50% for homozygous dominant (BB).

Incomplete Dominance

• Example with flower color (red and white producing pink).
• Genotypes: RW (both parents)
• Phenotypes: Pink if heterozygous.
  • Probability for pink: 50% (2 out of 4 combinations).

Codominance and Multiple Alleles

• Example with blood types (A, B, O).
• Blood Type Alleles:
  • A and B are codominant.
  • O is recessive.
• Parent Example:
  • One parent: AB genotype.
  • Other parent: AO genotype.
• Phenotype Possibilities:
  • AA, AB, AO, BO
  • Probability for A blood type: 50% (considering both AA and AO).

Dihybrid Crosses

• Example Traits:
  • Eye color: B (brown, dominant), b (blue, recessive).
  • Teeth size: T (big, dominant), t (small, recessive).
• Assumption: Independent assortment (different chromosomes).
• Parent Genotypes: BbTt for both parents.
• Possible Combinations:
  • 16 combinations in a 4x4 Punnett square.
  • Phenotypes:
    • Big teeth, brown eyes: 9/16
    • Big teeth, blue eyes: 3/16
    • Little teeth, brown eyes: 3/16
    • Little teeth, blue eyes: 1/16

Conclusion

• Punnett squares reveal all possible gene combinations.
• Applicable to multiple traits and types of inheritance.
• Useful for predicting phenotypic ratios in offspring.