🧬

Understanding Punnett Squares in Genetics

Feb 25, 2025

Lecture on Punnett Squares and Genetic Probability

Introduction to Genetics

  • Alleles: Variants of a gene.
    • Dominant Allele: Represented by a capital letter (e.g., B for brown eyes).
    • Recessive Allele: Represented by a lowercase letter (e.g., b for blue eyes).

Key Terms in Genetics

  • Homozygous Dominant: Two identical dominant alleles (e.g., BB).
  • Heterozygous: One dominant and one recessive allele (e.g., Bb).
  • Homozygous Recessive: Two identical recessive alleles (e.g., bb).
  • Genotype: Genetic makeup of an individual (e.g., BB, Bb, bb).
  • Phenotype: Observable physical characteristics (e.g., brown eyes or blue eyes).

Dominance in Genetics

  • Homozygous Dominant: Phenotype is determined by the dominant allele (e.g., BB = brown eyes).
  • Heterozygous: Phenotype displays the dominant trait (e.g., Bb = brown eyes).
  • Homozygous Recessive: Phenotype displays the recessive trait (e.g., bb = blue eyes).

Punnett Squares

  • A tool used to predict the probability of offspring inheriting certain traits.

Example Problems

Problem 1: Wolf Eye Color

  • Cross: Homozygous blue-eyed wolf (bb) x Heterozygous brown-eyed wolf (Bb).
  • Punnett Square:
    • Genotypes: Bb, Bb, bb, bb.
    • Probability of blue eyes: 50% (bb).
  • Phenotype Ratio: 1:1 (blue to brown).
  • Genotype Ratio: 1:1 (Bb to bb).

Problem 2: Cat Eye Color

  • Cross: Two heterozygous brown-eyed cats (Bb x Bb).
  • Punnett Square:
    • Genotypes: BB, Bb, Bb, bb.
    • Probability of brown eyes: 75% (BB, Bb, Bb).
  • Phenotype Ratio: 3:1 (brown to blue).
  • Genotype Ratio: 1:2:1 (BB:Bb:bb).

Problem 3: Flower Color (Incomplete Dominance)

  • Concept: RW (pink), RR (red), WW (white).
  • Cross: Two pink flowers (RW x RW).
  • Punnett Square:
    • Genotypes: RR, RW, RW, WW.
    • Probability of red flower: 25% (RR).
    • Probability of pink flower: 50% (RW).
    • Probability of white flower: 25% (WW).

Problem 4: Bear Fur and Eye Color (Dihybrid Cross)

  • Cross: Black fur & blue eyes (Ffbb) x White fur & brown eyes (ffBb).
  • Punnett Square:
    • Probability black fur & brown eyes: 25%.
    • Probability white fur & blue eyes: 25%.
  • Genotype Ratio: 1:1:1:1.
  • Phenotype Ratio: 1:1:1:1.

Conclusion

  • Understanding and using Punnett squares helps in predicting genetic trait probabilities.
  • Provides a foundational understanding for deeper study in genetics.

Additional Resources

  • Algebra, physics, and chemistry videos available for further study.

Note: This lecture covers basic genetic concepts and uses examples to illustrate how Punnett squares are used to solve genetic probability problems.

Full transcript