Basics of Punnett Squares

Introduction

• Punnett squares are used to measure the probability or likelihood of a genetic outcome based on a specific genetic cross.
• They predict potential outcomes, not certainties.

Key Concepts

• Genetic Cross: Crossing two genotypes for the same gene, which may or may not contain different alleles.
• Genotypes and Alleles: Genotypes can have dominant and recessive alleles (e.g., big G for dominant, little g for recessive).

Example 1: Eye Color Cross

• Genotypes:
  • Maternal: Big G, little g (green eyes - dominant)
  • Paternal: Little g, little g (blue eyes - recessive)
• Punnett Square Setup:
  • Draw a square split into four quarters, similar to a four-panel window.
  • Place one genotype on top and the other on the side.
  • Conventionally, write the dominant allele first.
• Results:
  • Possible Genotypes: Big G little g, little g little g
  • Genotype Probability: 50% big G little g, 50% little g little g
  • Phenotype Probability: 50% green eyes, 50% blue eyes

Example 2: Heterozygous Cross

• Genotypes: Big G, little g crossed with Big G, little g
• Punnett Square Setup:
  • Same setup as before, draw the square and place one gene on top and the other on the side.
• Results:
  • Genotype Ratio: 1:2:1
    • 25% homozygous dominant (Big G Big G)
    • 50% heterozygous (Big G little g)
    • 25% homozygous recessive (little g little g)
  • Phenotype Ratio: 3:1
    • 75% green eyes
    • 25% blue eyes

Conclusion

• Simple Punnett squares are a basic tool for understanding genetic crosses.
• Complicated genetic crosses exist and will be covered in future content.
• Viewers are encouraged to ask questions in the comments and interact with the content.

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Note: This content is part of a basic educational series on genetics and Punnett squares. Further topics will address more complex genetic interactions.