Understanding Punnett Squares

Introduction to Punnett Squares

• Purpose: Measure the probability or likelihood of genetic outcomes based on specific genetic crosses.
• Functionality: They predict potential outcomes rather than definitive results.

Basics of Punnett Squares

• Genetic Crosses:
  • Involves crossing two genotypes.
  • These genotypes relate to the same gene and might contain different alleles.

Example: Eye Color

• Genes and Alleles:

  • Green eyes (dominant) vs. Blue eyes (recessive).
  • Maternal genotype "Big G little g" (Gg).
  • Paternal genotype "little g little g" (gg).

• Setting Up the Punnett Square:

  • Draw a square divided into four quarters.
  • Place one parental genotype on top and the other on the side.
  • Conventionally write the dominant allele first if present.

• Calculating Probabilities:

  • Each quadrant represents a 25% probability of a genotype outcome.
  • Possible genotypes: Big G little G (Gg) and little g little g (gg).
  • Each genotype has a 50% chance to appear in offspring.
  • Phenotypes: 50% chance for green eyes and 50% for blue eyes.

Example: Heterozygous Cross

• Crossing Two Heterozygous Genes (Gg x Gg):

  • Draw a Punnett Square and assign one gene on top and the other on the side.

• Results:

  • Genotype Ratio:
    • 1:2:1
    • 25% Homozygous Dominant (GG)
    • 50% Heterozygous (Gg)
    • 25% Homozygous Recessive (gg)
  • Phenotypic Ratio:
    • 3:1
    • 75% Green Eyes
    • 25% Blue Eyes

Conclusion

• Further Learning:

  • More complex genetic crosses exist beyond simple Punnett Squares.
  • Future videos may explore these more complicated scenarios.

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