Lecture on Basics of Punnett Squares

Introduction to Punnett Squares

• Punnett squares measure the probability or likelihood of a genetic outcome based on a specific genetic cross.
• They do not predict the exact genetic outcome, only the possible outcomes.

Basic Setup for Punnett Squares

• Genotypes: Cross two genotypes, which may be for the same gene but contain different alleles.
• Example: Genetic cross for eye color:
  • Green eyes: Dominant allele.
  • Blue eyes: Recessive allele.
  • Maternal genotype: Big G Little g.
  • Paternal genotype: Little g Little g.

Constructing a Punnett Square

1. Draw a square divided into four quarters (like a simple four-panel window).
2. Place one parental genotype on top and the other on the side (order does not affect the outcome).
3. Conventionally write the dominant allele first if there is one.
4. Carry letters through to their respective quadrants to determine possible genotypes.

Outcomes

• Quadrants: Each quadrant represents a 25% chance of that genotype occurring in the offspring.
• Example outcome:
  • Genotypes: Big G Little g and Little g Little g.
  • Each genotype has a 50% probability.
  • Phenotypes: Green eyes and blue eyes with a 50% probability each.

Example of Heterozygous Genetic Cross

• Cross: Big G Little g with Big G Little g.
• Draw Punnett square and analyze 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).
• Phenotypic ratio: 3:1
  • 75% Green eyes.
  • 25% Blue eyes.

Conclusion

• Simple Punnett squares illustrate basic genetic probability.
• Future content will cover more complicated genetic crosses.
• Encourage engagement: Ask questions and interact (e.g., comments, likes, subscribe).