Punnett Squares - Basic Introduction

Understanding Punnett Squares

  • Before solving problems with Punnett squares, familiarize yourself with key concepts:

    • Alleles: Different versions of a gene, typically classified as dominant (capital letter, e.g., B for brown eyes) and recessive (lowercase letter, e.g., b for blue eyes).

Key Terms in Genetics

  • Homozygous: An organism with two identical alleles for a trait.

    • Homozygous Dominant: Two capital letters (e.g., BB).

    • Homozygous Recessive: Two lowercase letters (e.g., bb).

    • Heterozygous: One dominant and one recessive allele (e.g., Bb).

  • Genotype: The genetic makeup (alleles) of an organism.

  • Phenotype: The observable characteristics associated with the genotype (e.g., physical traits like eye color).

Example 1: Homozygous vs. Heterozygous Wolves

  • Cross between a homozygous blue-eyed wolf (bb) and a heterozygous brown-eyed wolf (Bb).

    • Punnett Square Setup: Draw a two-by-two grid, filling in the genotypes:

      • Left side: bb (homozygous recessive)

      • Top side: Bb (heterozygous)

    • Results:

      • Bb (Brown eyes)

      • Bb (Brown eyes)

      • bb (Blue eyes)

      • bb (Blue eyes)

  • Probability of Blue Eyes: 2 out of 4 (50%)

  • Phenotype Ratio: 1 Blue (bb) : 1 Brown (Bb)

    • Simplified ratio = 1:1.

  • **Genotype Ratio: ** 1 Bb : 1 bb

    • Simplified ratio = 1:1.

Example 2: Heterozygous Cats

  • Cross between two heterozygous brown-eyed cats (Bb).

  • Punnett Square Setup:

    • BB

    • Bb

    • Bb

    • bb

  • Probability of Brown Eyes: 3 out of 4 (75%)

  • Phenotype Ratio: Brown : Blue = 3:1

  • Genotype Ratio:

    • 1 BB : 2 Bb : 1 bb = 1:2:1

Incomplete Dominance in Flowers

  • Alleles for flower color:

    • R (Red), W (White); RW = Pink (heterozygous).

  • Cross between two pink flowers (RW):

  • Punnett Square Setup:

    • RR

    • RW

    • RW

    • WW

  • Probability of Red Flower: 1 out of 4 (25%)

  • Probability of Pink Flower: 2 out of 4 (50%)

  • Probability of White Flower: 1 out of 4 (25%)

Example 4: Mating Bears with Multiple Traits

  • Cross between a black-furred blue-eyed bear (Ff bb) and a white-furred brown-eyed bear (ff Bb).

  • Punnett Square Setup:

    • 16 squares total, four for each genotype combination.

  • Probability of Black Fur and Brown Eyes:

    • Identify combinations with at least one F and one B= 4 out of 16 (25%).

  • Probability of White Fur and Blue Eyes: 4 out of 16 (25%).

  • Probability of Homozygous Dominant for At Least One Trait: 0 out of 16 (0%).

  • Heterozygous for Both Traits: 4 out of 16 (25%).

  • Genotype Ratio: 1:1:1:1 (from combinations of genotypes).

  • Phenotype Ratio: 1 Black-Brown: 1 White-Brown: 1 Black-Blue: 1 White-Blue.