Punnet squares

• Probability of Brown eyes phenotype = 25% + 25% + 25% + 25% = 100%
• Passing alleles to Offsprings

  1. Meiosis: Will determine which allele is found in each gamete

  2. Fertilisation: Will determine which alleles are passed to offspring from sperm and egg
• Punnett Squares

  1. Used to show all possible genotypes of offspring that could result from a cross/fertilisation

  2. Determines how likely each genotype is to occur

  3. Represents only a probability
• Calculating Probability: Each of the 4 inner boxes in a Punnett square has the same chance of happening. This means each box has a 25% chance (or 1 in 4 chance) of containing that particular genotype of the offspring
• Phenotypic Ratio: All offspring will have brown eyes
• Example Inheritance of Alleles

  • Can have 2 alleles for eye colour: B allele (for brown eyes) is dominant over b allele (for blue eyes)
  • If mother has genotype of Bb, her eggs can contain either the B or b alleles. Her phenotype will be brown eyes because B allele is dominant
  • If father has genotype of BB, all of his sperm will contain B alleles
• The offspring will end up with 2 alleles for each gene (one from sperm and one from egg)
• Genotype of Offsprings


  Write the possible genotype of each offspring from the alleles of each gamete. Always place the dominant allele first (e.g. Ff, not fF)
• Calculation for phenotype

  1. Probability of BB genotype = 25% + 25% = 50%

  2. Probability of Bb genotype = 25% + 25% = 50%

  3. Probability of Brown eyes phenotype = 25% + 25% + 25% + 25% = 100%
• Probability and Ratio of Genotype

  1. Probability of genotype FF = 25%

  2. Probability of genotype Ff = 25% + 25% = 50%

  3. Probability of genotype ff = 25%

  4. Ratio: 1 FF : 2 Ff : 1 ff
• Ratio for 3 Possible Genotypes

  If both parents were heterozygous (Bb), then offspring would have 3 possible genotypes