Punnett Squares – Lesson Summary

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Learning Objective

In this lesson we will learn how Punnett squares can be used to predict genotypes and phenotypes of offspring.

Learning Outcomes

By the end of this lesson you will be able to:

  • Complete a Punnett square to determine the possible genotypes and phenotypes of offspring for:
  • Dominant and recessive inheritance
  • Incomplete dominance and codominance
  • X-linked traits
  • State the probability of particular genotypes or phenotypes in offspring, based on a Punnett square.
  • Explain why observed phenotype ratios can differ from predicted phenotype ratios.

 
punnett squares lesson contents

(Image: Sundry Photography, Adobe Stock)

Lesson Topics

1 | Introduction to Punnett Squares

2 | Steps for Completing a Punnett Square

3 | Punnett Squares for Incomplete Dominance and Codominance

4 | Punnett Squares for Sex-Linked Traits

5 | Summary

Lesson Summary

  • Punnett squares are diagrammatic methods for determining the possible genotypes and phenotypes of offspring, as well as their probability of occurring.
  • Punnett squares consist of a larger square divided into four smaller squares, in a 2×2 format.
  • The alleles of parents are written above and beside the Punnett square and the possible genotypes of offspring are written inside each smaller square.
  • Each smaller square represents a 25% probability.
  • The steps for completing a Punnett square are similar for all modes of inheritance:
  • Write the two alleles of one parent above the Punnett square and the two alleles of the other parent to the left of the Punnett square.
  • Determine the genotypes of offspring resulting from each parental allele combination in the four squares.
  • Determine the phenotypes of offspring for each genotype.
  • Determine the probability of each genotype and phenotype occurring in the offspring.
  • Punnett squares determine probabilities, not actual outcomes.
  • The larger the number of offspring produced, the greater the likelihood of observed ratios matching predicted ratios.

 
punnett square summary diagram

Punnett squares are a simple way of determining the possible genotypes and phenotypes of offspring.

 

(Drosophila images: SiberianPhotographer, Adobe Stock)

(Header Image: pixel2013, Pixabay)