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Punnett Squares

Predict the outcomes of single-gene crosses with step-by-step Punnett squares.

  • Define and explain Punnett Squares in your own words
  • Use key terms such as gene accurately
  • Apply what you have learned to new examples and questions
  • Avoid the common mistakes learners make with this topic

This lesson focuses on Punnett Squares: predict the outcomes of single-gene crosses with step-by-step Punnett squares.

Definition: Punnett Squares

Predict the outcomes of single-gene crosses with step-by-step Punnett squares.

Key ideas

Monohybrid inheritance

In a monohybrid cross, a Punnett square predicts offspring genotypes. Gametes carry one allele each, so a cross between two heterozygotes (Tt × Tt) gives TT, Tt, Tt and tt — a 3:1 phenotype ratio when T is dominant. The square shows probabilities, not certainties: each child independently has a 25% chance of being tt.

Inherited disorders

Some disorders are caused by single genes. Cystic fibrosis is recessive: a child must inherit the faulty allele from both parents to have the disorder, so two carrier parents have a 25% chance of an affected child. Polydactyly (extra fingers or toes) is dominant: only one faulty allele is needed, so an affected parent can pass it on even when the other parent is unaffected.

Key term — gene: A section of DNA that codes for a particular protein and therefore influences a characteristic.

Worked example: Punnett Squares

Two carriers of cystic fibrosis (Ff × Ff) have a child. What is the probability the child has cystic fibrosis (ff)? Show your reasoning.

The Punnett square gives FF, Ff, Ff, ff. Only ff is affected: 1 in 4, or 25%.

Answer: The Punnett square gives FF, Ff, Ff, ff. Only ff is affected: 1 in 4, or 25%.

Common mistakes
  • Putting two alleles in each gamete box Gametes are haploid and carry only one allele of each gene. Each row and column heading of a Punnett square holds a single allele.
  • Saying dominant means common Dominant describes how an allele behaves in a heterozygote, not how frequent it is. A dominant allele can be rare in a population.

Practice

What is the relationship between DNA, a gene and a chromosome?
Think of them as nested structures.

DNA is the molecule; a gene is a section of DNA coding for a protein; chromosomes are long DNA molecules, each carrying many genes.

Give one genetic and one environmental cause of variation in humans.
Think alleles and lifestyle.

Genetic: different alleles inherited from parents, e.g. eye colour. Environmental: e.g. scars, or muscle size affected by exercise and diet.

Define the terms homozygous and heterozygous.
Think about whether the two alleles match.

Homozygous means carrying two identical alleles (e.g. TT or tt); heterozygous means carrying two different alleles (e.g. Tt).

In humans, brown eyes (B) are dominant to blue eyes (b). What eye colour will a Bb person have?
Which allele is expressed?

Brown eyes, because the dominant B allele is expressed even with only one copy.

Quick check

Punnett Squares — quick check

Which of these best defines "gene"?

A section of DNA that codes for a particular protein and therefore influences a characteristic.

Why can two brown-eyed parents have a blue-eyed child?

Both parents can be heterozygous (Bb). Each can pass on the recessive b allele, and a child receiving b from both (bb) will have blue eyes — a 25% chance per child.
Key takeaways
  • Punnett Squares: predict the outcomes of single-gene crosses with step-by-step Punnett squares.
  • Monohybrid inheritance: In a monohybrid cross, a Punnett square predicts offspring genotypes.
  • allele: One version of a gene; for example, a flower-colour gene might have a red allele and a white allele.
  • Watch out for: putting two alleles in each gamete box
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