DNA, genes and inheritance: Punnett squares explained
Understand genes, alleles, dominant and recessive traits and how to complete a Punnett square, with worked examples for GCSE and high school.
GCSEA-LevelHigh SchoolAPCurriculum fit: GCSE Biology and Combined Science (inheritance, variation and evolution). US: high school life science (inheritance of traits); also AP Biology.
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What is it?
DNA is the molecule that carries genetic information. It is shaped like a double helix and is found in the nucleus. A gene is a short section of DNA that codes for a particular protein, and different versions of a gene are called alleles.
Offspring inherit one allele for each gene from each parent. Whether a characteristic shows depends on whether the allele is dominant or recessive.
Key formulas and ideas
- Dominant allele (capital letter): shows if at least one copy is present.
- Recessive allele (lower case): shows only if two copies are present.
- Homozygous: two identical alleles. Heterozygous: two different alleles.
- Genotype: the alleles present. Phenotype: the characteristic that shows.
Worked examples
Example 1: A Punnett square
Question: Two heterozygous parents (Bb × Bb) have a child. What are the possible genotypes and their probabilities?
- Parent gametes: B or b from each.
- Combinations: BB, Bb, Bb, bb.
- Probabilities: BB 25%, Bb 50%, bb 25%.
Answer: 25% BB, 50% Bb, 25% bb (3 dominant to 1 recessive phenotype)
Example 2: Carrier parents
Question: Cystic fibrosis is caused by a recessive allele (f). Two carriers (Ff) have a child. What is the probability the child has the condition?
- The child must be ff.
- In a Ff × Ff cross, the probability of ff is 1 in 4.
Answer: 25%
Example 3: Sex determination
Question: What is the probability that a baby is male?
- Females are XX and males are XY.
- A father gives X or Y with equal chance, so half of babies are XY.
Answer: 50%
Common mistakes
- Thinking a dominant allele is always the more common one.
- Writing alleles in the wrong case in a Punnett square.
- Saying probability is guaranteed, such as 'one in four children will have it'. It is a chance for each child.
- Confusing genotype and phenotype.
Practice questions
Frequently asked questions
Does dominant mean most common?
No. Dominant describes how an allele is expressed, not how common it is in the population.
How many chromosomes do humans have?
46 in body cells (23 pairs), and 23 in gametes.
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Reviewed by StudyMorrow editorial review on 5 Nov 2026.