Genetic Inheritance
Recap Genetic Inheritance for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Use Punnett squares and key terms — dominant, recessive, homozygous, heterozygous, genotype and phenotype — to predict inheritance.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Some characteristics are controlled by a single gene, and each gene may have different forms called alleles. An individual has two alleles for each gene, one inherited from each parent.
If the two alleles are the same, the individual is homozygous; if they are different, heterozygous. A dominant allele is expressed even if only one copy is present, whereas a recessive allele is only expressed if both alleles are recessive.
The genotype is the combination of alleles present; the phenotype is the characteristic actually expressed. A Punnett square is used to predict the outcome of a genetic cross, and the results should be expressed as a ratio or probability rather than a certainty.
Revision notes
Alleles, dominant and recessive
An allele is a different form of a gene. Each individual has two alleles for each gene, one from each parent.
A dominant allele is expressed if just one copy is present, and is written as a capital letter. A recessive allele is only expressed if both alleles are recessive, and is written as a lower case letter.
Genotype and phenotype
The genotype is the combination of alleles present, such as Bb.
The phenotype is the characteristic expressed, such as brown eyes. Homozygous means both alleles are the same; heterozygous means they are different.
Using a Punnett square
Write one parent's alleles along the top and the other's down the side, then combine them in each box.
The four boxes show the possible genotypes of the offspring. Express results as a ratio or probability — a 3:1 ratio means each offspring has a 75 per cent chance, not that exactly three of four will show the trait.
Key points
- An allele is a different form of a gene.
- Each individual has two alleles per gene.
- A dominant allele is expressed if one copy is present.
- A recessive allele needs both copies to be expressed.
- Genotype is the alleles; phenotype is the characteristic.
- Punnett square results are probabilities, not certainties.
Worked examples
Example 1
Explain the difference between genotype and phenotype. [2 marks]
Model answer
The genotype is the combination of alleles an organism has for a characteristic (1 mark). The phenotype is the characteristic that is actually expressed (1 mark).
Example 2
Two heterozygous parents (Bb) have a child. Use a Punnett square to find the probability the child is bb. [3 marks]
Model answer
The Punnett square gives the offspring genotypes BB, Bb, Bb and bb (2 marks). One of the four possible outcomes is bb, so the probability is 1 in 4, or 25 per cent (1 mark).
Example 3
Explain why a person can carry a recessive allele without showing the characteristic. [2 marks]
Model answer
A recessive allele is only expressed if both alleles present are recessive (1 mark). If the person also has a dominant allele, that dominant allele is expressed instead, so the recessive characteristic does not show (1 mark).
Common mistakes
Saying a 3:1 ratio means exactly three of four offspring.
It is a probability. Each offspring independently has a 75 per cent chance.
Using the same letter case for both alleles.
Capital for dominant, lower case for the same letter in recessive form.
Confusing genotype with phenotype.
Genotype is the alleles; phenotype is what is seen.
Forgetting that each parent contributes one allele.
This is what determines the possible combinations in the Punnett square.
Exam tips
- Use the same letter in capital and lower case for the two alleles.
- Draw the Punnett square fully — it usually carries marks.
- Express outcomes as ratios or probabilities.
- Keep genotype and phenotype clearly separate.
Key terms
- Allele
- A different form of a gene.
- Dominant
- An allele expressed even when only one copy is present.
- Recessive
- An allele expressed only when both alleles are recessive.
- Heterozygous
- Having two different alleles for a gene.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.