Why can two black mice have a brown pup? Why is a baby's sex a matter of chance? GCSE Biology expects you to answer questions like these using the language of genetics: alleles, genotype and phenotype, dominant and recessive, and the Punnett square.
This free quiz moves from the vocabulary to the skills that gain marks in the exam: completing a Punnett square, turning it into a probability, working out the genotype of a parent from its offspring and reading a family tree. Every answer comes with a short explanation, and questions that ask you to work out probabilities from the parents' genotypes are flagged as Higher so that Foundation students can leave them out.
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The Punnett square for two heterozygous pea plants (Tt × Tt) has the boxes TT, Tt, Tt and tt. T is the dominant allele for tall plants. What is the probability that an offspring is tall?
Answer: B, 3 in 4
Three of the four boxes (TT, Tt and Tt) have at least one T, so they show a tall plant. The probability is 3 out of 4, written as 3 in 4 or 3/4 or 75%.
Two parents who do not have an inherited condition have a daughter who does. What is the most reasonable conclusion?
Answer: A, The condition is caused by a recessive allele, and both parents carry it
If the condition were caused by a dominant allele, any parent carrying it would show it. Because the parents are unaffected but the daughter is not, the allele must be recessive, and the daughter must have inherited one copy from each parent. Both parents are therefore carriers (heterozygous).
Your genotype is the alleles you carry, such as Tt. Your phenotype is the characteristic they produce, such as being tall. A dominant allele is expressed even when only one copy is present, so TT and Tt both give a tall plant. A recessive allele shows only when there are two copies and no dominant allele, so a short plant must be tt.
This is why you cannot always work out a genotype from appearance: a tall plant could be TT or Tt. Homozygous means the two alleles match (TT or tt) and heterozygous means they are different (Tt).
First write down the gametes each parent can make: a Tt parent makes T and t, a tt parent makes only t. Put one parent's gametes along the top and the other's down the side, then fill each box with the two alleles that meet. Every box is equally likely.
To turn the square into a probability, count the boxes that match and divide by the total number of boxes, which is usually four. Three tall boxes out of four is a probability of 3/4, or 75%, or 3 in 4. For two children in a row, work out the probability for one child and then multiply the two probabilities together.
Circles are females, squares are males and shading shows who has the condition. Work from the clues. If two unaffected parents have an affected child, the allele must be recessive and both parents must carry it. If two affected parents have an unaffected child, the allele must be dominant and both parents must be heterozygous.
It is written to fit all of them. The genetic terms, Punnett squares and sex determination appear in the GCSE Biology specifications of all three boards. Family trees are named by AQA and Edexcel; OCR does not name them, but reading one uses the same ideas. This quiz leaves out the details of DNA structure and protein synthesis that only some courses require, so check your own specification for the exact content of your course.
The boards do not treat Punnett squares in exactly the same way. At least one board only asks students to work out probabilities from the parents' genotypes at Higher tier, so those questions in this quiz are marked as Higher. If you choose Foundation, the quiz leaves them out and uses the questions that suit both tiers. If you choose Higher, you get everything.
Each question uses one form in its answer options: a fraction such as 1/4, a percentage such as 25%, or "1 in 4". They mean the same thing, so practise moving between them. In your own work, answer in the form the question asks for.
No. Each attempt picks a set of questions from a larger bank and shuffles the order of the answers, so you cannot learn them by position. Your answers and score stay on your own device, in your browser, so you can carry on after a reload; you can clear them at any time with the "Clear my saved progress" button. We only count anonymous, cookie-free visits and whether a quiz was finished, never your answers or who you are.
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These practice questions were written by Cloudia Tutoring. They are not official AQA, Pearson Edexcel or OCR material, and Cloudia Tutoring is not affiliated with or endorsed by any exam board or the Department for Education. Always check your own exam board's specification.
Reviewed by Claudia Edwards on 19 October 2026 Last updated .
