What you'll learn
A genetic cross predicts the characteristics of offspring from their parents, and a Punnett square is the tool used to work out the probabilities. For AQA GCSE Biology you need to understand the key genetics terms, how to use a Punnett square for a monohybrid cross, how to work out ratios and probabilities, and how to use a genetic diagram to show inheritance. This guide covers the vocabulary of inheritance, how to construct a Punnett square, how to interpret the results, and inherited disorders and sex determination. By the end you should be able to complete a genetic cross and predict the outcomes.
Key terms and definitions
Gene — A section of DNA that codes for a characteristic.
Allele — A different version of a gene.
Dominant — An allele that is always expressed if present (shown with a capital letter).
Recessive — An allele only expressed if two copies are present (shown with a lower-case letter).
Genotype — The alleles an organism has for a characteristic (e.g. Bb).
Phenotype — The observable characteristic (e.g. brown eyes).
Homozygous — Having two identical alleles (e.g. BB or bb).
Heterozygous — Having two different alleles (e.g. Bb).
Core concepts
The vocabulary of inheritance
Characteristics are controlled by genes, and each gene can have different versions called alleles. For each gene, an organism has two alleles, one from each parent. Alleles can be:
- Dominant — expressed whenever present, shown with a capital letter (e.g. B).
- Recessive — only expressed when two copies are present, shown with a lower-case letter (e.g. b).
The combination of alleles is the genotype, and the characteristic you see is the phenotype. If the two alleles are the same, the organism is homozygous (BB or bb); if they are different, it is heterozygous (Bb). A heterozygous organism shows the dominant phenotype, because the dominant allele is expressed.
What a Punnett square does
A Punnett square is a diagram used to predict the possible genotypes and phenotypes of the offspring from a cross, and their probabilities. It shows all the possible combinations of the parents' alleles. To construct one for a monohybrid cross (one gene):
- Write the genotypes of the two parents.
- Work out the possible alleles in each parent's gametes.
- Put one parent's gametes along the top and the other's down the side of a 2×2 grid.
- Fill each box by combining the allele from the top and the side.
The four boxes show the possible genotypes of the offspring.
Working out ratios and probabilities
Once the Punnett square is filled in, you can read off the offspring:
- Count the genotypes (e.g. BB, Bb, bb) and the phenotypes (dominant or recessive).
- Express the results as a ratio (for example, 3 dominant : 1 recessive) or a probability (for example, a 1 in 4, or 25%, chance of the recessive phenotype).
For a cross between two heterozygotes (Bb × Bb), the offspring are 1 BB : 2 Bb : 1 bb, giving a phenotype ratio of 3 dominant : 1 recessive. The probabilities are predictions for each offspring, not guarantees.
Genetic diagrams
A genetic diagram sets out a cross fully, showing the parents' genotypes and phenotypes, the gametes, and the offspring genotypes and phenotypes. It is a clearer, written way of showing the same information as a Punnett square, and either can be used to answer a genetics question. Always label the alleles clearly and state which is dominant and which is recessive.
Inherited disorders
Some disorders are caused by alleles inherited from parents. For example, a disorder caused by a recessive allele only appears when a person inherits two copies of the allele; a person with one copy is a carrier who does not have the disorder but can pass the allele on. A dominant disorder appears in anyone with at least one copy of the allele. Punnett squares can be used to work out the chance of offspring inheriting such disorders, which is important in genetic counselling.
Sex determination
In humans, sex is determined by the sex chromosomes: females are XX and males are XY. A Punnett square with the parents XX and XY shows that half the offspring are expected to be XX (female) and half XY (male) — a 1:1 ratio. This is a special case of a genetic cross, and it explains why there is roughly an equal chance of having a boy or a girl.
Family trees (pedigrees)
As well as Punnett squares, inheritance can be shown using a family tree (pedigree), a diagram showing how a characteristic is passed down through the generations of a family. Circles usually represent females and squares males, and shaded shapes show individuals who have the characteristic. By studying a family tree, you can work out whether an allele is dominant or recessive and predict the genotypes of family members. For example, if two unaffected parents have an affected child, the allele must be recessive and the parents must both be carriers. Being able to read a family tree and use it to deduce genotypes and probabilities is a common exam skill that applies the same ideas as Punnett squares.
Variation from sexual reproduction
Genetic crosses also show why sexual reproduction produces variation. Because each parent contributes one of their two alleles at random to each gamete, and the gametes combine at random at fertilisation, offspring inherit a mixture of alleles from both parents. This is why offspring differ from their parents and from each other. A Punnett square shows exactly this mixing for a single gene. This variation is important because it is the raw material for natural selection — the differences between individuals are what allow some to be better suited to their environment. Linking genetic crosses to variation and its role in evolution shows a deeper understanding of why inheritance matters.
Worked examples
Example 1: A heterozygous cross
In pea plants, tall (T) is dominant to short (t). Cross two heterozygous tall plants (Tt × Tt). What is the phenotype ratio? The Punnett square gives 1 TT : 2 Tt : 1 tt, so the phenotype ratio is 3 tall : 1 short.
Example 2: Finding a probability
For the cross Tt × Tt, what is the probability that an offspring is short? Short is recessive (tt). Of the four boxes, one is tt, so the probability is 1 in 4 (25%).
Example 3: A carrier cross
Two parents are both carriers of a recessive disorder (Aa × Aa). What is the chance a child has the disorder? The disorder needs two recessive alleles (aa). The Punnett square gives 1 AA : 2 Aa : 1 aa, so the chance of aa (having the disorder) is 1 in 4 (25%).
Example 4: Sex determination
Use a genetic diagram to show the chance of a couple having a girl. The mother is XX and the father is XY. The mother's gametes are all X; the father's are half X and half Y. The offspring are XX (girl) and XY (boy) in a 1:1 ratio, so there is a 50% chance of a girl.
Common mistakes and how to avoid them
A common error is confusing genotype and phenotype. Genotype is the alleles (e.g. Tt); phenotype is the characteristic you see (e.g. tall). Keep them clearly separate.
Students often forget that a heterozygous organism (Tt) shows the dominant phenotype. Only two recessive alleles (tt) give the recessive phenotype.
Another mistake is not identifying the gametes correctly. Each gamete carries one allele, so a Tt parent produces gametes with T or t. Getting the gametes wrong makes the whole Punnett square wrong.
When giving results, be careful to distinguish a ratio (3:1) from a probability (1 in 4). Read the question to see which is asked for, and remember these are predictions, not guarantees.
Finally, use the correct case for alleles — capital for dominant, lower-case for recessive — and use the same letter for both (T and t), not two different letters.
Exam technique for "Genetic crosses and Punnett squares"
Set out crosses clearly: write the parents' genotypes, identify the gametes, draw the Punnett square, and then state the genotype and phenotype ratios or probabilities. Showing each step earns method marks.
Use a capital letter for the dominant allele and the lower-case of the same letter for the recessive one, and state which is which. Be ready to express results as both ratios and probabilities, and to work out the chance of an inherited disorder.
For sex determination, remember females are XX and males XY, giving a 1:1 ratio. Use precise terms — allele, dominant, recessive, genotype, phenotype, homozygous, heterozygous — throughout.
Quick revision summary
- Genes have different versions called alleles; each organism has two, one from each parent.
- Dominant alleles (capital letter) are always expressed; recessive alleles (lower-case) need two copies.
- Genotype = the alleles (e.g. Tt); phenotype = the characteristic; homozygous = two same alleles, heterozygous = two different.
- A Punnett square predicts offspring genotypes and phenotypes; Tt × Tt gives a 3:1 phenotype ratio.
- Results can be a ratio (3:1) or a probability (1 in 4 / 25%); they are predictions, not guarantees.
- Recessive disorders need two alleles (carriers have one); human sex is XX (female) or XY (male), a 1:1 ratio.