How Does Genetic Inheritance Work? Punnett Squares

Genetic inheritance works by passing alleles from parents to offspring. An allele is a version of a gene. Each parent passes on one allele for a particular gene in a gamete (an egg or sperm cell). At fertilisation, the two gametes join, giving the offspring an allele from each parent. A Punnett square shows the possible combinations.

This explanation assumes Higher tier AQA GCSE Biology, the separate-science qualification. The single-gene example below is a simple way to practise predicting inheritance.

How do alleles affect a characteristic?

For a characteristic controlled by one gene, a dominant allele is expressed when an offspring has at least one copy. A recessive allele is expressed only when the offspring has two copies. The pair of alleles is the genotype; the characteristic shown is the phenotype.

If the two alleles are the same, the genotype is homozygous. If they are different, it is heterozygous. These words describe the allele pair, not how an organism looks.

Worked Punnett square

Suppose a dominant allele, $B$, gives black fur in mice, while a recessive allele, $b$, gives brown fur. Both parents have the genotype $Bb$. Put each parent's possible gametes along an edge of the square, then combine the letters where the row and column meet.

Each parent can pass on either $B$ or $b$. The four boxes represent equally likely combinations. The possible genotypes are $BB$, $Bb$, $Bb$ and $bb$, giving a genotype ratio of $1:2:1$. Three boxes have at least one $B$, so the probability of black fur is $\frac{3}{4}$; the probability of brown fur is $\frac{1}{4}$. The phenotype ratio is therefore $3:1$.

Exam mistake to avoid: a Punnett square predicts probabilities, not the exact number of offspring with each phenotype. Count the boxes for the characteristic asked about, rather than confusing the genotype ratio with the phenotype ratio. This is a model for a single-gene characteristic; most characteristics involve multiple genes.