Genetic inheritance — AQA GCSE Biology

While most phenotypic features result from multiple genes interacting, certain traits are determined by just one gene. This simple inheritance model means the specific combination of alleles—alternative versions of the same gene—directly dictates the observed trait. For instance, in mice, inheriting a dominant allele for fur colour ensures that trait is expressed, even if a recessive allele is present. Similarly, red-green colour blindness in humans operates on a single-gene basis, where specific alleles dictate normal vision or the condition. Understanding these crosses allows students to predict offspring phenotypes using genetic diagrams, showing how dominant and recessive alleles interact to produce distinct physical traits.

The genotype is the combination of alleles an organism carries. Alleles are versions of a gene, and a gene is a length of DNA that codes for a protein. At the molecular level, the DNA base sequence of an allele determines the amino acid sequence of that protein, and therefore its shape and function. Proteins act as enzymes, structural components, carriers or signalling molecules, so they build the observable characteristics we call the phenotype. For example, an allele coding for a functional enzyme may produce dark pigment in hair or fur, while a different allele coding for a non-functional enzyme may leave the pigment absent. The phenotype also depends on the environment, so genotype is not the only influence.

An allele is dominant when its effect is seen in the phenotype even if only one copy is inherited. In a heterozygous organism, one dominant allele and one recessive allele are present, but the dominant allele determines the characteristic. This happens because the dominant allele codes for a functional protein, or because one working copy is enough to produce the characteristic. For example, in pea plants the allele for tall stems is dominant over the allele for short stems, so a plant with one tall allele and one short allele has tall stems. Genetic diagrams use a capital letter for the dominant allele, such as T for tall, and the same letter in lower case for the recessive allele, such as t for short.

A recessive allele is one whose effect on the phenotype is seen only when two copies are present, so no dominant allele is present. In a heterozygous organism, the dominant allele masks the recessive allele, and the recessive characteristic does not appear. The recessive phenotype therefore requires a homozygous recessive genotype, such as tt for short stems in pea plants. This happens because the recessive allele often codes for a non-functional protein, and one working dominant allele is enough to produce the dominant characteristic. Recessive alleles can be passed on by heterozygous carriers who do not show the characteristic, which is important when interpreting family trees and predicting the chance of offspring showing a recessive condition.

Homozygous and heterozygous describe whether the two alleles for a gene are the same or different. A homozygous organism has two identical alleles, such as TT or tt, and is described as homozygous dominant or homozygous recessive. A heterozygous organism has two different alleles, such as Tt, and shows the dominant characteristic. These terms apply to a particular gene or trait, so an organism can be homozygous for one gene and heterozygous for another. Correct notation uses one letter for the gene, with a capital for the dominant allele and lower case for the recessive allele. Being able to classify genotypes in this way is essential for predicting offspring ratios in genetic crosses.

This statement contrasts polygenic inheritance with single-gene inheritance. A single gene can produce clear-cut categories, such as the ABO blood group (which has three alleles producing four distinct blood groups). However, most characteristics, such as human height and mass, are polygenic, meaning they are controlled by multiple genes interacting. Several genes each contribute small effects, and their alleles interact so the phenotype forms a continuous spectrum rather than distinct classes. Environmental factors also heavily influence many of these features. In an exam, you may be asked to explain why a characteristic shows a continuous range of values, linking it to multiple genes and environmental effects.

This statement asks you to use probability to predict the outcomes of a single-gene cross while remembering that most phenotypic features are polygenic. In a monohybrid cross, each offspring inherits one allele from each parent, and the Punnett square shows the possible combinations. Probability describes the chance of each outcome, not a guarantee for a particular family. For example, a cross between two heterozygous parents, Aa × Aa, gives a 1 in 4 chance of aa offspring, or 25%, but a family of four children need not contain exactly one aa child. You must also recall that most characteristics, such as height, involve many genes, so simple single-gene ratios do not predict them. Use probability language carefully and distinguish predicted ratios from actual family results.

This statement requires you to express the outcome of a genetic cross as a ratio or as a proportion. In a monohybrid cross, the Punnett square gives the relative numbers of each genotype or phenotype. A cross between two heterozygotes, Aa × Aa, gives genotype ratio 1 AA : 2 Aa : 1 aa and phenotype ratio 3 dominant : 1 recessive. Direct proportion means comparing one part with the whole, such as 3 out of 4, or 0.75, or 75%. Simple ratios compare parts with each other, such as 3:1. You should be able to convert between these forms and to scale them to a predicted number of offspring, for example 3 out of every 4, or 75 out of 100. Always state what each number represents and keep the ratio in its simplest whole-number form.

This statement covers two related skills: completing a Punnett square and interpreting genetic crosses and family trees. To complete a Punnett square, write the alleles of one parent across the top and the alleles of the other parent down the side, then fill each cell with the allele from its row and column. Use capital letters for dominant alleles and lower-case letters for recessive alleles. From the completed square, identify genotype and phenotype ratios. Family trees use symbols to show inheritance across generations; you interpret them by tracking which phenotype appears in each generation and deducing parental genotypes. For example, if two unaffected parents have an affected child, the condition is recessive and both parents must be carriers. Always justify a deduced genotype using the evidence in the diagram.

A Punnett square is a grid that shows every possible combination of alleles from two parents. Write the gametes of one parent across the top and the other down the side, then fill each cell by combining the row and column alleles. For example, a cross between two heterozygous purple-flowered plants (Pp × Pp) gives cells PP, Pp, Pp and pp. The theory of probability then predicts the chance of each outcome: one cell in four is PP, so the probability of a purple-flowered offspring is 3/4 or 75%, and of a white-flowered offspring is 1/4 or 25%. Probabilities describe chance, not certainty, so predictions are about expected proportions in large numbers of offspring.