Alleles and inherited differences

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Alleles and inherited differencesSpec 3.12

In short

Alleles are different forms of the same gene, such as a purple allele and a white allele of a flower colour gene. Because chromosomes are in pairs, an organism usually has two alleles of each gene, one inherited from each parent. Alleles have slightly different base sequences, so they can code for slightly different versions of a protein.

Different forms of the same gene are called alleles. A gene for flower colour may have an allele for purple and an allele for white.

In body cells chromosomes are in pairs, so an organism usually has two alleles of each gene: one inherited from each parent. Individuals can have different combinations of alleles, so they have different inherited characteristics.

The alleles of a gene have slightly different base sequences, so they can code for slightly different versions of the same protein. This is why a difference in alleles can cause a difference in a characteristic.

Common mistake:

An allele is not a different gene. It is a different version of the same gene, found at the same position on a chromosome.

Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026

Frequently asked questions

What is a genotype and phenotype?

A genotype is the alleles an organism has for a gene, such as Tt. A phenotype is the characteristics that are expressed and can be observed, such as being tall. A heterozygous plant, Tt, has the phenotype of the dominant allele, so it is tall even though it also carries the allele for short.

What is a Punnett square used for in genetics?

A Punnett square is used to show the possible offspring of a genetic cross. The gametes from each parent are written along two sides, then combined in the boxes to give the possible genotypes. Counting the boxes gives the ratio, probability or percentage of each genotype and phenotype in the offspring.

What is the difference between homozygous and heterozygous?

Homozygous means having two identical alleles of a gene, such as TT or tt. Heterozygous means having two different alleles of a gene, such as Tt. A heterozygous organism shows the dominant characteristic, because a dominant allele is expressed in the phenotype even when only one copy is present.

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