Genetic terms

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Genetic termsSpec 2.15

In short

A gene is a length of DNA coding for a polypeptide, and an allele is one version of a gene. The genotype is the alleles an organism has, and the phenotype is its observable characteristics. A dominant allele shows in the heterozygote, a recessive allele only in the homozygote, and codominant alleles both show.

Gene: A sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain, found at a particular locus on a chromosome.
Allele: One of the different forms of a gene.
Genotype: The alleles that an organism has for a particular gene (or genes).
Phenotype: The observable characteristics of an organism, resulting from its genotype and the environment.
Dominant: An allele that is expressed in the phenotype when present in only one copy (in the heterozygote).
Recessive: An allele that is expressed in the phenotype only when two copies are present (in the homozygote).
Codominance: Both alleles are expressed in the phenotype of the heterozygote, because neither is dominant over the other.
Homozygote: An organism with two identical alleles of a gene.
Heterozygote: An organism with two different alleles of a gene.

A person has two alleles of each gene, one inherited from each parent. A dominant allele is shown by a capital letter (for example B) and a recessive allele by a lower-case letter (b). The genotype BB is homozygous dominant, bb is homozygous recessive, and Bb is heterozygous.

Codominance

In codominance, two alleles are both expressed. For example, the human ABO blood group gene has alleles Iᴬ, Iᴮ and Iᴼ. Iᴬ and Iᴮ are codominant, so a person with genotype IᴬIᴮ has blood group AB. Both are dominant over Iᴼ, so IᴬIᴼ has group A and IᴮIᴼ has group B, and only IᴼIᴼ has group O. When codominant alleles are used, the capital letter is the gene and the superscripts are the alleles.

Common mistake:

A dominant allele is not necessarily common or 'stronger'. It just shows in the phenotype in the heterozygote. Also, a 'carrier' has one copy of a recessive allele and does not show the condition.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between an allele and a gene?

A gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide. An allele is one of the different forms of that gene. A person has two alleles of each gene, one from each parent, and these may be the same or different.

How do you tell from a pedigree diagram whether an allele is dominant or recessive?

If two unaffected parents have an affected child, the allele for the condition is recessive and both parents are carriers. If two affected parents have an unaffected child, the allele is dominant and both parents are heterozygous. Always name the individuals and give their genotypes.

Why does cystic fibrosis cause problems in the lungs, digestive system and reproductive system?

The faulty CFTR chloride channel means less water moves out of cells by osmosis, so mucus is thick and sticky. It blocks the airways, causing infections, blocks the pancreatic duct so enzymes cannot reach the intestine, and blocks the tubes that carry sperm.

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