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The human genome has around 20,000 genes. Most of our genes contain instructions for the cell to make specific proteins. There are also some genes that contain instructions for the production of other molecules apart from proteins (for instance, short lengths of RNA, which regulate gene activation).

Gene location and size

Genes can vary in size from hundreds to millions of base pairs long. The size of the gene, however, does not reflect the importance of the molecule they provide the instructions for. In humans, genes are found on all 23 chromosomes. The homologues in each pair have the same genes located in the same position or locus along their length, which means that each individual has two copies of each gene, one inherited from the mother and the other from the father.

Sometimes the copies of each gene are not exactly the same and contain small differences in the DNA sequence. The term allele is used to describe each gene copy. If the two copies are exactly the same then it is the same allele, however if the two copies have variation they are called different alleles. The image below shows the loci of the gene that determines blood group and the two different alleles.

Figure 1: A homologous pair of chromosomes

The chromosome on the left came from the individual’s mother, the chromosome on the right from their father. Different alleles for blood groups are shown at the blood group locus. Blood group A is on the chromosome from the mother and group O on the chromosome from the father.

Structure of a gene

Genes have regions called exons and introns. In addition, there is a sequence in front of the gene known as a promoter.

Figure 2: Visualisation of a gene


Each gene has a sequence in front of it (upstream) that controls its expression through interaction with other molecules. This sequence is known as a promoter.


Exons are what we call the ‘coding sequence’, as they contain the information to make proteins.


Introns contain important regulatory information, but do not contain protein-coding information.

Key messages

  • Genes are stretches of DNA containing the instructions to build biological molecules such as proteins.
  • An individual usually has two copies of each gene.
  • Different versions of the same gene are called alleles.


For clinicians

Tagged: Core concepts

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  • Last reviewed: 09/05/2022
  • Next review due: 09/05/2024
  • Authors: Dr Ed Miller
  • Reviewers: Dr Siobhan Simpson