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The longest ‘arm’ of a chromosome, based on the distance from the centromere.

Use in clinical context

Each chromosome has a centromere. This can be found anywhere along the length of the chromosome, except within the telomeres. Although centromeres can be exactly in the middle of chromosomes this is not the case and the sections on either side of the centromere (the ‘arms’) are different lengths. The q arm is the longest arm. It can be large in comparison to the other arm – the p arm – or it can be similar size or any size in between. Naming the arms of the chromosomes is of benefit when noting the locations of a gene within the genome or the location of a variant associated with a condition. For example, the location of a gene, variants in which can cause cystic fibrosis, is found on chromosome 7 at position 31.2 on the p arm and would be written as 7q31.2.

Related terms

Centromere | Chromosome | p arm | Telomeres

Last updated on 30th May, 2019