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The shortest ‘arm’ of a chromosome, based on the distance from the centromere. The ‘p’ comes from the word ‘petit’, the French translation of ‘small’.

Use in clinical context

Each chromosome has a centromere, which can be found anywhere along the length of the chromosome, except within the telomeres. Although centromeres can be located exactly in the middle of chromosomes, this is not the case and the sections on either side of the centromere (the ‘arms’) are of different lengths. The p arm is the shortest and can be tiny in comparison to the other arm – the q arm – or it can be of similar size and 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 in which variants can cause familial hypercholesterolemia, is found on chromosome 2 at position 24.1 on the p arm and would be written as 2p24.1.

Related terms

Centromere | Chromosome | q arm | Telomeres

Last updated on 30th May, 2019