Penetrance and expressivity

Penetrance is the share of carriers of a genotype who actually show the trait. With full penetrance all carriers are affected, with incomplete penetrance only some. Expressivity describes how strongly the trait shows in those affected. Both explain why a risk allele is not a verdict.

PENETRANCE AND EXPRESSIVITY Penetrance the share of carriers who show the trait Example: 3 of 10 carriers show the trait, 7 stay healthy. Expressivity how strongly it shows in those affected from mild (left) to strong (right) A risk allele is therefore not a verdict: carriers often stay healthy.

Penetrance

Carrying a particular genotype does not mean the trait will appear. Penetrance measures in what share of carriers it does. For some rare hereditary diseases it is nearly complete. For most common risk variants, such as ApoE-e4 for Alzheimer's, it is low: many carriers never fall ill, many patients do not carry the variant at all.

Expressivity

Even when a trait appears, it can vary in strength. This range is called expressivity. The same variant can be barely noticeable in one person and pronounced in another. Age, other genes, environment and chance shift the picture. Penetrance asks whether, expressivity asks how strongly.

Why this matters for risk findings

Most genetic risks Genome presents have low penetrance. An elevated relative risk means a shifted probability, not a fixed outcome. A finding is therefore a pointer for attention and lifestyle, not a diagnosis. Genome shows the genotype; what it means depends on penetrance and context.

What Genome measures. Genome names a genotype. Incomplete penetrance is the reason a risk variant often does not become disease. The interpretation is given by the respective article.

Related topics

Allele, genotype and phenotypeApoEDisease associationsGWAS and polygenic scores

Sources

  1. 1Cooper et al., 2013 Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease. Human Genetics 132:1077–1130. doi.org/10.1007/s00439-013-1331-2
  2. 2Chen et al., 2016 Analysis of 589,306 genomes identifies individuals resilient to severe Mendelian childhood diseases. Nature Biotechnology 34:531–538. doi.org/10.1038/nbt.3514