For years, biologists have wondered how it is possible that not every person who carries a mutated gene expresses the trait or condition associated with the mutation. This common but poorly understood phenomenon, known as incomplete penetrance, exists in a wide range of organisms, including humans.

Many mutations in genes that are linked to diseases, including Parkinson’s disease and Type 1 diabetes, are incompletely penetrant. Some of this variation may be due to environmental factors and the influence of other genes, but not all: It has been shown that genetically identical organisms living in the same environment can show variability in some incompletely penetrant traits.

Now, a team of MIT biophysicists has demonstrated that some cases of incomplete penetrance are controlled by random fluctuations in gene expression.

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