Genetic heterogeneity refers to
either: a) a multiplicity of mutations within some gene and a spectrum of corresponding values for a trait (or “phenotype”), each of which varies little within the typical range of locations (i.e., allelic heterogeneity); or b) the trait exists if any one of a range of loci has the atypical form (i.e., locus heterogeneity).
For example, in the case of phenylketonuria (PKU)
differences remain among individuals with PKU because hundreds of mutations have been identified in the PAH gene; any individual with PKU may have one pair of mutations out of hundreds of thousands of possible pairs. Efforts are underway to classify mutations as mild, moderate, or severe and as responsive to the drug BH4, which allows a higher-protein diet. Tailoring the dietary modification will require acceptance and application of the diagnostic classification of mutations, secondary screening to place individuals in the right category, and maintenance of the appropriately calibrated diet despite the issues noted in the previous paragraph and some new ones—access to BH4 and keeping individuals who take it from going off the special diet altogether, which is not medically recommended (Paul and Brosco 2013, 108-109).
The complications are evident already above, as researchers look to make the distinctions between different variants of the PAH gene and medicine bases its actions on the resulting knowledge. Complementary complications arise from treating people with a mutation in a gene, e.g., PAH, as if they had the same condition.
Quotes are drawn from Taylor, P. J. (2014) Nature-Nurture? No: Moving the Sciences of Variation and Heredity Beyond the Gaps.
Paul, D. B. and J. P. Brosco (2013). The PKU Paradox: A Short History of a Genetic Disease. Baltimore: Johns Hopkins University.
(Introduction to this series of posts)