Genetic chameleons: remember the relapsing disorders.

Our genetic code is satisfyingly reliable. It does not suffer circadian fluctuations—there is no need for an ‘early morning’ or ‘fasting’ DNA test. It does not transmute over time—there is no need to resample or to compare results with an aged-matched sample. It does not suffer the vagaries of the test–retest variation as with an erythrocyte sedimentation rate or serum creatine kinase. The famous twisted-ladder of the double helix structure provides great physical and chemical stability. This inherent steadfastness is the tenet on which Michael Crichton’s Jurassic Park is based, as well as countless real and fictionalised court-room dramas. These features may predicate the optics through which we view a genetic disorder: indelible, inevitable, fixed and inert. In reality, no branch of medicine is as straightforward as we were initially taught—and medical genetics is unrecognisable from the undergraduate lectures of only a couple of decades ago. Somatic variation, mosaicism and heteroplasmy are but some of the mechanisms that undermine the superficial but false impression of genetic constancy.Consider the gamut of genetic disorders that break from this archetype, such as the paroxysmal conditions. There are the familial hemiplegic migraine disorders ( CACNA1A, ATP1A2, SCN1A, PRRT2 ) that produce spells on a spectrum …