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I work in neuroscience, mostly Alzheimer's disease (AD), with some work in autism spectrum disorder (ASD). The work is in gene regulation and epigenomics. I'm familiar with monozygotic twin (MZ) discordance for AD and ASD and the methylome studies done in discordant MZ for those disorders. I began to become curious.

What disorders have never had MZ discordance reported in the literature? Even sickle cell has MZ discordant progression (Amin et al. 1991. Hemoglobin. 15:247-56). Do any disorders lack MZ discordance?

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For those that don't know about it, could you expand a little on what MZ discordance is and what it means for a disease? – MattDMo Apr 9 '14 at 20:17
MZ discordance is when MZ twins do not have the same phenotype. Twin A gets Alzheimer's. Twin B does not. If Alzheimer's were fully genetic, both twins would get it, since their genomes are (very close to) identical. The higher MZ discordance for a disorder, the more effect environment is presumed to play. What is interesting is when a very Mendelian disease, like sickle cell, still shows MZ discordance on symptoms or a disorder with high genetic risk, like ASD, still shows non-patient/patient MZ discordance. My question means "Do any diseases have airtight evidence as being 100% 'genetic'"? – Bryan Apr 14 '14 at 18:52
Complete concordance is generally rare because environment will always have some influence on the trait. In general all disorders that are not quite lethal are susceptible to environmental effects and buffering – WYSIWYG Mar 18 at 15:18

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