Just shows that I know fuck all about genetics. Considering concordance I am not sure if it's true (again, can't comment as I just don't know). From wiki problems with concordance studies:
There are several problems with this assumption:
A given genetic pattern may not have 100% penetrance, in which case it may have different phenotypic consequences in genetically identical individuals;
Developmental and environmental conditions may be different for genetically identical individuals. If developmental and environmental conditions contribute to the development of the disease or other characteristic, there can be differences in the outcome of genetically identical individuals;
The logic is further complicated if the characteristic is polygenic, i.e., caused by differences in more than one gene.
Epigenetic effects can alter the genetic expressions in twins through varied factors. The expression of the epigenetic effect is typically weakest when the twins are young and increases as the identical twins grow older.[2]
Where in the absence of one or more environmental factors a condition will not develop in an individual, even with high concordance rates, the proximate cause is environmental, with strong genetic influence: thus "a substantial role of genetic factors does not preclude the possibility that the development of the disease can be modified by environmental intervention." So "genetic factors are assumed to contribute to the development of that disease", but cannot be assumed alone to be causal.[3]\
Edit: Another article:
https://www.scientificamerican.com/article/identical-twins-genes-are-not-identical/Edit: Another study:
https://pubmed.ncbi.nlm.nih.gov/22146048/
