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I think the Muscular Dystrophy Association gives the best explanation of the conundrum that MG is not hereditary but has a genetic component:
What is the genetic susceptibility in MG?
Although MG and other autoimmune diseases are not hereditary, genetic susceptibility does appear to play a role. It seems likely that genetic factors also contribute to the pathogenesis of MG. Certain human leukocyte antigen (HLA) types, cell-surface proteins that are responsible for the regulation of the immune system, have been associated with myasthenia, including HLA-B8, DRw3, and DQw2. MuSK antibody-positive myasthenia is associated with haplotypes (clusters of genes inherited together) DR14 and DQ5.Most studies suggest that if people have a relative with an autoimmune disease, their risk of getting an autoimmune disease is increased — the closer the relative, the higher the risk.
Even for identical twins, however, that risk is relatively small. Most studies suggest that when one twin has an autoimmune disease, the other has less than a 50% chance of getting the same disease.
Also, people who already have one autoimmune disease have a greater risk of developing another one. It is estimated that 5% to 10% of people with MG have another autoimmune disease that appeared before or after the onset of MG. The most common of these are autoimmune thyroid disease, rheumatoid arthritis, and systemic lupus erythematosus (a disease that affects multiple organs).
References- Carlsson, B., Wallin, J., Pirskanen, R., Matell, G. & Smith, C. I. E. Different HLA DR-DQ associations in subgroups of idiopathic myasthenia gravis. Immunogenetics (1990). doi:10.1007/BF02115001
- Niks, E. H. et al. Strong association of MuSK antibody-positive myasthenia gravis and HLA-DR14-DQ5. Neurology (2006). doi:10.1212/01.wnl.0000218159.79769.5c
Borrowed from the website: https://www.mda.org/disease/myasthenia-gravis/causes-inheritance
Hope this is helpful.
I believe I am in the category that I have another autoimmune disease (celiac), and MG piled on in 2014 at 57.