I’ve mentioned in the forum before my sister has identical twins, one who has high-functioning autism. So I shared this Business Insider article I just came across with her - she asked about heredity and autism. I thought it was an interesting question; I said possibly some of the genes for autism could still be hereditary if the de novo mutations are the ones that just tipped the scale on the phenotype for one of her sons. Correct me if I’m wrong in my understanding of that. I checked out the paper, but it’s mostly above my head. I’m posting in hopes someone finds it interesting and I can learn from the discussion. 
Despite the important role that monozygotic twins have played in genetics research, little is known about their genomic differences. Here we show that monozygotic twins differ on average by 5.2 early developmental mutations and that approximately 15% of monozygotic twins have a substantial number of these early developmental mutations specific to one of them. Using the parents and offspring of twins, we identified pre-twinning mutations. We observed instances where a twin was formed from a single cell lineage in the pre-twinning cell mass and instances where a twin was formed from several cell lineages. CpG>TpG mutations increased in frequency with embryonic development, coinciding with an increase in DNA methylation. Our results indicate that allocations of cells during development shapes genomic differences between monozygotic twins.
Phenotypic discordance between monozygotic twins has generally been attributed to the environment. This assumes that the contribution of mutations that separate monozygotic twins is negligible; however, for some diseases such as autism and other developmental disorders, a substantial component is due to de novo mutations28. Our analysis demonstrates that in 15% of monozygotic twins a substantial number of mutations are specific to one twin but not the other. This discordance suggests that in most heritability models the contribution of sequence variation to the pathogenesis of diseases with an appreciable mutational component is underestimated.