Just this month an article was published in the Journal of the American Medical Association which assessed the familial risk of autism. This was a gigantic population based study on Swedish children born between 1982 and 2006, which looked at trends in autism spectrum diagnosis amongst monozygotic (identical) and dizygotic (fraternal) twins, full siblings, half siblings, and cousins. Groups were broken down into multiplex (autism occurs more than once) and simplex (only a single child with the diagnosis), so as to look at differences in heritability.
They found that simplex cases of autism, whether twins, full siblings, half siblings, or cousins, ranged from 27-94 cases per 100,000 people per year, which is a fairly small spread, indicating little in the way of heritability (unsurprisingly). On the other hand, monozygotic twins concordant for autism occurred in approximately 6,274 cases per 100,000 people per year, calculated as 59.2% heritability. Meanwhile, dizygotic twins and full siblings exhibited equal rates of heritability, at 12.9% each. Maternally-related half siblings exhibited 8.6% heritability, paternally-related half siblings 6.8%, and cousins 2.6%. In short, the team concluded that, though autism was highly heritable, it was still about 50/50 genes vs. environment, and particularly postnatal environment, since dizygotic twins showed no differences in rates of heritability as compared to full siblings.
What I find absolutely intriguing is that if one looks at these percentages, particularly those of the dizygotic twins/full siblings vs. half siblings vs. cousins, these figures decline by 1/3rd or ~5% across these groupings (see graph below).
A graph based off the data reported by Sandin et al. (2014) on multiplex families. MMT = multiplex monozygotic twins, MDT = multiplex dizygotic twins, MMH = multiplex maternal half siblings, MPH = multiplex paternal half siblings, MC = multiplex cousins.
Maybe because we know autism in multiplex families is supposed to be heritable, this means that the idea of decreasing heritability the further we get in relatedness is a minor no-brainer. On the other hand, what it suggests to me is that there are discrete units of heritability that are either being inherited or not inherited which ultimately promote autism, with increasing probability the closer one comes to monozygotic twins. Therefore, to me, there’s no no-brainer about it.
Inherited genes are one type of discrete unit with which we’re all familiar and could well explain what were seeing here. However, I just want to remind the reader that genetic inheritance is not the only type of discrete unit of inheritance and that information can lie above the gene (epigenetics) and be inherited as well, though these processes are far less well understood at present.
Finally, this study offers estimates as far as prevalence of simplex versus multiplex cases. It suggests that in Sweden, and probably around the world, simplex cases are comparatively rare, meanwhile cases of autism which have an inherited component are in the large majority. This is a really important point to make, because it can help to pinpoint our research endeavors. Ultimately, we are probably looking for a combination of inherited and environmental triggers, both of which will likely vary by case.
This was an excellent, thorough, and well-designed epidemiological study on the incidence of autism spectrum conditions in a large population study. If you get a chance, I highly recommend a look at the original article by Sandin et al.