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September 04, 2019

Drawing upon state records, the researchers initially identified 1,156 twin pairs, with at least one member affected by an ASD, born to California mothers between 1987 and 2004. The children were all at least 4 years old, an age when autism can be reliably diagnosed. Ultimately, this group was winnowed to 192 twin pairs - 54 identical and 138 fraternal - for genetic analysis. Since autism disproportionately affects males, males outnumbered females by four to five times, with 80 of the pairs including both sexes.

Concordance for ASD was 77 percent among identical male pairs, and 31 percent among fraternal male pairs. In females, concordance for ASD was more closely spaced - 50% for identical and 36% for fraternal pairs. By contrast, previous studies had found concordance rates for fraternal twins that were much lower, ranging only in the single digits.

"Spectrum disorders traditionally thought to have less genetic loading turn out to stem from a similar mix of environmental and genetic heritability as narrowly defined autism," noted Thomas Lehner, Ph.D., chief of the NIMH Genomics Research Branch.

Yet, there can also be genetic influences that are not inherited from parents. New evidence emerged last month that rare, spontaneous mutations occur at abnormally high rates in autism.

"Such non-inherited genetic changes were proposed as a major mechanism of autism susceptibility, based on the very low concordance among fraternal twins found in earlier studies and evidence of increased risk associated with older parental age," explained Risch. "In light of the high fraternal twin concordance observed in our study, such new mutations may play a more limited role, since they would primarily occur in only one member of a fraternal pair, which would not lead to concordance."

Source: NIH/National Institute of Mental Health