to Autistic Spectrum Disorders:
Diversity and Insufficiency
There is strong evidence that genes underpin autism and autistic spectrum disorders. In other words, ASD only develops in susceptible children – and the susceptibility is dictated by particular gene variants (alleles) or combinations of variants. One task for modern genetic research is to identify these genes, with the hope that they might possibly indicate therapies or even preventative measures. However, this area is fraught with difficulty, not least the conclusion that the data clearly distinguish ASD from single-gene conditions like cystic fibrosis or sickle-cell anemia, and point away from an [autism gene] that might underpin most ASD cases.
The clearest line of evidence for a genetic predisposition comes from studying close relatives of subjects with ASD. Twins are particularly informative. Studies on identical (monozygotic, MZ) twins, where one is affected by ASD, show that most of the other co-twins have the same condition, i.e. are concordant. An early study put concordance at well over 60% in MZ twins, while there was no concordance (0%) between non-identical (dizygotic, DZ) twin pairs.1 Generally, monozygotic concordance in ASD exceeds dizygotic concordance by a large margin,1– 4 as reviewed.5