978-1572305441

Sally, Ann, and Danny 121
and has no play skills because of profound learning disability, the child
may display behaviors also seen in higher-functioning children with
autism. In these cases, the autistic features are a result of the severe cognitive
disability, not necessarily a result of autism. Why some children
with tuberous sclerosis develop autism and others do not is a mystery.
In general, the more severe the cognitive disability, whatever the cause,
the greater the likelihood that signs of autism will also be present,
though this is not always the case.
Of the other ninety percent of cases of ASD without an accompanying
neurological disorder we know even less, but certainly more than
we used to. We know the disorder is inherited in some fashion; autism
and ASD are genetic disorders, though what is inherited, and how, is
still very much an open question. About three to five percent of the siblings
of children with autism also have autism. Although this is a very
low rate, it’s much more common than in the general population
(roughly two per thousand), suggesting that autism runs in families.
But the best evidence for the genetic cause of autism comes from
comparisons of twins, at least one of whom has autism. Several studies
have compared the rates of autism in the co-twins of identical and fraternal
twins. Twins share the same intrauterine environment but differ
essentially in the number of genes they have in common. Identical
twins share one hundred percent of their genes, whereas fraternal twins
share, on average, fifty percent of their genes. The results of these twin
studies are conclusive; the identical co-twins of autistic children have
autism much more frequently than fraternal co-twins. This can be explained
only by the action of genes that confer susceptibility to autism
and ASD. The chance that non-twin siblings will have autism is three to
five percent, much lower than the incidence in identical twins. This
must be because multiple genes are involved in the etiology or some
environmental factor interacts with genetic susceptibility. This does not
mean that environmental factors are irrelevant; in fact there is good evidence
that thalidomide and maternal anticonvulsants taken during
pregnancy may cause autism. Other environmental risk factors may exist
(but have not yet been discovered in spite of years of research), and if
they do, they may exert their influence in the context of genetic vulnerability.
But the genetics of the disorder are complex. There are at least four
findings that cannot be explained by our current understanding. First,
not all identical co-twins are affected—usually about sixty percent (a
finding similar to many other developmental and neurological disor-