5.3 Autism genetics are complex

In autism, links are proposed between particular genetic variants, atypical development of the nervous system including the brain, and behavioural differences such as theory of mind difficulty and repetitive tendencies.

However, this is a complex and speculative field. Whereas certain conditions (e.g. cystic fibrosis) result from mutation of just a single gene, autism (except in fairly rare cases) involves the combined effects of variants in many different genes – it is said to be polygenic. Also, this combination of genes and variants may vary from one person or family to another, so autism is said to be heterogeneous. Researchers have found candidate genes (genes that may potentially transmit susceptibility to autism) on a very large number of chromosomes.

Besides this complex pattern of genetic influences, the heritability of autism (the extent to which it can be attributed to genetic factors) is not 100%. A parent may be on the autism spectrum without his or her children inevitably having autism. A child may develop autism without a family history – their genome may be altered by a new mutation, for instance arising during egg or sperm production, or by epigenetic influences, which control the action of certain genes. Other non-genetic factors may also influence the development of autism. For instance, exposure in the womb to Valproate, taken by a mother as epilepsy medication, may increase a child’s risk of developing autism (Christensen et al., 2013). Influences such as these, thought to affect the prenatal environment of the developing foetus, are not well understood at present.

(See Lai, Lombardo and Baron-Cohen, 2013 for an overview of autism findings like those discussed in this section)