Many researchers believe that autism may be a result of a combination of genetic and environmental factors. The identification of specific genes which may be responsible for the development of autism remains a major research focus with the objective of developing an understanding of the role genetics play in the underlying pathophysiology of the disorder.
The role of genetics in autism is supported by studies of monozygotic (identical) twins which have shown a concordance rate of 60 to 90% for autism. The incidence of autism in children who have an older sibling diagnosed with autism has also been shown to be 25 times higher than in the general population.
While researchers have identified a number of genes which may likely play a role in the development of autism, most genes which have been shown to be associated with autism are represented in less than 1% of individuals with autism spectrum disorders.
While autism is not typically the result of a single-gene mutation or a single chromosome abnormality, there are single gene disorders like Fragile X and Rett Syndrome which are associated with developmental delays in children.
While numerous studies have demonstrated a strong genetic basis for autism spectrum disorders, the identification of numerous genetic markers shown to be associated with autism demonstrate a complex inheritance pattern. Although genetic testing for autism is utilized to identify copy number variants (CNVs) having a strong association with autism, these CNVs are found in less than 10% of affected individuals. Adding de novo rare mutations to such CNVs or other chromosomal rearrangements only explain 10 to 20% of autism cases.
Recently a number of common genetic variants or SNPs (single nucleotide polymorphisms) have been strongly linked to autism. While individual SNPs are not on their own sufficient to be causal, recent studies have shown that the combination of autism associated SNPs may increase the risk of autism in siblings of affected children.
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