What is the risk of having a second child with autism? It is interesting to know that it is higher (7%) if the first child with autism was a girl than if it was a boy ( 4%). If a couple has 2 children with autism, the risk of having yet autistic child shoots up to 33-50%.
The genetic architecture of autism is complicated. Largely it is due to the presence of 2 or more low or moderate risk variants which are difficult to identify.https://dx.doi.org/10.1590%2FS1679-45082017RB4020
About 10% will have an abnormal chromosomal microarray. Clinical clues are dysmorphism, malformations & a family history of psychiatric disorders.
There are just a handful of monogenic disorders which are associated with autism.
The most important would be the Fragile X syndrome. It may be wise to recommend it in males with autism even in the absence of the classical phenotype and in girls who have a family history of fragile X or premature ovarian failure.
Girls with intellectual delay and autism need to be tested for the MECP2 gene.
Children with a large head > 2.5 SD need evaluation for the PTEN mutation.
If there are seizures, episodic deteriorations you may need to rule out IEM's. Some treatable IEM's like creatine deficiency disorders may be picked up on an MR spectroscopy.
The bottomline is that no single genetic test will make a diagnosis of autism. It is a clinical diagnosis but about 25% may have an underlying genetic disorder.
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