Genetic testing in autism: what a test can and cannot tell you

Who is offered a test?

Autism alone is not a reason for a genetic test on the NHS (National Health Service) in England. Nor is a mild learning disability on its own.¹

Since 1 April 2026, a child can have whole genome sequencing under the test code R27 (Paediatric disorders) if they have any of these:^1,2

• at least a moderate learning (intellectual) disability, with no known cause
• a moderately severe global developmental delay, meaning late in several areas such as movement, speech and play, with no known cause
• congenital anomalies (differences present from birth)
• dysmorphic features (unusual physical features).

Epilepsy on its own is not one of the R27 criteria.^1,2

A paediatrician can order the test directly for a child with at least a moderate learning disability. Referral to clinical genetics is needed only when there are signs of a syndrome, such as subtle differences in facial features or unusual growth. The doctor talks the test through with the family first and records that discussion on a standard form.¹

NICE (the National Institute for Health and Care Excellence) guideline CG128, Autism spectrum disorder in under 19s: recognition, referral and diagnosis, says medical tests should not be a routine part of an autism assessment. It says to consider genetic tests when a child has specific dysmorphic features, congenital anomalies or a learning (intellectual) disability.³

The NHS North Thames Genomic Medicine Service has short videos explaining whole genome sequencing. One is for families of children with a learning disability: Whole genome sequencing: learning disabilities (4 minutes). A longer general version is available in English, Bengali, Gujarati, Romanian and Urdu: all whole genome sequencing videos.

How is autism inherited?

Parents often ask whether they passed it on. There is rarely a single answer, because the genetics of autism works in several ways.

Many common variants

In most autistic people, the genetic part comes from many common variants. Each has a tiny effect. They come from both parents and act together with other factors. A clinical test does not report them.⁴

A new change in the child

Sometimes the cause is a single change that is new in the child. It was not inherited from either parent. This is called de novo. Not every new change causes a condition. If one does, the chance of it happening again in a brother or sister is usually low.

A single inherited change

Less often, the cause is one inherited gene change. If both parents carry a recessive change in the same gene, each pregnancy has a 1 in 4 chance. If a parent has a dominant change, each pregnancy has a 1 in 2 chance. These figures apply only once the specific cause is known. In X-linked conditions such as fragile X syndrome, boys are usually more severely affected. A genetics team gives the figures for your family.

How heritable is autism?

A study of 2 million children in five countries found that genetic factors explained about 80% of the differences in autism between people.⁵ This is a population figure. It does not predict what will happen in one family.

The chance for a younger brother or sister

An international study followed 1,605 babies who had an autistic older brother or sister. By age 3, 20% were autistic. The figure was 37% when two or more older siblings were autistic, and 21% when one was. It was 25% for boys and 13% for girls.⁶ These figures apply to families in general, not to families where a genetic cause has been found.

What the test looks for

Whole genome sequencing reads almost all of a child's DNA, about 3 billion letters. Since April 2026 it is the first test for children who meet the R27 criteria in England. The older test, microarray, now comes first only when a child's features strongly suggest a specific chromosome condition, such as Williams syndrome or 22q11 deletion syndrome.¹

The test can find three kinds of change:¹

• Large changes: a whole chromosome extra or missing, as in Down syndrome or XXY (Klinefelter syndrome), or a large piece moved or rearranged.
• Copy number variants (CNVs): a stretch of DNA missing (a deletion) or extra (a duplication).
• Single nucleotide variants (SNVs) and small insertions or deletions: one DNA letter changed, or a few letters added or lost, inside a gene.

The laboratory does not interpret every gene. It analyses an agreed list of genes linked to the child's difficulties, called a virtual panel. For R27 this is the Paediatric disorders panel.¹ A normal report means nothing relevant was found on that list.

The NHS England target for a whole genome result is 84 days. Laboratories warn that results may take longer because of their workload.¹ If you have heard nothing after three months, ask the doctor who ordered the test.

What a result can tell you

Most children who are tested do not get a clear answer. In a 2026 Belgian study of children and adults with developmental disorders, genome sequencing found a diagnosis in 40%.⁷

A clear result (pathogenic or likely pathogenic) explains, or very probably explains, the child's difficulties. It can lead to specific health checks and accurate advice about future pregnancies. It rarely changes autism support itself.

A variant of uncertain significance (VUS) is a change the laboratory cannot yet interpret. It is not a diagnosis.

No finding does not mean the cause is not genetic. The genome data are stored and can be looked at again as new genes are discovered.