Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by differences in communication, social interaction, and patterns of behavior. Many wonder if autism can be passed down through generations. Research has made progress in understanding autism’s contributing factors, with genetics playing a prominent role.
The Genetic Basis of Autism
Genetics are a primary factor contributing to autism spectrum disorder. Evidence for this connection comes from extensive studies comparing identical and fraternal twins. Identical twins share nearly all of their genetic material; if one identical twin has an ASD diagnosis, the likelihood of the other twin also being diagnosed ranges from approximately 60% to 90%.
Fraternal twins, who share about half of their DNA, similar to non-twin siblings, show significantly lower concordance rates, typically between 3% and 36%. This difference in concordance between identical and fraternal twins underscores a substantial genetic influence on autism development. These findings from twin studies are further supported by family studies that examine recurrence rates in siblings.
If a family has one child diagnosed with autism, the chance of a younger sibling also receiving an ASD diagnosis is considerably elevated compared to the general population. Recent research indicates that this sibling recurrence rate is around 18.7% to 20.2%, which is significantly higher than the general population prevalence of about 2.8%. The likelihood of recurrence can also be influenced by factors such as the infant’s sex (male infants having a higher rate) and the number of previously affected siblings in the family.
How Autism is Inherited
The inheritance of autism is complex, and it is not typically caused by a single “autism gene.” Instead, its genetic architecture often involves contributions from many different genes, each having a small effect on overall risk. This concept is known as polygenic inheritance, where common genetic variations distributed across an individual’s genome combine to influence susceptibility. Children with autism tend to inherit a higher cumulative “polygenic risk score” from their parents than would be expected by chance.
Beyond the combined effects of many common genetic variants, new genetic changes, called de novo mutations, also play a role in autism. These mutations are not inherited from either parent but arise spontaneously in the sperm or egg cells, or during the very early stages of embryonic development.
De novo mutations, particularly those that are likely to disrupt gene function, contribute to a notable percentage of autism cases, especially in families with no prior history of the condition. Estimates suggest that these mutations may account for approximately 30% to 45% of autism diagnoses in families where only one child is affected. While they are more prevalent in families with no known history of autism, de novo mutations can also occur in families with multiple affected members, though to a lesser extent.
Environmental and Other Contributing Factors
While genetics are a major contributor to autism, environmental factors can interact with an individual’s genetic predisposition to influence development. These environmental influences do not typically cause autism on their own but rather modulate risk in individuals who are already genetically susceptible.
One consistently identified environmental factor is advanced parental age. Studies have shown an increased risk of autism in children born to older parents. For every 10-year increase in a mother’s or father’s age, the risk of a child developing autism rises by approximately 18% to 21%.
Certain prenatal exposures and maternal health conditions during pregnancy have also been linked to increased autism risk. Maternal infections during gestation, such as viral infections in the first trimester or bacterial infections in the second, have been associated with increased odds of autism. Maternal health conditions like gestational diabetes can nearly double the risk. Exposure to specific medications during pregnancy, such as valproate, or environmental toxins like air pollution and certain pesticides, are areas of ongoing investigation for their influence on autism development.
Genetic Counseling and Testing
Genetic counseling provides families with valuable information regarding the genetic aspects of autism spectrum disorder. During a counseling session, a genetic counselor will evaluate the patient’s clinical history and assess the family’s medical background to identify any patterns or clues. This process helps families understand potential genetic contributions to autism and discuss the various testing options available.
Genetic testing itself does not provide an autism diagnosis, but it can identify specific genetic variations associated with the condition. Genetic testing is recommended as part of a comprehensive evaluation for all children diagnosed with autism or intellectual disability. One common initial test is Chromosomal Microarray Analysis (CMA), which can detect small missing or extra segments of chromosomes, known as copy number variants (CNVs). CMA offers a higher detection rate for such abnormalities, typically between 10% and 20%, compared to older chromosomal analysis methods.
If CMA results are negative, whole exome sequencing (WES) may be recommended, which examines the protein-coding regions of genes for smaller, more subtle changes or “spelling mistakes” in the DNA sequence. While genetic testing can identify a specific genetic cause in approximately 25% to 40% of autism cases, it is important to understand its limitations. When a genetic variation is identified, it can offer insights into potential risk factors for future children and, in some instances, guide specific medical management or therapeutic approaches.