What is the best description of the genetic contribution to the onset of autism?

The genetic contribution to autism, also known as Autism Spectrum Disorder (ASD), is a complex and multifaceted topic. The best description of the genetic influence on autism is that it is polygenic and multifactorial. This means that there are many genetic factors that contribute to the risk of developing autism, and these factors interact with environmental influences as well.

Autism is highly heritable, with genetics accounting for an estimated 50-90% of the risk. Studies of families, including twin and sibling studies, have shown that if one identical twin has autism, there's a high likelihood that the other will too, indicating a strong genetic component.

However, there is not a single "autism gene." Instead, there are many genes that have been identified as associated with autism. These genes are involved in various aspects of brain development and function. A few of these genes have a strong effect by themselves (such as mutations in the CHD8 or SHANK3 genes), whereas most have a smaller, additive effect.

Copy number variations (CNVs)—which involve duplications or deletions of large segments of DNA—have also been implicated in autism. Moreover, de novo mutations, which are new genetic changes that arise in an individual and are not present in their parents, play a significant role in some cases of autism.

Furthermore, genetic factors may interact with environmental factors, such as parental age at conception, prenatal exposure to certain drugs or chemicals, and very low birth weight, which can also influence the likelihood of developing autism. These complex interactions between multiple genetic and environmental factors make it challenging to predict the onset of autism based solely on genetics.