In Principles of Neural Science 5th edition, Chapter 3, It Is said that Autism Is a genetic disorder but which genes are involved Is not clear, how do we know there Is a genetic basis and Is not a response to environmental factors (or a combination)?
There is a clear heritable component that seems to be "highly genetically heterogeneous and may be caused by both inheritable and de novo gene variations" (Rylaarsdam, Gomboa, 2019). There has been hundreds of genes implicated in autism and a complex interaction of non-heritable gene variations as well, "genetic modifiers such as copy number variation, single nucleotide polymorphisms, and epigenetic alterations likely play a key role" (Rylaarsdam, Gomboa, 2019).
The Rylaarsdam review article summarises the current state of play in terms of the genetic and non-heritable gene factors as well.
Rylaarsdam, L., & Guemez Gamboa, A. D. (2019). Genetic Causes and Modifiers of Autism Spectrum Disorder. Frontiers in Cellular Neuroscience, 13, . https://doi.org/10.3389/fncel.2019.00385