Is asd hereditary

There is a large genetic component to autism. There seem to be two genetic systems at work. Firstly, there are many common genetic variations associated with autism, that occur in the population as a whole, and some of these appear to be beneficial - some are linked to higher than average educational achievement. The person with diagnosable autism just has a much higher concentration of these genetic variations than most of the population. The size of the individual variation tends to be small, relatively few base-pairs are affected in the case of each variant. Secondly, are rarer genetic variations, these can arise spontaneously, that is neither parent had them. These tend to be larger, such as duplication events or deletion events, where a length of DNA is  repeated, or it is removed completely. The autism caused by the rarer variations tends to be accompanied by more serious comorbidities, such as intellectual disability. Genetics is not the whole story, however, as environmental factors - such as a difficult birth -  can also contribute.

There is a large genetic component to autism. There seem to be two genetic systems at work. Firstly, there are many common genetic variations associated with autism, that occur in the population as a whole, and some of these appear to be beneficial - some are linked to higher than average educational achievement. The person with diagnosable autism just has a much higher concentration of these genetic variations than most of the population. The size of the individual variation tends to be small, relatively few base-pairs are affected in the case of each variant. Secondly, are rarer genetic variations, these can arise spontaneously, that is neither parent had them. These tend to be larger, such as duplication events or deletion events, where a length of DNA is  repeated, or it is removed completely. The autism caused by the rarer variations tends to be accompanied by more serious comorbidities, such as intellectual disability. Genetics is not the whole story, however, as environmental factors - such as a difficult birth -  can also contribute.

Yes, people tend to assume its all about inheritance and some even worry about passing it on, buts not that simple.  Also, autism isn't a specific gene, its a combination of genes that might affect the brains development.   Inheritance can pass them on, but random genetic variations could also mean these occur without any autism history. Even if you have autism yourself, any children you have might not develop enough difficulties or difference to be diagnosed with autism, or they may be more affected.

Differences in the brains development are usually in place before birth, but as the brain keeps developing after birth then yes environment factors can come into play - changes in the brains development due to stress or damage.  Gets complicated.

Diagnosable autism is quite widespread, 1 in 100 people have a diagnosis but many more could probably be diagnosed but may not realise or want to, or able to.  The genes responsible are much more widespread.  People can say "we are all a bit autistic" or "we are all on the spectrum" and that is kind of true in the sense we can all have the odd difficulty here and there, but to be diagnosed with autism takes more than one or two difficulties, and usually in different areas.