My husband is autistic. However, he was born and brought up in the 1960s, when the educational approach was much closer to 'normalisation' than is is today. That meant he learnt to sit on his hands to prevent himself from stimming, and he learnt that being bullied was the consequence of being 'strange'. He left school early and worked on farms, and later as a caretaker for municipal buildings. The solitary nature of the jobs and the routine really helped him. By this time he had found out it was a lot easier to socialise when he was drunk, so he spent a lot of his time in the pub.
We met and married when he could no longer work (anxiety and depression are close companions of masking). I knew nothing about autism, but there was a sweetness and vulnerability about my husband that I loved. Our eldest son was born.
It was obvious from the age of two that our son was developing atypically. He had speech and language therapy, but needed this on a one to one basis. He had one to one support throughout pre-school and school. He was diagnosed with autism when he was seven. After speaking to my mother in law, it became obvious that my husband had similar developmental problems in his early and primary years, but many of these were ignored or made light of. My husband was considered 'lazy' because he had toilet accidents frequently in school, and 'shy' because he did not talk very well or very frequently. My husband received his diagnosis in his early fifties.
If we compare my husband's and my son's lives superficially, it would seem that my son is much more 'severely' autistic because he has needed significant support from an early age. However, this would not be true. My husband needed the support but did not get it. He had the same needs, but these were not recognised.
My husband is now in a care home due to the effects of his very significant mental illness (late onset schizophrenia) which may or may not have been impacted by years of trying to conform to neurotypical expectations.
Yet, there are other factors. My husband and son have always been extremely anxious and prone to meltdowns and shutdowns. This means that the effects of the home environment and heritable (learnt) behaviours need to be considered when I reflect on my eldest son's development. There may also be a genetic and heritable component to mental illness, and my eldest spent three years in residential care as an adult due to the effects of his mental illnesses rather than autism. He is now in supported living. My youngest son also has serious mental health difficulties and is also awaiting assessment for autism/ADHD. I am sure there is a genetic component, but I also think the environment and his negative experience at school and possibly at home have impacted the presentation of his behaviours.
I am not suggesting that the situation in my family is the norm, but I think genetics, environment and the co-occurrence of other conditions and illnesses all play a part in terms of how 'severely' an individual appears to be affected.