Many studies show that IQ is highly heritable, about .75 out of 1, which is almost as high as height. Therefore if two people with high IQs have a child, the child is very likely to have a high IQ too, and likewise couples with average or low IQs will produce children with similar IQs most of the time.
But what about when their child has one of the genetic disorders that affect IQ? The potential IQ range for a child with Down syndrome, the most common syndrome that causes intellectual disability (ID), is huge: most have an IQ in the range of mild intellectual disability, moderate ID is also relatively common, but still others have a severe ID and some test at low-normal or even normal IQs.
Is IQ still heritable in this case? Does it follow that a Down syndrome child of two people with IQs of say 130 is likely to end up with a milder intellectual disability than a Down syndrome child of two people with IQs 95?
Huh. I wonder if there’s never been a study on this. Given the odd topics I’ve come across while looking up abstracts for articles one of the psychiatrists I work with is writing, it’s sort of hard to believe, though.
I saw this question when you first posted it and thought it was a good and interesting one. However, I have never seen any studies that have been done on this topic. I have seen studies on just about everything else but not this.
What is being discovered is that copy number variations (CNVs) are associated with behavioral and cognitive disorders (e.g., 1q21.1 deletion, 7q31.32 deletion). These are stretches of DNA (and thus genes) that are missing, causing problems. These CNVs often also have incomplete penetrance, meaning that not everyone who has a particular CNV has an observable effect. For instance, the 1q21.1 CNV has a penetrance of about 32%, meaning that of everyone that has the deletion, only 32% will show some effect (in this case, some combination of autism, macrocephaly, failure to grow and cognitive deficits). Thus, it is possible for parents who have the deletion to be unaffected, while their children are affected.
This points toward the fascinating idea of the genetics behind personality. It could be that CNVs (which also include duplications) or other genetic anomalies are responsible for various aspects of personality or cognitive ability, and are heritable like hair and eye color. Those genetic features might have higher penetrance, making for more consistent effects in descendents.