In general, the answer is yes. I can’t name specific examples, but logically it must have happened and still does happen.
Down’s syndrome, for example, does cause infertility, but not all developmentally delayed people suffer from Down’s syndrome. Some cases aren’t even genetic.
On Edit: To answer your second question, certainly if the problem isn’t caused by genetics then there is no reason they can’t have normal children.
There are many possible causes of mental retardation, and not all of them are correlated with fertility, nor are all of them genetic. Given that mentally retarded individuals will have a lot of contact with each other (through programs designed to help them, or just from a desire to be with what one perceives as one’s own kind), there are plenty of romantic pairings between such individuals, and so it’s a given that some of them will have children.
Down’s Syndrome specifically, one of the more common causes of mental retardation, does cause reduced fertility, and Wiki says that there have only been three confirmed cases of a man with Down’s fathering a child, so it’s probably unlikely that a child has been born to a pair of parents with Down’s. But as I said, there are other possible causes of retardation.
Lack of oxygen leading to brain damage during pregancy or delivery can be one cause. In utero stroke Exposure to drugs other than alcohol.
The number one cause of mental retardation in males is fragile x syndrome. It is genetic but it gets progressively worse with each generation, so a mildly retarded man can have more profoundly retarded sons.
Rubella (German measles) exposure while pregnant can cause mental retardation in the baby. Rubella is the “R” in the MMR vaccine that much of the anti-vaccine nonsense has been focused on, and the severity of the birth defects it can cause (hearing loss is another one) is one of the reasons why MMR vaccination is important. The rubella infection itself is pretty mild.
It’s actually probably happened often and with little records of it.
In my experience with the mental health population I notice providers and government agencies often use the term MR / DD. According to the DSM-IV (which is the most recent edition and thus authoritative for diagnostic purposes) Mental Retardation and Developmental Disorders (actually “Pervasive Developmental Disorders”) are two different things.
MR / DD to my knowledge started being lumped together by providers/State entities because persons with those diagnoses tended to need similar services and constituted a more or less natural “group” of individuals. Technically and in fact if you look at the diagnostic criteria for MR, and the diagnostic criteria for the Pervasive Developmental Disorders, it probably isn’t appropriate to replace Mental Retardation with Developmental Disorder.
My understanding is the “replacement” for Mental Retardation will eventually be “Intellectual Disability” because there is a genuine need to distinguish them from disorders which are Developmental in nature.
MR can be diagnosed as Mild (317), Moderate (318), Severe (318.1), Profound (318.2) and Unspecified (319.) Developmental Disorders are diagnosed as Autistic (299), Rett’s Disorder (299.80), Childhood Disintegrative Disorder (299.10), Asperger’s Disorder (299.80), Pervasive Developmental Disorder Not Otherwise Specified (299.80). (Down Syndrome is not listed at all because if you were to list all of the causes of Mental Retardation the DSM-IV would be many thousands of pages long, and some disorders are exceptionally rare and the treatment and services given to those individuals is typically not meaningfully different than that given to other people with MR diagnoses.)
What all that says is, while Down Syndrome is the most common mental retardation syndrome, and Down Syndrome does decrease fertility, there are many other disorders/syndromes that cause mental retardation. Many of those do not significantly depress fertility. Further, just because someone has an MR diagnosis does not mean they are closely monitored, in a group home, living in a facility or et cetera. Some very high functioning individuals with MR may live very unassisted lives, and those individuals are not only by and large free to live their lives and make their own decisions, but they are also not necessarily closely tracked or monitored by the “system.” Meaning it is entirely likely they meet persons similar to themselves and have children with them, and that it isn’t necessarily commonly reported.
I can’t find it now, but I remember a cover article of the New York Times Magazine, perhaps fifteen years ago, focusing on the non-retarded children of developmentally disabled parents. The point was as a consequence of mainstreaming and the encouragement of disabled people in general to live as normal lives as possible, many were now marrying and having children, some of whom were not developmentally disabled themselves. (These kids frequently had serious learning disabilities, IIRC.) It was hard as the kids grew older, and began to outpace their parents at a fairly young age.
*checks karyotype from Sept 1980. MR CAN be the result of a deletion or duplication or whatever…but not always. When I was little (and indeed up until I was a teenager and even now) the common wisdom was that kids with autosome (numbered chromosome) chromosome disorders were the stereotypical profoundly MR, deaf/blind with seizures, medically fragile/complex types. They are now finding that there are a lot more VERY mildly affected people out there, then they ever thought.
I thought just as with congentiial/early blindness and deafness/hearing loss, that the cause of most MR was unknown. There are causes in some cases, but others are a GIANT question mark.
I have known people who were MR because of extremely high fevers in infancy, or in one case a severe reaction to the MMR vaccine–one of the 1-in-a-million risks they tell you about on the info sheet.
No. The cause is unknown in 30 to 50% of cases. About 35 percent of cases are caused by a chromosomal abnormality (Down Syndrome 15%, Fragile X Syndrome 15%, Velocariofacial syndrome 5%). Roughly 15 percent are attributed to Fetal Alcohol Syndrome. (Cite from the American Academy of Family Physicians)
As a teacher, I’ve encountered more than one parent that I felt probably was technically MR. Lots of MR people are highly functional, and lots of them have very good social skills: through some miracle or social service they find a job that doesn’t require anything they can’t do, and they stay there forever. The reason people equate MR=Downs is the the physical characteristics of Downs are so obvious. You’ve probably met any number of high functioning MR people and never known it. Some of them may never know it, if they went to a crappy school.
My uncle is MR due to brain damage during the birth process - the umbilical cord was around his neck and cut off blood supply to the brain. No chromosomal damage, he would be as fertile as any other man.
He’s in his mid-50’s now, and is very functional, lives on his own in a house owned by my grandparents, and has a job. I know he has dated also, though he has never married.
My cousin’s uncle is MR, about the same age as Maggie the Ocelot’s uncle. The guy I know I believe had problems with his skull not forming properly and not closing properly after birth. He doesn’t particularly look different than a normal-functioning person (you wouldn’t confuse him with someone with Downs Syndrome) but you can tell right away upon speaking with him that he is retarded. He’s got a job and I think lived in assisted living for a while, but I don’t believe he’s ever had a girlfriend. Nice fella - and a Special Olympics medalist!
