Find Information About:

Drugs & Supplements

Get information and reviews on prescription drugs, over-the-counter medications, vitamins, and supplements. Search by name or medical condition.

Pill Identifier

Pill Identifier

Having trouble identifying your pills?

Enter the shape, color, or imprint of your prescription or OTC drug. Our pill identification tool will display pictures that you can compare to your pill.

Get Started
My Medicine

My Medicine

Save your medicine, check interactions, sign up for FDA alerts, create family profiles and more.

Get Started

WebMD Health Experts and Community

Talk to health experts and other people like you in WebMD's Communities. It's a safe forum where you can create or participate in support groups and discussions about health topics that interest you.

  • Second Opinion

    Second Opinion

    Read expert perspectives on popular health topics.

  • Community


    Connect with people like you, and get expert guidance on living a healthy life.

Got a health question? Get answers provided by leading organizations, doctors, and experts.

Get Answers

Sign up to receive WebMD's award-winning content delivered to your inbox.

Sign Up

Children's Health

Font Size

Chromosomal Abnormalities Cause of Some Unexplained Mental Retardation

WebMD Health News

Nov. 12, 1999 (New York) -- Subtle alterations to the ends of pieces of genetic material may explain a significant number of cases of moderate to severe mental retardation that currently have no known cause. In the Nov. 13 issue of the journal The Lancet, British researchers conclude that testing for these chromosomal abnormalities is warranted in children for whom there is no apparent explanation for their mental handicap.

In moderate to severe cases of mental retardation, 30% to 40% can be explained by known genetic or chromosomal disorders and 10% to 30% can be explained by environmental factors, leaving about 40% of cases unexplained. Evidence has suggested that minor changes on chromosomes, which are responsible for transmitting genetic information, may be an important cause of mental retardation, but much of this evidence has come from case reports or small studies.

In the new study, researchers from John Radcliffe Hospital in Oxford, England, used a modification of an existing laboratory test, called the fluorescence in-situ hybridization or FISH, to analyze chromosome ends in about 450 mentally retarded children and 75 normal men. The children's retardation varied from mild to severe.

Among the 284 children with moderate to severe mental retardation, the frequency of chromosomal abnormalities detected was 7.4%. Among children with mild retardation, the frequency was 0.5% The normal adult male controls had no chromosomal abnormalities. In approximately 50% of children with the detected chromosome alterations, a family link was established by performing the same testing on parents, siblings, or other family members.

Study author Samantha J.L. Knight, PhD, and colleagues suggest that the family association is important since it may provide explanations in some families for children with unexplained retardation in which another cause, such as brain damage, has previously been given. The authors estimate that the population prevalence of these chromosomal abnormalities is 2.1 per 10,000 people. They recommend this type of testing, the multiprobe FISH, for children with mild to severe unexplained mental retardation. Such children have a variety of disabilities and generally have an IQ below 50. An average IQ is considered to be about 100.

Today on WebMD

child with red rash on cheeks
What’s that rash?
plate of fruit and veggies
How healthy is your child’s diet?
smiling baby
Treating diarrhea, fever and more.
Middle school band practice
Understanding your child’s changing body.

worried kid
jennifer aniston
Measles virus
sick child

Child with adhd
rl with friends
Syringes and graph illustration