| amniocentesis | A clinical procedure that can identify abnormal chromosomes in a developing fetus.
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| brain plasticity | The brain's ability, during infancy, to be altered by environmental stimulation.
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| congenital disorders | Disorders acquired during prenatal development but not transmitted genetically.
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| cretinism | A congenital disorder marked by serious mental retardation and physical disabilities; caused by lack of iodine in the pregnant woman's diet or damage to the thyroid during birth.
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| Down syndrome | A form of mental retardation caused by an extra chromosome. Individuals with this condition usually have IQs of 50 or less and distinctive physical characteristics, such as an extra fold of skin on the upper eyelid, a flat nose, and poor muscle tone.
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| echolalia | A speech deficit, characteristic of autistic children and some catatonic schizophrenics, in which the child aimlessly repeats what other people say.
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| fetal alcohol syndrome (FAS) | A complex of physical and behavioral defects found in many children of alcoholic women. The defects include distinctive facial characteristics, retarded physical growth, and, frequently, mental retardation.
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| fragile X syndrome | A condition in which an individual's X chromosome shows a weak spot; the most common genetic cause of mental retardation.
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| infantile autism | A disorder in children in which the primary symptom, apparent from infancy, is the inability to relate to anyone outside of oneself.
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| mental retardation | A condition that is characterized by subaverage intellectual functioning, by serious deficits in adaptive skills, and by onset before age 18.
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| phenylketonuria (PKU) | A genetic defect caused by a deficiency in a liver enzyme, phenylalanine 4-hydroxylase, which results in severe retardation, hyperactivity, and erratic behavior.
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| savant syndrome | A disorder in which a person with greatly diminished mental skill shows extraordinary proficiency in one, isolated skill.
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| sheltered workshops | Special work centers designed to meet the needs of mentally retarded people employed in them.
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| Tay-Sachs disease | A genetic disorder of lipid metabolism marked by the absence of the enzyme hexosominidase A in brain tissues; causes mental retardation, muscular deterioration, convulsions, and death before the age of six.
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| theory of mind | The ability, lacking in autistic children, to appreciate the existence of purely mental states, such as beliefs or desires, and to predict or understand behavior based on such states.
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| trisomy 21 | A condition in which there is an extra chromosome in pair 21 in the human cell; the genetic basis of Down syndrome.
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