Down syndrome (DS) is easily recognized by the lay population and perhaps thought to be synonymous with mental retardation. It is one of the most common genetic causes of mental retardation and perhaps the best known. It is estimated that DS occurs about 1 to 1.5 times every 1000 births. However, other estimates reveal DS may occur once in every 600 births. The risk of having a child with DS increases as the age of the mother increases. It is estimated that males are affected slightly more than females (1.3 to 1.0), which may be due, in part, to a higher mortality rate in females during infancy.
The most common cause of DS involves an extra chromosome existing at the 21st position, known as nondisjunction or trisomy 21. Nondisjunction results from an inappropriate separation of chromosomes during meiotic division that results in a total of 47 chromosomes. This accounts for approximately 95% of the cases of DS. However, DS can also be caused by the chromosomal abnormalities of translocation or mosacism. Translocation (responsible for 2-4% of cases of DS) occurs when part of a chromosome is attached or translocated to another location or chromosome, often to chromosome 14. Mosaicism (responsible for 1-4% of cases of DS) occurs when there is an uneven pattern of dissimilar cells with 46 or 47 chromosomes.
DS results in a cluster of physical traits that include short stature; flat, broad face with small ears and nose;
brachycephaly; short, broad hands and feet; tongue protrusion and/or fissured tongue; heart defects; hypotonia; slanted and almond-shaped eyes; epicanth-ic folds in the inside corners of the eyes; a wide space between the first and second toe; and a single crease that transverses the palm. These physical traits can vary from individual to individual and may become less pronounced as an individual grows and develops.
Most individuals with DS often have some degree of intellectual impairment, with mild to moderate mental retardation being most prevalent. As children with DS age, there seems to be a general decline in their developmental rate and progression, with verbal short-term memory skills being most vulnerable. Some research has suggested that individuals with mosaicism have higher average cognitive scores (10-30 points higher) than individuals with trisomy 21 on standardized measures of IQ. Children with mosaicism have also demonstrated average visual-perception skills.
The accepted stereotype of persons with DS is that they are happy, social, and friendly; however, this is not scientifically based. Instead, studies have shown that children with DS have temperament profiles that are comparable to those of typically developing children. Children with DS have difficulty in the area of social competence, perhaps due to concomitant motor, cognitive, and language delays. Individuals with DS exhibit other comorbid disorders, including attention deficit/hyperactivity disorder, conduct/op-positional disorder, aggression, phobias, stereotypic behavior and self-injurious behaviors.
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