Every time a pregnancy occurs there is a chance of having a child that is mentally retarded. Mental retardation represents the lower end of the distribution of intelligence. An IQ of less than 70 is generally regarded as mental retardation. The most common form of mental retardation appears in the form of the incurable Down Syndrome.
Down Syndrome--An Explanation.
Down Syndrome is a birth defect where there is a congenital malformation accompanied by moderate to severe mental retardation, and caused by a chromosomal abnormality. Down Syndrome is the most common genetic birth defect associated with mental retardation and occurs equally across all races and levels of society. The effects of the disorder on physical and mental development are severe and expressed throughout the persons life. (Genetic Disorders Source Book pg. 2).
Down Syndrome is characterized by a genetic defect in the chromosome pair 21. Normal cells in the human body, except the Ova and Sperm cells have 46 chromosomes. There are 44 autosomes and two sex chromosomes. Normal reproductive cells contain 23 chromosomes, 22 of them are autosomes and there is one sex chromosome. In all other no reproductive body cells, chromosomes occur in pairs. The 22 pairs of autosomes are designated by a number with the two sex chromosomes being represented by a XX for a female or a XY for a male. The defect associated with Down Syndrome is the presence of an extra material on the chromosome pair designated 21. Other defects may be associated with an extra chromosome but only Down Syndrome is characterized by extra chromosome 21 material. There are three forms in which extra material can appear as, and they are Trisomy 21, Translocation, and Mosaicism. (Genetic Disorder Source Book pgs. 3-4).
Relationship of Down Syndrome to Maternal Age.
Surveys have shown that the average incidence of Down Syndrome is about one in every 800 live births.