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Down syndrome the most common genetic birth defect associated with mental retardation occurs equally across all races and levels of society The effects of the disorder on physical and mental development are severe and are expressed throughout the life span The individuals family is also affected emotionally economically and socially Bellenir 1996 Characteristics associated with Down syndrome include epicanthal folds unilateral squints a flat saddle nose flat jaw bones large fissured tongue macroglossia hyptonic lower lip cheilosis receding chin microgenia protruding flat ears dry and cracked lips dry skin delays in skeletal maturation shorter than average stature delays in motor skill acquisition flat feet intellectual disability delayed language abilities and numerous others Whitaker 1998 It has only been known since as recently as 1959 that Down syndrome first described in 1866 by Langdon Down is caused by extra genetic material carried on the 21st chromosome All normal cells in the human body except for eggs and sperm cells have 46 chromosomes These being 44 autosomes and two sex chromosomes Normal reproductive cells contain one half of that number- 22 autosomes and one sex chromosome The genetic effect associated with Down syndrome is the presence of extra material on the chromosome pair 21 This extra material in chromosome pair 21 is usually another chromosome This is called Trisomy 21 see figure 1 It occurs in 95 of all Down syndrome cases Trisomy 21 results from as error in cell division during the development of the egg or sperm or during fertilization Bellenir 1996 Translocation is an interchange of chromosomes or parts of chromosomes which may result in a mismatched pair Translocation occurs in about four percent of the cases Children with translocation Down syndrome have an extra number 21 chromosome that has broken and become attached to another chromosome Bellenir 1996 In a few cases a person can carry a broken chromosome 21 without showing any symptoms of Down syndrome because the correct amount of
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