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Medical Definition of Chromosome 21
1. Down syndrome is the most common chromosome disorder seen in live births and is a leading cause of mental retardation. The frequency of the syndrome is estimated at 1 per 700 births and the risk increases exponentially with maternal age. Some of the manifestations are mental retardation, flat facies, oblique palpebral fissures, epicanthic folds, short broad hands, flattened occiput, cardiac abnormalities, hypotonia, Brushfield spots of the iris, abnormal dermatoglyphics, seizures, increased susceptibility to respiratory infections, increased incidence of leukaemia and premature ageing with Alzheimer-like brain degeneration. 92% to 95% of children born with Down syndrome have a free trisomy as a result of maternal nondisjunction (95%) or paternal nondisjunction (5%). Translocations account for the rest of the trisomies with t(14q21q) or t(15q2q) representing 54.2% of the cases, and t(21q21q) or t(21q22q)5 accounting for 40.9% of the cases. There are a few cases involving other chromosomes. In 55% of the cases, translocations involving the D group chromosomes arise de novo and in 45% of cases a parent carries the translocation. When a G group chromosome is involved t(21q21q) occurs 83.3% of the time. In the vast majority of cases t(21qGq) occurs de novo. Partial trisomy or tetrasomy 21 due to duplication of the distal segment 21q22 results in the classical Down phenotype. Partial trisomy of the proximal segment of 21q21 shows a fairly normal phenotype with moderate mental retardation. Monosomy-21 is not compatible with life. Deletions of chromosome 21 are associated with psychomotor and growth retardation, congenital heart disease, holoprosencephaly, microphthalmia, skeletal malformation, genital hypoplasia and other dysmorphisms. Infants with ring chromosome 21 syndrome, a countertype of trisomy-21, show hypotonia, growth retardation, microcephaly and protruding occiput. Other malformations include ocular, cardiac, digestive, urogenital and skeletal malformation. Mental retardation is severe. (05 Mar 2000)
Lexicographical Neighbors of Chromosome 21
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