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CHROMOSOME 21

Down syndrome is the most general chromosome mess seen in live births and is a leading reason of mental retardation. The frequency of the syndrome is estimated at 1 for 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, emergency dermatoglyphics, seizures, heighten susceptibility to respiratory infections, heighten 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 little cases inclusion another chromosomes. In 55% of the cases, translocations inclusion 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 usual phenotype with temperate mental retardation. Monosomy-21 is not compatible with life. Deletions of chromosome 21 are associated with psychomotor and growth retardation, congenital heart malady, holoprosencephaly, microphthalmia, skeletal malformation, genital hypoplasia and another dysmorphisms. Infants with ring chromosome 21 syndrome, a countertype of trisomy-21, show hypotonia, growth retardation, microcephaly and protruding occiput. Another malformations include ocular, cardiac, digestive, urogenital and skeletal malformation. Mental retardation is severe.
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