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.