The most common chromosomal defect is:
Down’s syndrome is the most common of the chromosome disorders and a major cause of mental retardation.
Cytogenetic abnormality
Non disjunction (MC) = 95%
Translocation (robertsonian) = 4 %
Mosaicism = 1%
Increased maternal age is the main risk factor .
Translocation carriers carry high recurrence risk.
Clinical features:                               Cardiac → endocardial cushion defect (MC)
Hypotonia                                          GIT → duodenal atresia, TEF, IHPS, Hirschprung
Mental retardation                              disease, annular pancreas
Short stature                                      musculoskeletal system → atlantoaxial
Dislocation
Microcephaly/ brachycephaly           Respiratory → recurrent aspiration pneumonia
Open AF                                           Endocrine → primary gonadal defect,
Hypothyroidism
Oblique palpebral fissure                 Haematopoietic → increased ALL and AML
Bushfield spot                                 . But AML has a better
Depressed nasal bridges                 prognosis than normal individuals
Low set ears                                     CNS Alzheimer's in patients older than 40 years
Short neck
Simian crease, clinodactyly
Sandal gap between 1st and 2nd
Toes
Kennedy crease
Down’s
Turner
Edward
Patau