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