<prompt> The most common chromosomal defect is:DownsTurnerEdwardPatau  <response>: Downs syndrome is the most common of the chromosome disorders and a major cause of mental retardation.Cytogenetic abnormalityNon disjunction MC 95Translocation robertsonian 4 Mosaicism 1Increased maternal age is the main risk factor .Translocation carriers carry high recurrence risk.Clinical features: Cardiac endocardial cushion defect MCHypotonia GIT duodenal atresia, TEF, IHPS, HirschprungMental retardation disease, annular pancreasShort stature musculoskeletal system atlantoaxialDislocationMicrocephaly brachycephaly Respiratory recurrent aspiration pneumoniaOpen AF Endocrine primary gonadal defect,HypothyroidismOblique palpebral fissure Haematopoietic increased ALL and AMLBushfield spot . But AML has a betterDepressed nasal bridges prognosis than normal individualsLow set ears CNS Alzheimers in patients older than 40 yearsShort neckSimian crease, clinodactylySandal gap between 1st and 2ndToesKennedy crease 