AP BIOLOGY

Detailed explanation-1: -The most common cause of Down syndrome is trisomy 21 (i.e., the presence of three copies of chromosome 21), a condition that results from a meiotic nondisjunction event, usually in the mother. The risk of this type of trisomy 21 increases with maternal age.

Detailed explanation-2: -Abnormal karyotype – approximately 95% of cases result from chromosomal non-disjunction of chromosome 21 (47, XX, +21 or 47, XY, +21) at conception. Translocation trisomy 21 (2% of cases) is often familial, and commonly involves chromosomes 14 and 21.

Detailed explanation-3: -Potentially serious complications-The most serious complications of Down syndrome include heart defects, blood disorders that can include leukemia (cancer of the blood), and immune system problems. Heart defects-Approximately half of all babies with Down syndrome are born with (often repairable) heart defects.

Detailed explanation-4: -Fourteen (2%) of the cases with DS had an obstructive anomaly of the renal pelvis, including hydronephrosis. The other most common anomalies associated with cases with DS were syndactyly, club foot, polydactyly, limb reduction, cataract, hydrocephaly, cleft palate, hypospadias and diaphragmatic hernia.