AP BIOLOGY

Detailed explanation-1: -About 95 percent of the time, Down syndrome is caused by trisomy 21-the person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell.

Detailed explanation-2: -A medical term for having an extra copy of a chromosome is ‘trisomy.’ Down syndrome is also referred to as Trisomy 21. This extra copy changes how the baby’s body and brain develop, which can cause both mental and physical challenges for the baby.

Detailed explanation-3: -The classic form is the most common chromosomal disorder, in which there is one extra X chromosome resulting in the karyotype of 47, XXY [18]. Double aneuploidy was first described in a patient with both Down and Klinefelter (48, XXY, +21) syndromes.

Detailed explanation-4: -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-5: -Affected people have two normal copies of chromosome 21 plus extra material from chromosome 21 attached to another chromosome, resulting in three copies of genetic material from chromosome 21. Affected individuals with this genetic change are said to have translocation Down syndrome.