NEET BIOLOGY

Detailed explanation-1: -Typically, a baby is born with 46 chromosomes. Babies with Down syndrome have an extra copy of one of these chromosomes, chromosome 21. A medical term for having an extra copy of a chromosome is ‘trisomy. ‘ Down syndrome is also referred to as Trisomy 21.

Detailed explanation-2: -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-3: -Down syndrome is caused by a random error in cell division that results in the presence of an extra copy of chromosome 21.

Detailed explanation-4: -Double aneuploidy (Down-Klinefelter syndrome) is rare, with an estimated incidence of 0.4-0.9 cases per 10, 000 male births and 11.7 cases per 10, 000 cases of trisomy 21 [7]. It is characterized by the presence of a third copy of chromosome 21 plus an extra X chromosome, resulting in 48 chromosomes (48, XXY, +21).

Detailed explanation-5: -Down syndrome is usually caused by an error in cell division called “nondisjunction.” Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate.