Trisomy 21 or Down syndrome occurs when there is a normal diploid chromosomal complement of 46 chromosomes plus one (extra) chromosome

True/False

True

Explanation:

Down syndrome is also called trisomy 21. This means that the affected individual has three copies of chromosome 21. A normal human being carries two copies of chromosome 21. Therefore, a person affected with Down syndrome has a total of 47 chromosomes instead of normal chromosomal number 46.

Down syndrome may be caused when the two copies of homologous chromosomes 21 do not separate from each other during anaphase-I. This would form some of the gametes having two copies of chromosome 21. Fertilization of these gametes with the normal gametes would result in zygotes having trisomy 21.