Zoology - Principles of Inheritance and Variation Question with Solution | TestHub

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. As the embryo develops, the extra chromosome is replicated in every cell of the body. Thus Down syndrome is called trisomy of the 21^stchromosome.