Which of the following genetic disorders is characterized by the presence of an extra twenty-first chromosome, resulting in developmental delays and distinct physical features?

💡 Solution & Explanation

Down syndrome is a chromosomal condition caused by trisomy of chromosome 21, typically resulting from non-disjunction during meiotic cell division where homologous chromosomes fail to separate properly. Klinefelter syndrome involves an extra X chromosome in males, Turner syndrome is characterized by monosomy of the X chromosome in females, and phenylketonuria is an autosomal recessive metabolic disorder. The incidence of chromosomal non-disjunction increases with maternal age.