Which syndrome is caused by an extra chromosome 21?

Down syndrome, also known as Trisomy 21, is caused by the presence of an extra copy of chromosome 21. This genetic condition results from nondisjunction during meiosis, where chromosomes fail to separate properly, leading to an individual having three copies of chromosome 21 instead of the usual two. The additional genetic material disrupts normal development and can cause a variety of physical and intellectual disabilities.

Individuals with Down syndrome often present with characteristic features such as a flat facial profile, slanted eyes, and a single transverse palmar crease, among other traits. Understanding this condition involves recognizing both its genetic basis and its implications for health and development throughout the individual's life.

The other conditions mentioned, such as Klinefelter syndrome (characterized by an extra X chromosome in males), Edwards syndrome (associated with an extra chromosome 18), and Turner syndrome (resulting from a missing X chromosome in females), are distinct genetic disorders with different chromosome abnormalities. Thus, these do not relate to the presence of an extra chromosome 21.