Final answer:
Down syndrome, or Trisomy 21, is characterized by the presence of three copies of chromosome 21. It is not associated with an extra Y chromosome, which is a distinguishing feature of Klinefelter syndrome, not Down syndrome.
Step-by-step explanation:
The characteristic that is NOT a feature of Down syndrome is D.) An extra Y chromosome. Down syndrome, also known as Trisomy 21, is characterized by an individual having three copies of chromosome 21, instead of the usual two. The resulting chromosomal abnormality leads to a variety of symptoms including intellectual disability, physical deformities, and a distinctive facial appearance.
Individuals with Down syndrome typically have a round face, a flattened skull, and intellectual and motor disabilities, and they may also experience vision problems, congenital heart disease, and an increased susceptibility to cancer. These characteristics are direct consequences of the extra genetic material from chromosome 21 affecting the normal development of the body and brain. Down syndrome is a result of chromosomal nondisjunction during meiosis, and the likelihood of having a child with Down syndrome increases with maternal age, particularly in women over 36.
Klinefelter syndrome is a different genetic condition where a male has an extra X chromosome, resulting in an XXY karyotype. It is unrelated to Down syndrome and is not characterized by an extra chromosome 21. In Klinefelter syndrome, affected males may have underdeveloped sex organs, elongated limbs, and difficulties with learning new things.