Down syndrome results from a specific chromosomal error. What is that error?

Down syndrome, also known as Trisomy 21, occurs due to the presence of an extra chromosome on the 21st pair. In normal human genetics, individuals typically have two copies of each chromosome; however, in Down syndrome, individuals have three copies of chromosome 21. This extra genetic material affects the development and can lead to the characteristic physical features and cognitive challenges associated with Down syndrome.

The reason this answer is correct lies in the direct relationship between the chromosomal anomaly and the condition itself. The presence of the additional chromosome results from nondisjunction during meiosis, which is the failure of homologous chromosomes or sister chromatids to separate properly. This leads to an embryo with three copies of chromosome 21 when fertilization occurs.

The other options describe errors involving different chromosomes or missing chromosomes, none of which are related to Down syndrome. A missing chromosome affects conditions such as Turner syndrome or Klinefelter syndrome, while a rearrangement or deletion of chromosomes relates to other genetic disorders but not to Down syndrome specifically. Understanding the chromosomal basis for Down syndrome is crucial for recognizing how genetic abnormalities can lead to developmental differences.