Down syndrome is primarily caused by abnormalities in which pair of chromosomes?

Down syndrome is primarily caused by the presence of an extra copy of chromosome 21, which results in a condition known as trisomy 21. This genetic anomaly occurs when there is a failure in normal cell division during the formation of eggs or sperm, leading to an embryo with three copies of chromosome 21 instead of the typical two. The extra genetic material from the third copy of chromosome 21 results in the various physical and intellectual characteristics associated with Down syndrome.

Understanding this genetic basis is crucial not only for identifying and diagnosing Down syndrome but also for guiding interventions and support strategies tailored for individuals with this condition. The other chromosome pairs listed—22nd, 18th, and 23rd—are not associated with Down syndrome and do not contribute to its characteristics or diagnosis. Thus, focusing on the 21st pair is essential for comprehending the underlying cause of this genetic disorder.