How does an extra copy of chromosome 21 cause Down syndrome?

Down syndrome is a congenital disorder caused by the presence of extra genetic material from chromosome 21. This condition, known as trisomy 21, occurs when an individual has three copies of chromosome 21 instead of the typical two.12

The extra genetic material typically results from nondisjunction—a random error during cell division where chromosome pairs fail to separate properly during the formation of egg or sperm cells. When such a gamete is fertilized, the resulting zygote carries the additional chromosome.13

The presence of this extra chromosome disrupts normal development, leading to the characteristic physical and cognitive signs of the disorder, such as intellectual disability, poor muscle tone, and potential heart or kidney malformations. While the most common form involves all cells containing the extra chromosome, rarer types include mosaic Down syndrome (where only some cells are affected) and translocation Down syndrome (where the extra chromosome attaches to another chromosome).23