Why isn't down syndrome hereditary?

Down syndrome is primarily caused by an extra copy of chromosome 21, a condition known as trisomy 21. This typically occurs due to a random error during cell division in the formation of eggs or sperm, rather than being inherited from a parent. While there are rare hereditary forms of Down syndrome, most cases arise sporadically and are not passed down genetically. Therefore, Down syndrome is not considered hereditary in the conventional sense.