Final answer:
Aneuploidies are harmful due to nondisjunction during meiosis, which leads to gametes with abnormal chromosome numbers, causing genetic imbalances and various abnormalities.
Step-by-step explanation:
Aneuploidies are detrimental to individuals primarily because of nondisjunction, which is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis. This failure leads to the formation of gametes with an abnormal number of chromosomes, a condition known as aneuploidy. Gametes containing an extra chromosome (n+1) or lacking a chromosome (n-1) can lead to genetic imbalances when they fuse with a normal gamete, often resulting in physical and mental abnormalities in the offspring, or even lethality in diploid organisms.