Final answer:
Aneuploidy is the gain or loss of individual chromosomes to or from a complete set. It includes monosomy (loss of one chromosome) or trisomy (gain of an extraneous chromosome). Trisomy 21, also known as Down Syndrome, is the most common trisomy among viable births.
Step-by-step explanation:
Aneuploidy is the gain or loss of individual chromosomes to or from a complete set. An individual with an error in chromosome number is described as aneuploid, which includes monosomy (loss of one chromosome) or trisomy (gain of an extraneous chromosome). Monosomic zygotes missing any one copy of an autosome invariably fail to develop to birth because they lack essential genes. Trisomic individuals suffer from an excess in gene dose, which can lead to developmental challenges. The most common trisomy among viable births is Down Syndrome, which results from the trisomy of chromosome 21.