Final answer:
Mosaicism is when an individual has two or more chromosome complements, and it is distinct from aneuploidy, which includes monosomy and trisomy such as Down Syndrome caused by an extra copy of chromosome 21.
Step-by-step explanation:
When two or more chromosome complements can be found within an individual, this is known as mosaicism. This biological condition differs from aneuploidy, which is characterized by an abnormal number of chromosomes within a cell and includes both monosomy (loss of one chromosome) and trisomy (gain of an extraneous chromosome). Aneuploidy can often result in genetic disorders such as Down Syndrome, which is specifically caused by trisomy 21, where individuals have three copies of chromosome 21, leading to physical and developmental challenges. Conditions like mosaicism show how gene dosage and chromosome number are crucial for proper development and function in organisms.