Final answer:
Polyploidy is the presence of complete extra sets of chromosomes, leading to robust plant individuals, while aneuploidy, affecting humans with conditions like Down syndrome, involves an incorrect number of chromosomes due to monosomy or trisomy.
Step-by-step explanation:
The presence of complete extra sets of chromosomes in an organism is referred to as polyploidy. This condition, which is common in plants, leads to individuals that are larger and potentially more robust than those with the normal number of chromosome sets, known as euploids. In humans and other diploid species, having the appropriate number of chromosomes is referred to as euploidy. In contrast, aneuploidy describes a situation where the number of chromosomes is incorrect, due to either monosomy or trisomy. Down syndrome, also known as Trisomy 21, is an example of a condition arising from an extra chromosome, where individuals have 47 chromosomes instead of the typical 46.