Final answer:
An organism with only one copy of a particular chromosome is monosomic, a condition that generally results in nonviable zygotes in humans due to a lack of essential genes. Trisomy 21, leading to Down syndrome, is an example of aneuploidy where an individual has three copies of chromosome 21.
Step-by-step explanation:
Understanding Aneuploidy
An organism that has only one copy of a particular chromosome instead of the normal two copies is said to be monosomic. This condition is one type of aneuploidy, which occurs when an individual does not have the normal chromosomal number for their species, known as euploidy. In humans, a euploid individual has 22 pairs of autosomes and one pair of sex chromosomes. Aneuploidy includes conditions such as monosomy (the loss of one chromosome) and trisomy (the gain of an extraneous chromosome).
Monosomic human zygotes are typically nonviable because they lack essential genes, emphasizing the importance of gene dosage. Most autosomal trisomies also result in nonviable zygotes, with the exception of trisomies of some smaller chromosomes. Trisomy 21, which leads to Down syndrome, is the most common trisomy among viable births, characterized by specific physical features and developmental delays. The incidence of Down syndrome is higher in children of older mothers.