Final answer:
Down syndrome is a chromosomal disorder characterized by an extra copy of chromosome 21, known as trisomy 21, which is a variation in chromosome number. It leads to a total of 47 chromosomes in an individual's cells. This is distinct from structural chromosome variations such as deletions or translocations.
Step-by-step explanation:
Down syndrome is an example of variation in chromosome number, not chromosome structure. It is caused by the presence of an extra copy of chromosome 21, a condition also referred to as trisomy 21. Disorders such as Down syndrome arise from nondisjunction events during meiosis, leading to an embryo with an abnormal number of chromosomes, also known as aneuploidy. This particular genetic abnormality results in a total of 47 chromosomes in the affected individual's cells.
Similar to Down syndrome, Klinefelter syndrome is another disorder related to an abnormal number of chromosomes, where a male has an extra X chromosome, resulting in an XXY genotype. While chromosomal disorders can include structural variations like deletions, insertions, inversions, or translocations, Down syndrome specifically involves an extra full chromosome rather than changes to the chromosome's structure.