[Campbell Biology P.357] What is the specific chromosomal abnormality that causes Down syndr... | Practice Question

What is the specific chromosomal abnormality that causes Down syndrome?

  • A: A deletion on chromosome 21
  • B: Trisomy 21
  • C: An extra X chromosome (XXY)
  • D: A translocation between chromosome 9 and 22

Explanation

The text explicitly states that Down syndrome is 'usually the result of an extra chromosome 21, so that each body cell has a total of 47 chromosomes.' This condition is specifically called trisomy 21, as shown in the karyotype in Figure 15.15.