Which disability is caused when abnormal cell division results in an extra full or partial copy of chromosome 21?

Which disability is caused when abnormal cell division results in an extra full or partial copy of chromosome 21?

• A. Autism spectrum disorder
• B. Fragile X syndrome
• C. Reactive attachment disorder
• D. Down syndrome

Answer: (D)

An extra full or partial copy of chromosome 21 leads to Down syndrome. This happens when cell division goes awry during conception, usually through nondisjunction, so the resulting zygote has three copies of chromosome 21 instead of two. The extra genetic material alters development, producing the characteristic features of Down syndrome—distinct facial traits, low muscle tone, and a range of intellectual abilities—along with possible health issues such as heart problems. Sometimes only part of chromosome 21 is duplicated or the extra copy is present only in some cells (mosaic Down syndrome), but the outcome is the same: increased gene dosage on chromosome 21. The other conditions listed have different causes: autism involves a complex mix of genetic and environmental factors; Fragile X syndrome results from a mutation on the FMR1 gene on the X chromosome; Reactive attachment disorder stems from early relational trauma rather than chromosomal changes.

An extra full or partial copy of chromosome 21 leads to Down syndrome. This happens when cell division goes awry during conception, usually through nondisjunction, so the resulting zygote has three copies of chromosome 21 instead of two. The extra genetic material alters development, producing the characteristic features of Down syndrome—distinct facial traits, low muscle tone, and a range of intellectual abilities—along with possible health issues such as heart problems. Sometimes only part of chromosome 21 is duplicated or the extra copy is present only in some cells (mosaic Down syndrome), but the outcome is the same: increased gene dosage on chromosome 21. The other conditions listed have different causes: autism involves a complex mix of genetic and environmental factors; Fragile X syndrome results from a mutation on the FMR1 gene on the X chromosome; Reactive attachment disorder stems from early relational trauma rather than chromosomal changes.