What is trisomy 21 and its typical consequence?

What is trisomy 21 and its typical consequence?

• A. Turner syndrome results from a missing chromosome.
• B. Down syndrome; an extra copy of chromosome 21 leading to developmental and physical differences.
• C. Klinefelter syndrome is caused by an extra X chromosome.
• D. Down syndrome is caused by an environmental toxin.

Answer: (B)

Trisomy 21 means having three copies of chromosome 21 in the body's cells. That extra copy increases the dose of the genes on that chromosome, which disrupts normal development. The typical consequence is Down syndrome, presenting as developmental and physical differences such as mild to moderate intellectual disability, characteristic facial features, and hypotonia, along with a higher likelihood of congenital heart defects and other health issues. Most cases arise from nondisjunction during egg formation, though translocation or mosaicism can also be involved. The other options describe different chromosomal conditions or imply an environmental cause, which don’t explain Down syndrome.

Trisomy 21 means having three copies of chromosome 21 in the body's cells. That extra copy increases the dose of the genes on that chromosome, which disrupts normal development. The typical consequence is Down syndrome, presenting as developmental and physical differences such as mild to moderate intellectual disability, characteristic facial features, and hypotonia, along with a higher likelihood of congenital heart defects and other health issues. Most cases arise from nondisjunction during egg formation, though translocation or mosaicism can also be involved. The other options describe different chromosomal conditions or imply an environmental cause, which don’t explain Down syndrome.