What chromosomal abnormality results from nondisjunction in meiosis and is associated with Down syndrome?

What chromosomal abnormality results from nondisjunction in meiosis and is associated with Down syndrome?

• A. Monosomy X
• B. Turner syndrome
• C. Trisomy 21
• D. Klinefelter syndrome

Answer: (C)

Nondisjunction in meiosis produces gametes with an abnormal number of chromosomes because homologous chromosomes (or sister chromatids) fail to separate properly. If a gamete carries an extra chromosome 21 and fertilizes a normal gamete, the resulting zygote has three copies of chromosome 21. That extra copy is what leads to trisomy 21, the chromosomal basis of Down syndrome. Other listed conditions involve different chromosomal counts, such as monosomy X (Turner syndrome) or XXY (Klinefelter syndrome), which are not Down syndrome.

Nondisjunction in meiosis produces gametes with an abnormal number of chromosomes because homologous chromosomes (or sister chromatids) fail to separate properly. If a gamete carries an extra chromosome 21 and fertilizes a normal gamete, the resulting zygote has three copies of chromosome 21. That extra copy is what leads to trisomy 21, the chromosomal basis of Down syndrome. Other listed conditions involve different chromosomal counts, such as monosomy X (Turner syndrome) or XXY (Klinefelter syndrome), which are not Down syndrome.