Trisomy is best described as which of the following?

• A. A condition in which a chromosome is missing in the cell nuclei.
• B. A condition in which an extra copy of a chromosome is present in the cell nuclei, causing developmental abnormalities.
• C. A rearrangement of chromosomes within the nucleus.
• D. A translocation of chromosomes between non-homologous pairs.

Answer: (B)

Trisomy is having an extra copy of a chromosome in the cell nuclei, so instead of two copies there are three. This abnormal chromosome number alters gene dosage, meaning too much of many genes on that chromosome are expressed, which can disrupt development and cause various abnormalities. It usually happens when paired chromosomes fail to separate properly during meiosis, creating gametes with an extra chromosome that, if fertilized, results in a zygote with trisomy. The other descriptions describe different chromosomal problems: missing an entire chromosome (monosomy), changes in chromosome structure (deletions, duplications, inversions), or movement of chromosome segments between nonhomologous chromosomes (translocations). The hallmark of trisomy is the presence of an extra whole chromosome, leading to dosage imbalance and developmental effects.