chromosomal abnormalities

Meaning: Chromosomal abnormalities are changes in chromosome number or structure. Aneuploidy is an extra or missing chromosome (trisomy, monosomy). Polyploidy is extra whole sets, which is common in plants but rarely survives in humans. Aneuploidy usually arises from nondisjunction, a failure of homologs to separate in meiosis I or of sister chromatids to separate in meiosis II. Structural changes are deletion, duplication, inversion, and translocation, caused by chromosome breakage and rejoining.

On the MCAT: Passages show a karyotype or a described cross and ask which error occurred and when. Nondisjunction in meiosis I makes all four gametes abnormal. In meiosis II, two gametes are abnormal and two are normal. A gamete with an extra chromosome fertilized by a normal gamete gives a trisomy (2n + 1). A gamete missing one gives a monosomy (2n − 1). Know trisomy 21, XXY, and XO as examples. The risk of nondisjunction rises with maternal age.

Common confusion: Aneuploidy (one chromosome off) is not polyploidy (whole sets extra), and neither is a point mutation, which is too small to see on a karyotype. A balanced translocation or inversion changes arrangement without losing genes, so the carrier may be healthy, while a deletion loses genetic material.