In addition to chromosome rearrangements, chromosome mutations also include changes in the number of chromosomes. Variations in chromosome number can be classified into two basic types: changes in the number of individual chromosomes (aneuploidy) and changes in the number of chromosome sets (polyploidy).

Aneuploidy can arise in several ways. First, a chromosome may be lost in the course of mitosis or meiosis if, for example, its centromere is deleted. Loss of the centromere prevents the spindle fibers from attaching; so the chromosome fails to move to the spindle pole and does not become incorporated into a nucleus after cell division. Second, the small chromosome generated by a Robertsonian translocation may be lost in mitosis or meiosis. Third, aneuploids may arise through nondisjunction, the failure of homolo gous chromosomes or sister chromatids to separate in meio-sis or mitosis (see p. 000 in Chapter 4). Nondisjunction leads to some gametes or cells that contain an extra chromosome and others that are missing a chromosome ( FIGURE 9.20).

Types of Aneuploidy

We will consider four types of relatively common aneuploid conditions in diploid individuals: nullisomy, monosomy, trisomy, and tetrasomy.

1. Nullisomy is the loss of both members of a homologous pair of chromosomes. It is represented as 2n — 2, where n refers to the haploid number of chromosomes. Thus, among humans, who normally possess 2n = 46 chromosomes, a nullisomic person has 44 chromosomes.

2. Monosomy is the loss of a single chromosome, represented as 2n — 1. A monosomic person has 45 chromosomes.

(a) Nondisjunction in meiosis I Gametes

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