A metacentric chromosome has its centromere in or near its middle. Robertsonian translocations occur when two acrocentric chromosomes fuse to form a single metacentric (see Fig. 1 of Acrocentric Chromosome). Dissociation occurs when a metacentric chromosome breaks at the centromere to form two acrocentrics. This type of chromosome fusion and breakage is a major source of variation in chromosome numbers within various taxa. Species with high numbers of metacentric chromosomes tend to have fewer chromosomes than those with high numbers of acrocentrics. Metacentric and submetacentric chromosomes (where the centromere is near the middle) have two arms and appear as V- or J-shaped structures in metaphase preparations. The shorter arm of a submetacentric is called the ‘p’ arm, and the longer arm is the ‘q’ arm (Figure 1).
Figure 1. The basic organization of a mammalian chromosome, indicating p and q arms, telomeres, and centromeres.
