Acrocentric Chromosome
 
An acrocentric chromosome has a single centromere that is localized at or near one end of the chromosome (acro = extremity). One of the most common forms of chromosomal translocation occurs when two acrocentric chromosomes fuse at the ends containing the centromeres (1). This is called a Robertsonian event (Fig. 1). This generally leads to the formation of a metacentric chromosome, where the new centromeric region appears at or near the middle of the new chromosome. Robertsonian events lead to a decrease in the apparent number of chromosomes. Much of the variation in the number of chromosomes in the members of a family or genus is related to this type of acrocentric chromosomal fusion. Whereas the number of chromosomal arms remain relatively constant, counting two for a metacentric chromosome and one for an acrocentric, the number of chromosomes can vary widely. For example, most dogs have either 32 to 38 or 18 to 21 autosomes (any chromosome other than the X- or Y-chromosomes), but 34 to 38 chromosomal arms. Those with the greater number of autosomes have larger numbers of acrocentric chromosomes (2, 3). In genomes whose karyotypes contain exclusively acrocentric chromosomes (e.g., that of the house mouse, Mus musculus), rapid chromosome identification can be achieved via whole-chromosome painting techniques, e.g., spectral karyotyping (4). 

Figure 1. Robertsonian translocation occurs when two acrocentric chromosomes fuse to create a single metacentric chromosome.
References
1. W. Robertson, J. Morph. 27, 179–197 (1916). 
2. R.K. Wayne, W.G. Nash, and S.J. O''Brian, Cytogenet. Cell Genet. 44, 123–133 (1987). 
3. R.K. Wayne, W.G. Nash, and S.J. O''Brian, Cytogenet. Cell Genet. 44, 134–141 (1987). 
4. T. Ried, E. Schrock, Y. Ning, and J. Wienberg, Hum. Mol. Genet. 7, 1619–26 (1998).