acrocentric chromosome pairs (number II in the image), where the short arms are fairly short but not very short. A: Short arm (p arm) B: Centromere C: Long arm (q arm) D: Sister chromatids In humans,<unk> s occur in the five acrocentric chromosome pairs (chromosome pairs where the short arms are fairly short), namely 13, 14, 15, 21 and 22. The participating chromosomes break at their centromeres and the long arms fuse to form a single, large chromosome with a single centromere. A<unk>  is a type of<unk>  involving two homologous (paired) or non-homologous chromosomes (i.e. two different chromosomes, not belonging to a homologous pair). A feature of chromosomes that are commonly found to undergo such<unk> s is that they possess an acrocentric centromere, partitioning the chromosome into a large arm containing the vast majority of its genes, and a short arm with a much smaller proportion of genetic content. The short arms also join to form a smaller reciprocal product, which typically contains only nonessential genes also present elsewhere in the genome, and is usually lost within a few cell divisions. This type of<unk>  is cytologically visible, and can reduce chromosome number (from 23 to 22 pairs, in humans) if the smaller chromosome that results from a<unk>  is lost in the process of future cellular divisions. However, the smaller chromosome lost may carry so few genes (which are, in any case, also present elsewhere in the genome) that it can be lost without an ill effect to the individual.
Robertsonian translocation
Caudal duplication
Trisomy
Germline mosaicism
Postzygotic mutation
Uniparental disomy
Isodisomy
Chromosomal translocation