Chromosomal mutations include changes in chromosome number and structure. Monosomy occurs when an individual has only one of a particular type of chromosome. Trisomy occurs when an individual has three of a particular type of chromosome. Deletions are a type of mutation in which an end of a chromosome breaks off or when two simultaneous breaks lead to the loss of a segment. Cri du chat syndrome is deletion in which an individual has a small head, is mentally retarded, has facial abnormalities, and abnormal glottis and larynx resulting in a cry resembling that of a cat.
Duplications are a doubling of a chromosomal segment.
A broken segment from one chromosome can simply attach to its homologue. Unequal crossing-over may occur. Multiple copies of genes can mutate differently and provide additional genetic variation for a species.