If crosses are performed for three alleles on a chromosome, only one map order explains map units. Changes in chromosomes or genes that pass to offspring if they occur in gametes. Mutations increase the amount of variation among offspring. 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.