A mutation may result in the addition or the deletion of one or two chromosomes in the diploid chromosome. The loss of a single chromosome from a diploid genome (2n) is called monosomy (2n-1) and the loss of two chromosomes from a diploid genome is called nullisomy (2n-2). The gain of one chromosome is called trisomy (2n+1) and gain of two chromosomes is known as tetraploidy (2n+2).