It is change in number of chromosome which can be either due to loss of one or more chromosomes or due to addition or deletion of one or more chromosomes. It leads to variations in chromosome number and do not involve the whole of the Karyothype. The nuclei of the aneuploids contain chromosomes whose number is not true mullyple of the basic number (n). The aneuploidy arises due to non disjunction. The loss of one chromosome produces a Monosomic (2n—1) and the condition is termed as Monosomy. The gain of one chromosome produces Trisomic (2n—1) and the condition is known as Trisomy. In the same way the addition of two or more chromosomes is respectively known as Tetrasomy and Pentasomy, the individuals are known as Tetrasomic and Pentasomic. In some cases a pair of homologous chromosomes is lost (2n—2), such indivividuals are termed as Nullisomic and the condition is called Nullisomy.
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