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Aneuploidy occurs when there is an abnormal number of chromosomes, such as in Down’s syndrome (trisomy 21).
Ref: NCERT Class 12 Biology Chapter 4: Principles of Inheritance and Variation Sex Determination Mutation and Genetic Disorders
Aneuploidy is caused by nondisjunction, where chromosomes fail to separate properly during meiosis.
Ref: NCERT Class 12 Biology Chapter 4: Principles of Inheritance and Variation Mendel's Laws - Dominance Segregation Independent Assortment
Variation concerning individual chromosome gain or loss while maintaining most diploid complement termed aneuploidy distinct from euploidy involving whole sets. Aneuploidy subdivided hypoploid forms monosomy 2n-1 and nullisomy 2n-2 versus hyperploid forms trisomy 2n+1 and tetrasomy. Origin predominantly meiotic nondisjunction where homologs or sister chromatids fail to separate due spindle defects, producing n+1 and n-1 gametes. Phenotypically often deleterious due to unbalanced gene dosage disrupting regulatory networks. Many cases embryonic lethal except sex chromosome aneuploidies and small autosomes like 21 that tolerate dosage imbalance partially.
Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 8: Aneuploidy Individual Chromosome Gain Loss
