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Klinefelter’s syndrome (47, XXY) results from the non-disjunction of sex chromosomes during meiosis.
Ref: NCERT Class 12 Biology Chapter 4: Principles of Inheritance and Variation Sex Determination Mutation and Genetic Disorders
In meiosis II, homologous chromosomes have already segregated in meiosis I, and only sister chromatids must separate at anaphase II. If one pair of chromatids fails to divide, that meiosis II division yields one n+1 chromatid pair and one n-1 chromatid lacking that chromosome. The other meiosis II division, derived from the normal secondary gametocyte, proceeds correctly and forms two haploid normal gametes. Thus only two of four products are abnormal, with 50% normal haploids retained, contrasting with meiosis I nondisjunction where all four gametes are aneuploid.
Ref: Hartl & Jones, Genetics: Analysis of Genes and Genomes, 8th ed., Chapter 4: Meiosis and Nondisjunction
During meiosis I, homologous chromosomes normally disjoin to opposite poles at anaphase I. Nondisjunction occurs when a bivalent fails to separate, so both homologs migrate to one pole and none to the other. Consequently both secondary oocytes or spermatocytes become aneuploid. After meiosis II, sister chromatids separate normally, but all four resulting gametes carry unbalanced numbers: two gametes are n+1 disomic and two are n-1 nullisomic for that chromosome. Subsequent fertilization produces trisomic or monosomic zygotes with characteristic syndromes and developmental failure.
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 3: Chromosome Segregation and Aneuploidy
Non-disjunction describes error during cell division where homologous chromosomes in meiosis I or sister chromatids in meiosis II or mitosis fail to separate properly, both copies moving same spindle pole due defective cohesin cleavage, spindle assembly checkpoint failure, or merotelic kinetochore attachment lacking correction. Result two daughter cells aneuploid one with n+1 chromosomes one with n-1, reciprocal products. Fertilization with normal haploid gamete produces trisomic and monosomic concepti. Mechanism responsible for Down, Turner, Klinefelter syndromes, majority spontaneous abortio
Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 8: Non-Disjunction Failure of Chromosome Segregation Mechanism
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