Principles of Genetics and Breeding

6.1.2 Chromosome mutations

Normally chromatids (mitosis, meiosis) or chromosomes (meiosis), separate at anaphase by the action of spindle fibres. Failure to separate (nondisjunction) results in abnormal ploidy levels.

• Thus, the phenomenon in which two homologous chromosomes fail to separate or disjoin with each other during anaphase of either mitotic or meiotic cell division is called non-disjunction.
• In Drosophila and man various cases of sex-chromosomal non-disjunctions have been reported.
• They basically cause trisomic and monosomic aneuploids which ultimately affect the sexual phenotype and metabolism of the organisms.
• Sometimes this failure is caused by a malfunction of the cell.
• It can also be induced by chemicals such as colchicine, which cause rapid depolymerization of the microtubules.
• Aneuploids, are usually deleterious.
• Similarly, autopolyploids and polyploids with odd numbers of sets of chromosome are usually sterile, because pairing at meiosis may result in unbalanced gametes with aneuploid numbers of chromosomes.
• Chromosome imbalance is also the reason why polyploidy is rare in animals. XXXX or XXY individuals are usually sterile.