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Consider what happens in cell division. During mitosis (producing two genetically identical daughter cells); sister chromatids must separate and move to opposite poles of a cell (being ‘pulled’ apart by the spindle fibres). This is disjunction. In meiosis, disjunction is the point at which two homologous chromosomes separate during Anaphase I. The two members of the homologous pair of chromosomes go to the opposite poles of the cell. This ensures the chromosome number is now no longer diploid but haploid. Disjunction takes place again in Anaphase II of meiosis, when the sister chromatids separate and move to the opposite poles. To compare this with non-disjunction is when this process of separation of chromatids (mitosis) and homologous chromosomes (meiosis) fails to occur. This has serious consequences in gamete formation of meiosis as the resulting cells may have gained an extra chromosome or lost one; eg a sperm cell may have an X and a Y chromosome or none at all. The abnormal number of chromosomes will either lead to death of the cell but in some cases, the resulting abnormal gamete may still go on to fuse with another gamete during fertilisation. The resulting offspring will have an abnormal number of chromosomes in its genotype. Down syndrome is a condition where an extra chromosome exists.

and non-dijunction*

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