The pairing up of homologous chromosomes during Prophase I is called synapsis. Each resulting pair is called tetrad or bivalent.
The separation of the tetrads during Anaphase I is called disjunction. Failure of chromosomes to separate can lead to aneuploidy which means having one extra chromosome or missing one chromosome.
In extreme cases all homologous pairs fail to separate – total disjunction.
Best known example of aneuploidy:
Down syndrome - If non-disjunction occurs in either parent, one of…