(4) Non-disjunction and Down Syndrome

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…