What happens if a homologous pair of chromosomes fails to separate, and how might this contribute to genetic disorders such as Down syndrome

If a homologous pair of chromosomes fails to separate, gametes with abnormal chromosome number are formed. Down syndrome is caused due to trisomy chromosome 21.

Explanation:

During anaphase-I of meiosis-I, homologous chromosomes separate from each other and move to the opposite poles. If one or more chromosomes fail to do so, this is called non-disjunction. Non-disjunction of chromosomes results in the formation of gametes with abnormal chromosome numbers and causes many genetic disorders.

For example, when chromosome 21 fails to separate from its homolog during anaphase-I, some of the resultant gametes have two copies of this chromosome and some lack chromosome 21 at all. The fertilization of gamete carrying two copies of chromosome 21 with normal gamete results in the formation of a zygote with three copies of this chromosome and causes Down's syndrome.