Definition of Meiotic Nondisjunction
A meiotic nondisjunction is an event in which the chromosomes are unable to separate equally from each other during the process of meiosis. When these chromosome pairs are not segregated properly, they tend to go to the same daughter cell and produce abnormal conditions thereby. A number of birth defects and miscarriages are caused by this abnormality. Meiotic nondisjunction occurs during meiosis I as well as meiosis II. In meiosis I, there would be four products.
Two out of these four would have two copies of chromosomes in them whereas the other two would be empty. No chromosomal recombination takes place in this. On the other hand, in meiosis II, two products out of four would be normal whereas the other two would be abnormal. Down syndrome is caused due to meiotic nondisjunction and in this, trisomy occurs on chromosome 21.