Re: How do non- disjunctional chromosome mutations occur?

During meiosis (or sometimes during mitosis as well), homologous chromosomes recombine, or physically exchange, stretches of DNA. This process is carried out by specific proteins whose job are to mediate the exchange of the DNA strands between the chromosomes. If the exchanged regions of the chromosome are not physically separated and "resolved" correctly, nondisjunction can occur.

Nondisjunction of homologous chromosomes can cause abberant chromosome segregation during the following cell divison. One of the daughter cells (or gametes) contains too many chromosomes and one not enough resulting in what is called aneuploidy. An aneuploid individual is one who differs from a "normal" individual by part of a chromosome set.

In meiotic nondisjunction, the chromosmes fail to separate at either the first or second cell division producing gametes that are either n + 1 or n - 1. If an n-1 gamete is fertilized by a normal gamete, a monosomic zygote results that is 2n-1. In humans, Turner syndrome is the result of nondisjunction of the X chromosome and is characterized by 44 autosomes and a single X chromosome.

If an n+1 gamete is fertilized by a normal gamete, a trisomic zygote occurs. Some examples of trisomy in humans is Down syndrome (trisomy 21) and Kleinfelter's syndrome (XXY individuals).