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In the field of genetics, "nondisjunction" is a technical term for a specific defect in the way the chromosomes of a cell separate. This defect creates a genetic mutation or defect in the cells produced from the process. The resulting cells are called aneuploid. There are two crucial stages of cellular division when this defect can occur. The first stage is meiosis I, when the homologous chromosomes fail to separate, while the second is meiosis II, commonly known as mitosis, when the sister chromatids do not separate properly.
When nondisjunction occurs during meiosis I, all of the resulting gametes, called daughter cells, are affected. Two of the daughter cells will have an extra chromosome, and the other two daughter cells will be missing one chromosome.
Only half of the resulting daughter cells will be affected when this defect occurs during meiosis II. Of the four resulting gametes, two will be normal, one will have an extra chromosome, and one will be missing a chromosome.
When carried through in genetics, nondisjunction can result in either the sperm or the egg cells of the parent possessing 24 chromosomes instead of the normal 23. In these cases, the child conceived will have 47 chromosomes, which is the common variation known as trisomy, seen in children with Down syndrome.
When cells are missing a single chromosome, it is called monosomy. This form of genetic mutation can lead to birth defects such as Turner syndrome, a syndrome marked by developmental delays. It also can lead to infertility caused by the lack of genetic material.
Genetically, nondisjunction is often a death sentence for the fetus. As a result of the abnormal cellular structure, the immune system of the mother will often kick in, destroying the rogue cells and causing a miscarriage. In other cases, it leads to genetic defects, referred to as chromosomal anomalies, that result in conditions such as Down syndrome, Edwards syndrome, Patau syndrome, Klinefelter syndrome, and Turner syndrome.
While genetic testing, including antenatal testing, is used to detect genetic disorders, there are no known cures for many of the conditions caused by this defect. Medical science continues to study this mutation process in hopes of understanding why the cells fail to replicate properly. Mapping out the variations found in the cells produced by this condition may help researchers to find cures for genetic diseases and defects.