What is nondisjunction?

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. This results in daughter cells with an abnormal number of chromosomes (aneuploidy). It can occur during meiosis (the formation of gametes – sperm and egg cells) or mitosis (cell division in somatic cells).

Here's a breakdown of key aspects of nondisjunction:

Types:

• Meiotic nondisjunction: This occurs during meiosis I or meiosis II. Meiosis I nondisjunction involves the failure of homologous chromosomes to separate, while meiosis II nondisjunction involves the failure of sister chromatids to separate. The consequences are more severe in meiosis I because all four resulting gametes will have an abnormal chromosome number.
• Mitotic nondisjunction: This occurs during mitosis, leading to some daughter cells having an extra chromosome and others having one chromosome missing. This can lead to mosaicism, where an individual has a mixture of cells with different chromosome numbers.

Consequences:

• Aneuploidy: The most common consequence. This is an abnormal number of chromosomes in a cell. Examples include:
  • Trisomy: Having three copies of a particular chromosome (e.g., Trisomy 21, Down syndrome; Trisomy 18, Edwards syndrome; Trisomy 13, Patau syndrome).
  • Monosomy: Having only one copy of a particular chromosome (e.g., Monosomy X, Turner syndrome).
• Miscarriages: A significant number of pregnancies resulting from gametes with aneuploidy end in miscarriage.
• Birth defects: Babies born with aneuploidy often have significant developmental delays, physical abnormalities, and health problems. The severity varies greatly depending on the chromosome involved and the specific aneuploidy.
• Cancer: Nondisjunction in somatic cells can contribute to the development of cancer by leading to chromosomal instability and the accumulation of genetic changes that promote uncontrolled cell growth.

Causes:

The exact causes of nondisjunction aren't fully understood, but several factors are thought to contribute, including:

• Maternal age: The risk of nondisjunction increases significantly with increasing maternal age, particularly for Down syndrome.
• Genetic predisposition: Some families have a higher incidence of nondisjunction.
• Environmental factors: Exposure to certain chemicals or radiation may increase the risk.

Diagnosis:

Nondisjunction can be diagnosed before birth through prenatal testing such as amniocentesis or chorionic villus sampling (CVS). After birth, chromosomal analysis (karyotype) can confirm the diagnosis.

In summary, nondisjunction is a significant source of genetic variation and can have serious consequences for human health. While some cases are associated with increased risk factors, many cases occur spontaneously.