What Is Nondisjunction

22 February 2024

Venkat S. R.

This image explains how and where nondisjunction occurs during meiosis I and meiosis II. The image also shows how nondisjunction affects the chromosomes count in the four daughter cells depending on when it occurs.

Nondisjunction is a term used to describe a type of genetic abnormality that can occur during the process of cell division. In normal cell division, chromosomes are separated and distributed evenly between the two daughter cells.

However, in cases of nondisjunction, the chromosomes fail to separate properly, resulting in an uneven distribution of genetic material. This can lead to a number of health problems and genetic disorders.

This article explains the different types of nondisjunction, its symptoms, causes, and how it can be diagnosed and treated.

Table Of Contents

What Is Nondisjunction?
What Are the Types of Nondisjunction?
Nondisjunction Causes and Risk Factors
Nondisjunction Diagnosis and Treatment
Living with Nondisjunction
Frequently Asked Questions
- Sources:

What Is Nondisjunction?

To understand how nondisjunction occurs, we first need to understand a bit about chromosomes. Chromosomes are structures made up of DNA and proteins that contain genetic information.

Humans typically have 23 pairs of chromosomes, for a total of 46 chromosomes. When a cell divides, the chromosomes replicate and then line up along the center of the cell.

Chromosomes then break up and are pulled to opposite ends of the cell by spindle fibers. Finally, the cell splits in two, with each daughter cell containing a complete set of chromosomes.

When nondisjunction occurs, the chromosomes do not separate properly. This can happen during either meiosis, which is the process that produces gametes (or sex cells) like sperm and eggs, or during mitosis, which is the process that produces all other cells in the body.

What Are the Types of Nondisjunction?

There are several different types of nondisjunction, each of which can involve different combinations of chromosomes and genetic material. The most common type of nondisjunction is called autosomal nondisjunction.

This occurs when chromosomes fail to separate properly during the division of non-sex cells, resulting in an abnormal number of chromosomes in the resulting cells. Autosomal nondisjunction can lead to a variety of genetic conditions, including Down syndrome, Edwards syndrome, and Patau syndrome.

Another type of nondisjunction is called sex chromosome nondisjunction, which is when chromosomes don’t separate during the formation of reproductive cells. This leads to an abnormal number of sex chromosomes in the resulting cells. Sex chromosome nondisjunction can lead to a variety of conditions, including Turner syndrome and Klinefelter syndrome.

There is also a type of nondisjunction known as mosaicism. This occurs when some cells in the body have the correct number of chromosomes, while others have an abnormal number. Mosaicism can occur as a result of errors during cell division early in fetal development, and can lead to a variety of conditions depending on which cells are affected.

Finally, there is a type of nondisjunction known as uniparental disomy. This occurs when an individual inherits two copies of a chromosome from a single parent, rather than one copy from each parent. Uniparental disomy can occur as a result of errors during meiosis, or as a result of chromosomal rearrangements that result in two copies of a chromosome from one parent.

Nondisjunction Causes and Risk Factors

While the precise causes of nondisjunction are not well understood, there are a number of factors that may increase an individual’s risk of developing this condition.

One of the primary risk factors for nondisjunction is advanced maternal age. As women age, their eggs are more likely to have errors in chromosome division, which can lead to nondisjunction. ^{¹^}

In particular, women over the age of 40 are at an increased risk of having a child with Down syndrome, a common genetic condition associated with nondisjunction. ^{²^}

There may also be a genetic component to nondisjunction, as certain genetic mutations and conditions may increase an individual’s risk of developing this condition. For example, individuals with a family history of Down syndrome or other genetic conditions associated with nondisjunction may be at an increased risk of developing these conditions themselves. ^{²^}

Early diagnosis and management of genetic conditions associated with nondisjunction can be important for ensuring the best possible outcomes for affected individuals.

Nondisjunction Diagnosis and Treatment

Nondisjunction can be detected through a variety of diagnostic tests, including amniocentesis and chorionic villus sampling. Depending on the specific chromosome abnormality that results, treatment options can include surgery, hormone therapy, and various supportive therapies. ^{³^}

Living with Nondisjunction

Individuals and families affected by nondisjunction can benefit from a variety of coping strategies and support resources. These can include counseling, support groups, and various types of therapy.

Scientists and medical professionals are constantly working to better understand nondisjunction and its underlying causes. Ongoing research efforts are focused on developing new diagnostic tools and treatments, as well as identifying new risk factors and preventative measures.

Frequently Asked Questions

Q) What is the difference between nondisjunction and aneuploidy?

A) Nondisjunction is an error that can occur during cell division when homologous chromosomes or sister chromatids fail to separate correctly. This can result in an abnormal distribution of chromosomes in the daughter cells, leading to aneuploidy, which is a condition characterized by an abnormal number of chromosomes. In short, nondisjunction is the cause, while aneuploidy is the effect.

Q) What are the two results of nondisjunction in meiosis?

A) The two possible results of nondisjunction in meiosis are the production of cells with an extra chromosome (called trisomy) or cells missing a chromosome (called monosomy). For example, if nondisjunction occurs during meiosis I of a human cell, one daughter cell will have an extra chromosome, and the other will be missing a chromosome.

If nondisjunction occurs during meiosis II, then two of the four daughter cells produced will be abnormal — one will have an extra chromosome, and the other will be missing a chromosome.

Q) What is nondisjunction of chromosome 21?

A) Nondisjunction of chromosome 21 is a type of aneuploidy that occurs when the homologous chromosomes or sister chromatids fail to separate correctly during meiosis. This can result in the production of gametes (eggs or sperm) with an extra chromosome 21 or with a missing chromosome 21.

When fertilization occurs with one of these abnormal gametes, it results in a zygote with either trisomy 21 (Down syndrome) or monosomy 21 (which is usually lethal). Down syndrome is characterized by intellectual disability, distinct facial features, and other medical conditions.

Q) At which stage of meiosis does nondisjunction occur?

A) Nondisjunction can occur during both meiosis I (anaphase I) and meiosis II (anaphase II). In meiosis I, nondisjunction occurs when homologous chromosomes fail to separate, resulting in the production of gametes with an extra chromosome and gametes missing a chromosome.

In meiosis II, nondisjunction occurs when sister chromatids fail to separate, leading to the production of gametes with either an extra or a missing chromosome. The specific stage of meiosis at which nondisjunction occurs will determine the type of aneuploidy that results.

Q) Is nondisjunction always harmful?

A) No, not all cases of nondisjunction result in health problems or genetic disorders. Some cases of nondisjunction can lead to the production of viable offspring, although these offspring may have health issues or developmental abnormalities.

However, in many cases, nondisjunction can lead to serious health conditions or even death. For example, trisomy 21 (Down syndrome), which is caused by nondisjunction of chromosome 21, can cause intellectual disability, developmental delays, and various medical problems.

Q) Can nondisjunction be prevented?

A) While it is not always possible to prevent nondisjunction, certain factors can increase the risk of this error occurring. For example, advanced maternal age is a risk factor for nondisjunction, particularly for trisomies such as Down syndrome.

Genetic counseling and prenatal testing can help identify individuals who may be at higher risk for nondisjunction and can help couples make informed decisions about their reproductive options. Additionally, maintaining a healthy lifestyle and avoiding exposure to environmental toxins may help reduce the risk of chromosomal abnormalities.

Sources:

1 – Cell: “Cohesin and the Maternal Age Effect.” –

2 – Current Opinion in Pediatrics: “Maternal age and chromosomally abnormal pregnancies: what we know and what we wish we knew.” –

3 – Gottlieb, S. F., Tupper, C., Kerndt, C. C., Tegay, D. H. Genetics: Nondisjunction.