Nondisjunction in Meiosis: Definition & Examples

Nondisjunction during meiosis produces eggs or sperm cells that don’t have the normal number of chromosomes. In this lesson, learn about the process of nondisjunction and problems it can cause in humans.
What is Nondisjunction in Meiosis?
All of your body cells, except eggs and sperm cells, have two sets of 23 chromosomes, for a total of 46 chromosomes. Having two sets of chromosomes is called diploid and designated as 2n. Eggs and sperm cells are called gametes, and they have only one set of 23 chromosomes, which is called a haploid, or n. When two haploid gametes (one egg and one sperm cell) fuse during fertilization, a diploid cell is produced. This diploid cell has one set of chromosomes from the egg and one set of chromosomes from the sperm cell. This diploid cell will divide and develop into a baby.

When two haploid cells fuse, one diploid cell is produced. In this figure, n = 4 and 2n = 8.
Fertilization Produces Diploid Cell

Meiosis is the process of creating haploid gametes from a diploid cell. If everything goes smoothing during meiosis, chromosomes will be separated and distributed evenly to produce four haploid gametes. However, sometimes chromosomes do not separate properly. This is called nondisjunction and results in gametes with either too many or too few chromosomes. In humans, nondisjunction becomes more common the older one gets.

Production of Normal Gametes
To get ready for meiosis, a diploid germ cell must first undergo DNA replication. This process duplicates every strand of DNA so that there are two copies of every chromosome. The two copies are called sister chromatids and are attached to each other. Once DNA replication is complete, two rounds of chromosomal divisions take place, one each in meiosis I and meiosis II. During meiosis I, homologous chromosomes pair up, line up, and then are split apart.

Homologous chromosomes are those that are the same size and carry the same genes. You get one from your mom and one from your dad. In human diploid cells, there are 23 pairs of homologous chromosomes. After meiosis I, two cells are present, each with 23 different chromosomes that are still in duplicate as sister chromatids.

Both the cells produced in meiosis I undergo meiosis II. During this round, chromosomes line up and sister chromatids are pulled apart into two sets. Now, a total of four haploid cells are present. The pulling apart of either homologous chromosomes or sister chromatids is called disjunction and occurs during anaphase of meiosis I or meiosis II, respectively.

Normal disjunction during both rounds of meiosis produces four haploid cells. In this figure, n = 4.
Disjunction During Meiosis

Nondisjunction Produces Abnormal Gametes
Sometimes during anaphase, chromosomes will fail to separate properly. Remember, this is called nondisjunction. This can happen either during meiosis I or meiosis II.

If nondisjunction occurs during anaphase I of meiosis I, this means that at least one pair of homologous chromosomes did not separate. The end result is two cells that have an extra copy of one chromosome and two cells that are missing that chromosome. In humans, n + 1 designates a cell with 23 chromosomes plus an extra copy of one for a total of 24 chromosomes. n – 1 designates a cell missing a chromosome for a total of only 22 chromosomes in humans.

Nondisjunction in meiosis I occurs during anaphase I when one pair of homologous chromosomes fails to separate. In this example, one cell with 5 chromosomes and one cell with 3 chromosomes are produced. Each cell undergoes meiosis II, resulting in two cells with n + 1, or 5, and two cells with n – 1, or 3.
Nondisjunction in Meiosis I

If nondisjunction occurs during anaphase II of meiosis II, it means that at least one pair of sister chromatids did not separate. In this scenario, two cells will have the normal haploid number of chromosomes. Additionally, one cell will have an extra chromosome (n + 1) and one will be missing a chromosome (n – 1).

When nondisjunction occurs during meiosis II only, disjunction occurs normally during meiosis I. In this example, this results in two cells with normal numbers of chromosomes (n), and two cells with abnormal numbers of chromosomes. The cell with an extra chromosome is n + 1, and the cell missing a chromosome is n – 1. (In this figure: n = 4, n + 1 = 5, n – 1 = 3)
Nondisjunction in Meiosis II

Fertilization with Abnormal Gametes
If one of these abnormal gametes undergoes fertilization, then a baby with an abnormal number of chromosomes in its cells could be born. Trisomy is the condition of having 3 copies of one chromosome type. It is designated as 2n + 1 because the cell has the normal two sets of each 23 types of chromosomes plus an extra copy of one chromosome. Monosomy is the condition of having only 1 copy of a chromosome and is designated as 2n – 1.