Nondisjunction | Definition, Types & Examples


Nondisjunction Definition:

What is Nondisjunction: It lies in cell division when the chromosome does not properly divide. The chromosomes consist of DNA of a cell that needs to perform a function and reproduce. Nondisjunction occurs When the chromosomes cannot separate properly. The result of this is that the daughter cells have an incorrect number of chromosomes, as one can have too many, and others may have too few. This problem causes cell function due to a cell cannot function properly without the right numbers of chromosomes.

Types of Nondisjunction:

Image result for Nondisjunction

This problem can occur during the process of mitosis, meiosis I, or meiosis II.

  • During Mitosis:

During mitosis, the division of somatic cells takes place. Two identical or similar daughter cells produce from each original parent cell. When the nondisjunction occurs, the chromatids do not separate, and the result comes that one cell gains both chromatids, and gain no one. Due to this problem, each daughter cell receives an abnormal number of chromosomes after the complete process of mitosis as, one cell may receive extra chromosomes, and the other receives few chromosomes.

During the process of meiosis I, the gametes are created. One cell divided into four daughter cells by the combined process of both meioses I and meiosis II. The nondisjunction occurs in anaphase of meiosis I when pair of homologous chromosomes not separated. In the result of this problem, one cell copies a chromosome, while other copies no chromosome.

Nondisjunction can still happen in meiosis II, even if in meiosis I the cell divides accurately. During the process of meiosis II, each cell splits and goes from the diploid to haploid for the preparation of fertilization. If the pair of sister chromatids cannot separate accurately during the anaphase of meiosis II, the result will be that one daughter cell has an extra chromosome, and one daughter cell missed the chromosome.

Examples of Nondisjunction Disorder:

Some examples of nondisjunction disorder given below:

  • Down Syndrome:

Down syndrome happens in the result of nondisjunction during the process of meiosis I which produces an egg cell having an extra copy of 21 chromosomes. The people infected with Down syndrome, have three copies of chromosomes 21 in all their somatic cells, two copies from mother and one copy from father.

This extra chromosome 21 causes the host characteristics as delays in physical growth, certain facial features, and mild intellectual disabilities.

  • Sex Chromosome Aneuploidy:

Sex chromosome aneuploidy is the form of abnormal numbers in sex chromosomes. Typically, females consist of two X chromosomes and males consist of one X and one Y chromosome. Nondisjunction is the cause of the individuals to be born female with one X, female with three X chromosomes, males with XXY, or male with XYY. The rare combination can also happen such as five X chromosomes.

Image result for nondisjunction example Aneuploidy

Other Types of Trisomy:

Many other forms of trisomy occur in the result of miscarriage during the first trimester of pregnancy because the fetus cannot survive chromosomal abnormality. The trisomy 16 happens in over 1% of pregnancies and it is the most common trisomy, but many individuals having this trisomy do not survive.

There are three common kinds of trisomy which are survivable; Trisomy 21 (Down Syndrome), Trisomy 18 (Edwards Syndrome) and Trisomy 13 (Patau Syndrome).