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What happens if Non-disjunction occurs in meiosis I, II or mitosis?

From a single cell, billions of cells are formed in an adult human being by mitosis. Even in adults, many cells like skin cells are continuously replaced by mitosis. The precision of the process is extremely important for maintaining an individual and also the maintenance of a species. But think of meiosis, gametes are formed by meiosis that unites to form the first cell zygote. Zygote has the complete program or everything to become a fully functional complete individual. If any fault occur during meiosis, that can be disastrous as the first cell itself is a faulty cell and all cells derived from it by mitosis carry the same mistake. But as I mentioned the living process are almost perfect. Error rate is often one in millions or billions in these vital processes. But nothing in this world is fully perfect. That is the case with cell division also. In this post we will discuss ‘non disjunction’ a major reason for variation in chromosome number.


What are the reasons for aneuploidy?
Some times cells are formed with variation in the basic chromosome number, referred as aneuploidy. In our case 46+1 or 46-1 like that. The reason may be
  • A chromosome may be deleted during mitosis or meiosis. Generally centromere is deleted. So spindle fibres cannot attach and leading to loss of that chromosome during anaphase.
  • Translocation is another reason for loss of chromosome especially Robertsonian translocation. A small chromosome is generated due to Robertsonian translocation and this chromosome often fails to segregate during anaphase. We will discuss this separately later.
  •  The third reason is called as non-disjunction. Non-disjunction is the inability or failure of homologous chromosomes or sister chromatids to separate in meiosis or mitosis. As a result some gamete or cells may contain an extra chromosome and others that are missing a chromosome. This is the most common reason for many syndromes or genetic defects in humans. In this post we will discuss in detail the effect of nondisjunction in different stages of cell cycle.
Look at the figure first
Meiotic and Mitotic Non disjunction
Meiotic and Mitotic Non- disjunction

Case 1: What happens if Non-disjunction occurs in meiosis I?
Explanation of first part of the figure:
  • Nondisjunction in meiosis I=out of 4 gametes formed, two will be (n+1) and the other 2 will be n-1.
  • Fusion of n+1 gamete with normal gamete (n)= 2n+1 or trisomic
  • Fusion of n-1 gamete with normal gamete (n)=2n-1 or monosomic
In short, Nondisjunction in meiosis I lead to trisomy (2n+1) or monosomy (2n-1)
Case 2: What happens if Non-disjunction occurs in meiosis II?
Explanation: see the figure 
  • Nondisjunction in meiosis II=out of 4 gametes formed, one will be (n+1), other will be (n-1) and the other 2 will be n (normal).
  • Fusion of n+1 gamete with normal gamete (n)= 2n+1 or trisomic
  • Fusion of n-1 gamete with normal gamete (n)=2n-1 or monosomic
  • Fusion of n gamete with normal gamete (n)=2n (normal diploid)
In short, Nondisjunction in meiosis II leads to trisomy (2n+1), monosomy (2n-1) and normal diploids.
Case 3: What happens if non-disjunction occurs in mitosis?

  • Of the cells formed one may be 2n+1 or trisomic and other may be 2n-1 monosomic.
  • All cells derived from 2n+1 may lead to population of trisomic cells. (somatic clones of trisomics)
  • All cells derived from 2n-1 may lead to population of monosomic cells. (somatic clones of monosomics)

Nondisjunction in mitosis may lead to mosaicism
In mitosis, nondisjunction lead to the formation of monosomics or trisomics and all cells derived from that cell carry this mistake resulting in a population of cells, but restricted to that site or tissue only. Non-disjunction may lead to mosaicism. 
Mosaicism refers to the condition in which a tissue may contain patches of cells with chromosome abnormality and other cells with normal karyotype. Or Presence of different chromosome constitution within the cells of an individual is called as mosaicism.
Nearly 50% with Turner syndrome exhibit mosaicism. Some cells are normal 46,XX cells and others possessing 45X cells.
Glossary:
Nullisomy: loss of a homologous pair of chromosomes, represented as 2n-2.In humans, a nullisomic person has 44 chromosomes (2n=46, 2n-2 ie 46-2=44)
Monosomy is the loss of a single chromosome, represented as 2n-1. A monosomic person has 45 chromosomes
Trisomy is the gain of single chromosome, represented as 2n+1. A trisomic person has 47 chromosomes. 

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