Chromosomal Instability: Definition, Types, and Examples

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What is Chromosomal Disorder or Chromosomal Instability?

Many types of disorders are arising in humans, each are due to various reasons such as genetic, acquired or through mutated conditions.

These mutated conditions are mostly due to any abnormality or defect in the chromosome of an individual.

Humans usually have 23 pairs of chromosomes in that 22 pairs are autosomes and the last pair is known as allosome or sex chromosome.

Chromosomal disorders occurs if there is any addition of chromosomes to the particular number or reduction in this particular set of numbers.

Any change in the number of chromosomes is known as ploidy. And this disorder occurs when there is any change in the structure of the chromosome.

Why Chromosomal Disorder or Chromosomal Instability?

Chromosomal disorders are due to any abnormality or change in structure or function of a chromosome.

A change in a structure of any one of the chromosomes or missing of a part of a chromosome or a whole chromosome or mutation or any defects in a particular gene in a chromosome may be due to chromosomal disorders.

These disorders are detected with the help of banding patterns which helps us to identify the abnormalities in the chromosome by staining the chromosomes with colour stains.

Karyotyping also helps us to detect the missing pair of chromosomes. In karyotyping the chromosomes are arranged in a definite manner and seen under a light microscope.

Idiogram also helps in karyographing of chromosomes. Any small change in the chromosome can lead to vigorous effects in the genes and affect the enzyme producing capacity; which leads to various diseases.

These chromosomal abnormality happening in an allosomes are definitely passed to the next generations, where as the abnormalities of autosomes may or may not pass to the next generation.

Types of Chromosomal Instability

Generally, there are two types of chromosomal instability which leads to chromosomal disorders.

1. Structural Chromosomal Instability

2. Numerical Chromosomal Instability

1. Structural Chromosomal Instability

The structural chromosomal instability are due to a change in structure of a chromosome or missing of any part of the chromosome.

As said, there are about 23 pair of chromosomes which are 46 in number.

It also occurs when there is a change in structure or missing of any part of the chromosome or when the missing parts are translocated to any other areas.

This condition causes many birth effects due to addition or deletion of a part of a chromosome.

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All chromosomes are made up of many segments which are commonly divided into short and long arm, where the short arm forms upper half of the chromosome and the long arm forms a lower part of the chromosome.

The short arm is known as “p” arm and the long arm is known as “q” arm.

i. Chromosomal Instability: Deletion of Chromosome

In deletion of chromosomes the part of a chromosome is deleted or went missing.

Even a small piece of a missing chromosome can lead to vigorous disorders or misleading of gene encoding, as a small part contains various information for coding different genes and enzymes.

When these conditions happen in a pregnant mother this condition is being inherited to an offspring.

The best example for deletion is Cri-du-chat syndrome where there is the damage or missing of chromosome number five.

Cri-du-chat Syndrome

Cri-du-chat syndrome is usually seen among one in 20,0000 people. It occurs mostly in the children of America.

It is caused due to a defect or deletion in the chromosome number five. Which occur especially in the p arm and it is said as 5p.

The characteristic feature of this syndrome is that the children’s have a high pitch cry, poor tone of muscles and the heads of these children are very small.

They are usually underweight. They also face the problem while speaking, they express their emotions with the help of signs.

Other related health problems are with feeding, hyperactivity of an individual, and disability in their intellectual activities.

Mostly the people affected with this Cri-du-chat syndrome have a normal life span in most of the cases.

They result in series condition only when they have organ defects along with this syndrome, which leads to short life span of an individual.

The children with this type of syndrome are involved in language and physical therapy to improve themselves.

ii. Chromosomal Instability: Duplication of Chromosome

The term itself provides a complete meaning for this syndrome, here the duplication of chromosomes occurs where the same chromosome forms a copy of the other, which results in two copies of the same chromosome, as a result two copies of the same genetic material are formed.

Even if the genes are present in many copies it leads to non-functioning of the chromosomes.

Or by giving extra instruction to the same enzyme production which leads to errors in developing a baby.

This duplication leads to a Pallister-Killian syndrome.

Pallister-Killian Syndrome

In this syndrome the extra copy of chromosome 12 is present. Which leads to mosaicism.

Infants with this condition has many serious problems such as intellectual disability, poor muscle tone, poor feeding, stiffness of joints, cataracts in eye.

Usually, a people in this syndrome have a very short life span. These people have only their life span till 40 years.

iii. Chromosomal Instability: Inversion of Chromosome

This occurs when the part of the chromosome results in breakage and reuniting by combining by rotating at 180º with its own axis.

This results in rearrangement of genes. But it is not severe as much as other structural abnormalities.

iv. Chromosomal Instability: Translocation of Chromosome

The transfer of chromosomes with a non-homologous pair is known as translocation. The addition or deletion or rearrangement of genes may occur in this condition.

Which leads to a phenotypic change resulting in abnormalities in the development of egg, sperm or zygote. It often results in miscarriage of a pregnant women.

2. Numerical Chromosomal Instability

Any abnormalities in the chromosomal number leads to this condition. It is divided into two types as aneuploidy and euploidy

i. Chromosomal Instability: Aneuploidy

It is a condition where loss or gain of a chromosome occurs. It mostly occurs due to a non-disjunction in a chromosome, during cell division. Aneuploidy is further classified as Trisomy (2n+1) and monosomy (2n-1).

The disorders caused due to aneuploidy is Turners syndrome, Klinefelter’s syndrome and Downs syndrome.

ii. Chromosomal Instability: Euploidy

Here the loss or gain of whole set of the chromosomes occurs. This condition mostly occurs in plants.

Haploid: In this condition loss of one set of the chromosomes occurs.

Polyploid: In this condition the loss of one or more sets occurs.

Chromosomal Instability Citations


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