[Biology Class Notes] on Chromosomal Abnormalities Pdf

Chromosomes are a thread-like formation of protein, and nucleic acid found extensively in most living cells. These chromosomes are responsible for carrying genetic information and tend to play a vital role in formulating the genetic makeup of new generations. Needless to say, any kind of abnormality or disruption in their functioning tends to hamper the healthy genetic formation or transfer of vital genetic data. Keeping this in mind, let’s proceed to find out more about chromosome abnormalities, their types and causes. Doing so, you will further understand the importance of chromosomes in general

What are Chromosomal Abnormalities?

Chromosomal abnormalities are genetic disorders which occur due to a change in chromosomes or owing to the abnormal arrangement of chromosomes. The abnormalities in chromosomes can occur during the early stages of development in the fetus. There are ways to detect such abnormalities but all cannot be detected in prenatal screening or testing. Typically, there are several types of chromosomal abnormalities which can be categorised as either structural or numerical abnormality.

Numerical Abnormalities

In this condition, one of the chromosomes is absent from the pair and such a condition is known as monosomy. Similarly, when an individual has more than two chromosomes in a pair then it is known as trisomy. 

The most common types of numerical abnormalities are as follow –

Polyploidy 

This particular condition is widely observed in plants. Typically, it occurs when the chromosome count of a set increases owing to the malfunctioning of cytokinesis.

Aneuploidy 

It typically arises due to increase or decrease of chromosomes. Notably, it is a result of abnormal segregation of genes. On the other hand, structural abnormalities arise due to breakage or incorrect arrangement of chromosomal segments. For example, ring chromosome abnormality can occur when a particular chromosome breaks twice, and then its broken ends join into a circular chromosome. Likewise, an isochromosome is formed when a chromosome’s arm goes missing, and the remaining one starts to duplicate. This image below shows the common numerical and structural chromosomal aberrations.

Mixoploidy

In this condition, one cell is diploid and the other is polyploid coexisting within the same organism.  It is also found in living adults and children. It is considered an essential topic of cytology.  

Structural Abnormalities

When the chromosomal segments break and rejoin in the fetus, it results in structural abnormalities. It may result in diseases. 

Structural abnormalities can be defined in two ways – 

1. Balanced – A structural chromosome is said to be balanced when the total chromosomal set is present and rearranged. 

2. Unbalanced – A structural chromosome is said to be unbalanced when the material or information is additional or absent.   

The common kinds of Structural abnormalities are as follows – 

Deletions

Most chromosomal abnormalities occur due to accidents in the egg or the sperm. In this kind of abnormality, a portion of the chromosome is absent or deleted. 

Duplications

In this kind of abnormality, a portion of the chromosome is duplicated thereby, resulting in additional genetic material. 

Translocations

When a portion of chromosome transfers to another chromosome then it results in translocation. There are two kinds of translocations and they are as follows:

1. Reciprocal translocation 

In this type of translocation, the segments of two different chromosomes are exchanged in the fetus. 

2. Robertsonian translocation 

In this type of translocation, a chromosome is entirely attached to another chromosome in the centromere.

Rings 

This may happen with or without the loss of genetic material but in this kind of abnormality, the chromosome breaks off in the fetus to form rings.

Inversion

When a chromosome is broken off in the fetus, turns upside down and reattaches to another chromosome, it results in inverted genetic materials. 

Dicentric Chromosomes

A chromosome that contains two centromeres instead of one like a normal chromosome is known as a dicentric chromosome. It may be a result of the abnormal fusion of chromosomes with one centromere each.  

Chimerism

A chimera is an animal or a human that contains the cells of two more different individuals. It is a rare condition that can also affect animals causing two different colours of eyes. Such an instance is microchimerism, a fetus absorbs the cells of the womb or vice versa. 

Isochromosomes

These are the chromosomes with two identical arms. So instead of a normal chromosome with one p arm and one q arm, there is an extra set of p arm and q arm resulting in an additional copy of information or absence of information of genes on missing arms. 

It is also to be taken into account that several cancerous cells also develop changes in chromosomal structure. They occur in the somatic cells during the progression of cancerous tumours. 

Treatment of Structural Abnormalities in Chromosomes 

There is no condensed treatment for structural chromosomal abnormalities, in fact, the chromosomal abnormalities carried by men requires no therapy. However, if an individual is impaired fertility then the said individual can seek medical attention with artificial reproductive techniques along with genetic counselling is advised strongly to the male individual. 

Women with balanced structural chromosomal abnormalities are generally fertile. 

It is to be noted that with structural abnormalities, there are higher risks of birthing disabled children. 

DIY: What is chromosome disorder? Provide suitable examples.

Types of Chromosomal Disorders

Some of the most common examples of chromosomal abnormalities include Down syndrome, 

Turner syndrome and Klinefelter syndrome. Details of such disorders are mentioned below.

Down Syndrome

This particular chromosomal abnormality occurs due to an additional chromosome 21, which in turn occurred due to abnormal cell division. In a broader sense, it can be divided into three types – translocation, trisomy 21 and mosaicism.It tends to hamper the physical characteristics and cognitive development in individuals. Also, the most noticeable symptoms of Down syndrome include small height, short neck,
low muscle tone, eyes which slant upwards, etc. 

Turner Syndrome

In this case, there is one X chromosome absent in females which decreases the chromosome count to 45 instead of 46. The prevalent symptoms of turner syndrome include sterility and absence of secondary sexual traits.Other than that, heart disorders, high blood pressure, skeletal abnormalities and kidney ailments are other prominent indicators of turner syndrome.

Klinefelter Syndrome

An extra X chromosome in males is the reason behind the occurrence of Klinefelter. Owing to this, the chromosome count stands at 47 instead of the standard 46. This chromosomal aberration tends to manifest in males in the form of absent or delayed puberty, sterility, development of breast, etc.  It is also said to affect intellectual development in males.

Test Your Knowledge: Look at the chromosomal abnormalities list below and distinguish them as structural and numerical aberrations.

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