Chromosomal Disorders - Notes | Study Biology Class 12 - NEET

What are Chromosomal Abnormalities (Disorders)?

Chromosomal abnormalities are the type of genetic disorders caused due to the change in one or many chromosomes or the abnormal arrangement of the chromosomes. There are different types of chromosomal abnormalities as follows:

1. Aneuploidy – It is a condition in which there is a loss or gain of chromosomes due to abnormal segregation of genes during cell division.

2. Polyploidy – It is a condition in which the count of the entire set of chromosomes increases due to the failure of cytokinesis in cell division. It is mostly observed in plants.

In humans, when there is an extra copy of a chromosome in one of the pairs, it is called trisomy and when one of the chromosome from the pair is lacking, it is called monosomy.

Aneuploidy v/s Polyploidy

Aneuploidy

Normal human beings have forty six chromosomes arranged in twenty three pairs. However, even a slight variation from this pattern causes abnormalities. Chromosomal disorders are caused either due to changes in chromosomal number or changes in chromosomal structure. Changes in chromosomal number occur due to non-disjunction of chromosomes which is the failure of chromatids to dis-join during cell division leading to either aneuploidy or euploidy. Aneuploidy is a condition where one or more chromosomes are either gained or lost. Aneuploidy is of two types – trisomy and monosomy. A normal diploid individual has 2n number of chromosomes. However, when an extra chromosome is added, the condition is known as trisomy. On the other hand, when a chromosome is lost or absent, the condition is known as monosomy.

Polyploidy

Euploidy is a polyploidic condition, where more than two haploid sets of chromosomes are formed due to the failure of cytokinesis. Depending on the number of chromosomal sets added, euploidy can be triploidy, tetraploidy, pentaploid and so on.

Other common examples of chromosomal abnormalities are down syndrome, Klinefelter syndrome, and Turner syndrome.

1. Down’s Syndrome 

It is caused due to the presence of an extra copy of the twenty first chromosome.The symptoms are a swollen face, bulging and slanting eyes, a small mouth and a protruding and furrowed tongue.

2. Klinefelter’s Syndrome

Klinefelter’s Syndrome is a condition where males have an additional X-chromosome, which results in a karyotype of forty seven, that is, XXY. It is one of the most common sex chromosome disorders in males and shows symptoms such as poor beard growth, narrow shoulders, gynaecomastia or breast development and under-developed testis. Moreover, such males are sterile.

3. Turner’s Syndrome

Turner’s Syndrome

Turner’s Syndrome where one of the X-chromosomes is absent in females. Chromosomes in these females will be forty five with XO. Females with this syndrome show symptoms like a webbed neck, constriction of the aorta, poor breast development, under-developed ovaries and short stature.

Changes in chromosomal structure can take several forms. Some of these changes include a portion of a chromosome getting deleted, duplicated or transferred to another chromosome. While some of these structural changes in a chromosome are inherited others take place accidentally when reproductive cells are being formed or during early foetal development. Jacobsen Syndrome and Cri-du Chat Syndrome are some disorders caused due to changes in chromosomal structure. Therefore, chromosomes hold the genetic keys to all the functions of our body and any change in the number or structure of a chromosome can lead to chromosomal disorders.