Chromosomal defects and their disorders

Introduction to chromosomes

Chromosomes are the coloured bodies (Chromos= coloured) containing genetic material within them. The genetic material is present in the form of genes. Chromosomes are present within the nucleolus- central part of a cell. Human beings have 46 (23 pairs of) chromosomes. out of which, 22 pairs are called somatic and the last pair is the sex chromosome. the sex chromosome is also known as allosome represented by XX in females and XY in males.  A healthy chromosome assist in cell division and protein synthesis, however, there are many chromosomal defects both by birth or acquired. Most Common chromosomal conditions can be classified as numerical defects, structural defects, and syndromes. The following section will elaborate on the chromosomal defects in detail.

 

Table of Content

1. Introduction to chromosomes

 

2. Most common chromosomal syndromes

    2. 1  Aneuploidy

    2.2  Monosomy 

    2.3  Trisomy

    2.4  Autosomal polyploidy 

    2.5  Polyploidy

 

3. Structural defects of chromosomes 

     3.1  Deletions 

    3.2 Translocations 

    3. 3 Inversions

    3.4 Mosaicisms

    3.5 Chimerism

    3.6 Isochromosomes formation 

    3.7 Dicentric chromosomes 

    3.8  Ring chromosomes 

Most common chromosomal syndromes

Aneuploidy

Aneuploidy refers to the abnormal numbers of chromosomes presenting with either excessive numbers or lesser numbers against the normal 46 chromosomes. Common types of aneuploidy are Monosomy and Trisomy explained as below.

Monosomy 

Monosomy is when an individual has 45 chromosomes instead of 46. There is one chromosome missing out of 46. The most lethal type of monosomy is autosomal monosomy. One of the well-known autosomal monosomy is (X-chromosome monosomy) OR Turner syndrome.

Trisomy

Trisomy is a condition in which there are 47 chromosomes ( one additional chromosome) instead of normal 46 ( 23 pairs). Depending upon which pair of the chromosome has an extra piece, trisomy can be Trisomy -18, Trisomy-21, and Trisomy-13. Trisomy-21 ( Down`s syndrome) is the most common and highly potent in developing children. 1 in 800- 1000 newborns will born with Down`s syndrome. On the other hand, trisomy 18 is known as Edwards’s syndrome seen rarely ( 1 in 8000 newborns). Trisomy 13 is also called Patau syndrome affecting 1 in 20,000 newborns. 

Autosomal polyploidy 

Autosomal polyploidy is the condition in which there is one extra chromosome paired with XX or XY.i,e XXX or XXY . If the combination is made up of XXY, then it is called Klinefelter’s syndrome. The  XXX syndrome is present in females characterised by variable intelligence and mental problems. XYY syndrome kids are phenotypically tall but persist with learning disabilities and aggressiveness.

Polyploidy

Polyploidy is a little different than trisomy. In this case, there may be a huge difference in the number of chromosomes against the normal 46 numbers. Some of the common types are Partial hydatidiform mole (69 chromosomes) occurs when the extra set of chromosomes is paternal. The affected person might have intrauterine growth retardation of the fetus and the placenta (69 chromosomes) occurs when the extra set is of maternal origin.​

Structural defects of chromosomes

 1. Deletions 

They are also called known as mutant genes or gene deletion or gene deficiency indicated by the sign: Δ. It is associated with a mutation (a genetic aberration) through the chromosomal structure. Some of the common deletions can be deletion of 4p or Wolff-Hirschhorn syndrome and deletion of 5p - Cri du Chat syndrome. Deletions can also happen as microdeletion of 22q which is called  Shprintzen syndrome.

2. Translocations 

They are the reciprocal breaks at two points of chromosomes leading to the transfer of genetic material before it could heal. This causes a wrong presentation of gene order in the daughter cells. Translocations can be balanced translocation characterized by equal amounts of genetic materials are exchanged  OR they can be abnormal translocations with one-quarter of gametes are normal, one-half are characterised by duplications and deletions, and one-quarter are made up of translocation carriers.

3. Inversions

Inversions are the breaks that occur at two points resulting in the inversion of the genetic sequence before it gets healed completely. Inversion can be Paracentric where the breaks occur away from the centromeres or they can be Pericentric inversions that occur with the breaks include the centromere.

4. Mosaicisms

Mosaicism is when an individual has 2 or more genetically different sets of cells in his or her body. n a normal person,  there are 46 chromosomes grouped in 23 pair but in a mosaicism case,  some cells may have 46 chromosomes and few others have 47.

5.Chimerism

Chimerism is when there are 2-cell lines in the body made up of different chromosomal complements from two different zygotes. Fraternal twins with exchange of hematopoietic stem cells may lead to an individual with two different DNAs on blood testing.

6. Isochromosomes formation 

The abnormal chromosomes being formed with identical arms - either two short (p) arms or two long (q) arms. The arms lie on the same side of the centromere, with or without an equal length, and possess identical genes, most common Isochromosomes defect is Pallister-Killian syndrome.

7. Dicentric chromosomes 

Dicentric chromosomes result from the abnormal fusion of 2-p chromosome pieces where both the chromosomes will have a centromere.

8. Ring chromosomes 

Ring chromosomes form when there is a deletion of ends of both arms of the same chromosome and the remaining pieces lead to  "sticky" ends. These sticky ends then join together to make a ring shape.

 

General rules for identifying chromosomal defects

1. Chromosomes are visible only during the stage of the cell cycle when the genetic material transfer occurs within the nucleus. At this stage, chromosomes get condensed to become microscopically visible.

2. New staining techniques create bands on chromosomes. Each technique develops a distinctive banding pattern so that minute areas of the chromosome may be visibly distinguished and compared.

3. There may be visible clues of extra pieces or missing pieces through microscopic vision. A normal microscope may not be able to tract this, instead, an ultra-microscope is essential.

4. The presence of an extra chromosome or chromosome segment can result in a distinctive genetic condition or a peculiar phenotype.

The three major single-chromosome mutations: deletion (1), duplication (2) and inversion (3).  Chromosomal deletion (1), duplication (2) and inversion (3)-source-(Wikipedia)

  

Insertion and translocationInsertion and translocation defects-source( wikipedia)

                        

Balanced translocation(Source-US National Library)

 

Unbalanced translocation (Source-US National Library)

     

   Formation of ring chromosomes source( US National Library)

                            

Dicentric chromosmesDicentric chromosome formation source( wiki commons)



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