Disorders And Defects Of Chromosomes

Introduction to chromosomes

Chromosomes are the coloured bodies (Chromos= coloured) accommodating genetic material within them. They are responsible for the genetic inheritance of the traits in the form of genes. Chromosomes are embedded inside the nucleolus- central part of a cell. Human beings have 46 (23 pairs of) chromosomes. Out of which, 22 pairs are called somatic chromosomes or autosomes and the last pair is the sex chromosome otherwise known as the allosomes. The composition of sex chromosomes in the males and females vary, XX indicates females and XY belongs to males. Sex chromosomes determine gender in human beings. The fundamental responsibilities of the chromosomes are the cell division and protein synthesis, however, they also assist in some cellular processes. In many cases, chromosomes can be found abnormally with a number of numerical and structural defects. The following section will elaborate on the chromosomal defects in detail.

 

Chromosomal syndromes

Aneuploidy

Sometimes chromosomes are found with an extra chromosome in addition to the 46 chromosomes, this is known as trisomy. One of the most common trisomy includes Down`s syndrome. In some cases, a chromosome may be missed, in other words, there may be 45 or less called monosomy. The most lethal type of monosomy is autosomal monosomy. One of the well-known autosomal monosomy is (X-chromosome monosomy) OR Turner syndrome.

Trisomies

Trisomy is a condition in which the chromosomes contain an extra pair making it 47 chromosomes ( 44+XXX or 44+XXY). Depending upon the affected pair, 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 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 an additional chromosome is paired with XX or XY, therefore the resulting pair is presented as  XXX or XXY . The  XXY combination is called Klinefelter’s syndrome. The  XXX syndrome is present in females characterized by mental retardation and some emotional problems.  Children with XYY syndrome are phenotypically tall but persist with a number of learning disabilities as well as poor emotional control with aggressiveness.

Polyploidy

Polyploidy is just the opposite to aneuploidy. In a poly policy case, the number of chromosomes largely differ. For instance, in a  Partial hydatidiform mole, there are as many as 69 chromosomes instead of 46 chromosomes. The affected person might have intrauterine growth retardation and some abnormalities of the placenta (69 chromosomes).

Structural defects of chromosomes

 1. Deletions 

It is one of the types of mutation characterised by the deficiency of a part of the chromosome. It is indicated by the sign Δ. Some of the common types of deletions are deletion of 4p or Wolff-Hirschhorn syndrome and deletion of 5p - Cri du Chat syndrome. The alternative form of deletions microdeletion ( 22q) h is called  Shprintzen syndrome.

2. Translocations 

They are the reciprocal breaks at two points of chromosomes leading to the transfer of genetic material from one chromosome to the other. This can lead to the 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 being normal, and the one-half is characterized by duplications and deletions, and the residual one-quarter may experience 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 with the breaks happening along with the centromere.

4. Mosaicisms

Mosaicism is when an individual has 2 or more genetically different sets of chromosomes in the cells. In 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

genetic chimerism or chimaera is a single organism composed of cells with more than one distinct genotype. In animals, this means an individual derived from two or more zygotes.

6. Isochromosomes 

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. Depending upon the deposition of the centromere, the 2 arms can be equal or unequal in their length. The most common Isochromosome defect is Pallister-Killian syndrome.

7. Dicentric chromosomes 

The dicentric chromosomes are the abnormal chromosomes having two centromeres. They are formed through the fusion of the 2 chromosome segments, each with a centromere, resulting in the loss of acentric fragments (lacking a centromere) instead they form dicentric fragments.

8. Ring chromosomes 

Ring chromosomes are formed if the ends of both arms are deleted so that the deleted ends form sticky ends. These sticky ends then join together to make a ring-shaped chromosomes.

 

General rules for identifying chromosomal defects

Chromosomes are visible only during the specific stage of the cell cycle at the time of transfer of the genetic material within the nucleus. At this stage, chromosomes are condensed to become shorter and thicker so that they are microscopically visible. New staining techniques create bands on chromosomes enabling easy appearance through the electron microscope. Each technique develops a distinctive banding pattern so that minute areas of the chromosome may be visibly distinguished and compared. There may be obvious traces of extra pieces or missing pieces through microscopic vision however, a normal microscope may not be able to track this, instead, an ultra-microscope is essential.

 

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)

 

 

Read more

1.   

Evidences for evolution

2.

Structure and functions of chromosomes

3.

Chromosomal theory of inheritance

4.

Introduction to genetics and heredity

 

 

Important Questions

  1. What are autosomes and allosomes?

  2. Define trisomy and explain any one type.

  3. What are ring chromosomes and how do they form?

  4. Explain about translocation defects in chromosomes

  5. What are the causes of chromosomal defects?

 



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