What Are Chromosomal Aberrations?

Introduction

A change in chromosome number or a change in the shape or destruction of a part of a chromosome can cause chromosomal aberration. An improper cell cycle and cell division results in chromosomal aberration. Chromosomal aberration can lead to the loss of functional genes or the addition of a few functional genes hence the addition of some characters in the case of aneuploidy.

The major reason for chromosomal aberration among many others includes errors during cell division (that is during Mitosis and Meiosis). In case of errors during mitosis division, it occurs in somatic chromosomes, while errors during meiosis occur only in sex chromosomes or it may be due to exposure to certain substance that cause birth defects (Teratogens).

What are Chromosomes?

Chromosomes are condensed form of DNA which contain coding and non-coding parts of the gene. They are self-reproductive and play a vital role in the hereditary, mutation, variation, and evolutionary development of the species. Chromosome are condensed by binding with an octamer protein named Histones.

• Diploid Chromosomes: Somatic cells of most organisms contain a set of chromosomes (two chromosomes in pair), and this condition can be said as a diploid condition.

• Haploid Chromosomes: The gametic cells i.e., sperm and ova in the case of humans have single chromosome thus showcasing a haploid condition.

The shape of the chromosome is changeable depending on the phase that it is present in the continuous process called cell cycle and cell division. The basic structure of a chromosome is ‘X’ shaped with two chromatids which can be said as sister chromatids. Both chromatids are connected with kinetochore which by joining forms the chromosome structure.

Types of Chromosomal Aberrations

Namely, there are two types of Chromosomal Aberrations −

• Numerical Chromosomal Aberrations.

• Structural Chromosomal Aberrations.

Numerical Chromosome Aberrations (Euploidy)

In the Greek language, the term euploidy means true unit. Numerical abnormalities are a type of chromosome defect that happens when the number of chromosomes in the cells of the body is different from what is commonly found.

There are 2 types of Euploidy −

• Aneuploidy (Monoploidy)

• Polyploidy

Aneuploidy (Monoploidy)

It is the condition in which there will be an abnormal number of total chromosomes in the whole genome. In humans, it includes −

• Monosomy (2n-1),

• Disomy (n+1),

• Trisomy (2n+1), and

• Nulisomy (2n-2)

Monosomy

It describes the absence of one chromosome from the total chromosomal number, which will be 45 instead of 46 in total in human beings.

For example: Turner's syndrome - It is found in only females. It causes various medical and developmental conditions such as short height, failure of ovaries, heart diseases, etc.

Cri-du-chat syndrome - It is also known as 5p minus as there is a loss of one piece of chromosome number 5. It causes high-pitched cry sounds in infants which resembles cat.

Disomy

It occurs when a person receives two copies of a chromosome or a part of it.

For Example: Angelman syndrome - It affects the development of speech, balance, and intellectual ability and sometimes causes seizures.

Prader-Willi syndrome - It is a rare genetic disorder causing various physical, mental and behavioral problems, major indication is constant hunger.

Trisomy

It results from an extra copy of chromosome that is 47 in humans instead of 46.

For example: Down's syndrome: It is also called 21 trisomy as there is an extra copy of chromosome of 21^st pair in humans. It can be characterized by the flat nasal bridge, protruding tongue, low muscle tone, and short stature.

Edward's Syndrome: Trisomy 18 is often resulting the abnormal intellectual ability, underdeveloped internal organs such as lungs and heart, overlapping and/or clubfeet.

Nulisomy

The condition in which there will be an absence of a pair or both of the chromosome numbers resulting in the condition of 2n-2.

Polyploidy

A condition in which the total number of chromosomes is doubled than that of normal condition.

There are 2 distinct types of polyploidies −

• Autopolyploidy.

• Allopolyploidy.

Autopolyploidy

The situation resulting in the rise of two (Double) structurally homologous chromosomes (Genome) i.e., AAAA. For example, Piggy bank plant, Acipenser transmontane.

Allopolyploid

Individual having either two or more complete sets of chromosomes belonging to different species.

There are various types of allopolyploids; Segmental Allopolyploid, Genome Allopolyploid, and Auto-Allopolyploid.

Structural Chromosomal Aberrations

Alteration in the structure of chromosomes without any change in the number refers to structural chromosomal aberrations. There are mainly 4 types of structural chromosomal aberrations, namely -

• Deletion

• Inversion

• Duplication

• Translocation

Deletion

Deletion/absence of tiny or more portion of chromosomal information. For example: Cri-Du-Chat syndrome.

Inversion

Flipping of genetic sequence inside the same chromosomal strand, less likely to cause disease.

Duplication

Condition in which information or genetic material gets repeated, resulting in extra genetic material in the chromosome. For example: Charcot-Marie-Tooth-Disease Type-1.

Translocation

Breaking of some part of the chromosomal strand and getting attached to another chromosomal strand.

Causes of Chromosomal Aberrations

It may be the result of an error in the cell division process.

The crossing-over often occurs during the meiosis phase of cell division. At this phase, errors can occur at the time of chromosomal segregation, which can result in a decrease or increase in the number of chromosomes, producing aneuploidy. If aneuploidy is present in the sex chromosomes, it will be passed on to the next generation.

Furthermore, structural chromosomes can result in monoploidy. Different cell lines can occasionally create mosaic effects in chromosomal crossing over, resulting in chromosomal abnormality.

Diagnosis

Various diagnostic methods to identify chromosomal abnormalities include −

• During prenatal screening i.e., before birth or in the early stages of pregnancy, the trisomy will be detected by a void or fluid-filled area behind the neck.

• Detecting the levels of the hormones hCG, AFP, and uE3 can also be beneficial for detection.

• Tissue samples obtained from the placenta or amniotic fluids are also valuable.

• Karyotyping and Fluorescent in-situ hybridization (FISH) are other viable options.

Precautions to Prevent Chromosomal Aberrations

Precautionary steps to follow include −

• Adequate nutritional demands must be met during the pregnancy.

• Prevent the consumption of Alcoholic beverages and any type of abusive substances.

• Testing for chromosomal abnormalities throughout pregnancy.

• Adequate vitamin absorption

Conclusion

Chromosomal Aberrations are chromosomal disorders that result from mistakes during mitosis and meiosis cell division processes. This consists of two categories, namely numerical aberrations and structural aberrations, each of which includes subtypes.

Both varieties of aberrations are capable of causing dramatic physical and mental changes in individuals, and can even render them infertile. Many methods, such as FISH and Karyotyping, can effectively diagnose such chromosomal aberrations.