Chromosomes are thread-like structures present within the nucleus that carry hereditary information within the sort of genes which are passed from parents to offspring. Every species features a characteristic structure and number of chromosomes present. Due to certain irregularities at the time of cell division, alteration in the structure or number of chromosomes may happen. Even the slightest alteration can lead to various abnormalities. Changes in one chromosome parts, whole chromosome or chromosomal sets are known as “chromosomal aberrations”.
There are 46 chromosomes in each human cell present as 23 pairs (n pairs), out of which 22 are autosomes and 1 pair of sex chromosomes.
Chromosomal disorders are caused because of the structural changes or numerical changes in chromosomes.
A. Chromosomal Disorders Due to Numerical Abnormalities
Chromosomal disorders are caused due to the change in the number of chromosomes present. This can be categorised into various types:-
Aneuploidy: loss or gain of a chromosome. This happens due to non-disjunction of chromatids when chromatids fail to separate during cell division. This results in one gamete having two copies of one chromosome and the other having no chromosome.
Trisomy: The cell has one extra chromosome (2n+1)
Monosomy: The cell has one chromosome less (2n-1)
Aneuploidy can be due to nondisjunction of autosomes i.e. chromosomes 1-22 or sex chromosomes.
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Chromosomal Disorders Due to Aneuploidy: This is the cause of most of the genetically inherited disorders and abortion during pregnancy
Euploidy: Loss or gain of the entire set of chromosomes. Mostly occurs in plants.
Haploid: Loss of one set of the chromosomes, i.e. ‘n’ number of chromosomes
Polyploid: Addition of one or more set of chromosomes, e.g. ‘3n (triploid)’, ‘6n (hexaploid)’ etc.
B. Chromosomal Disorders Due to Structural Abnormalities
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This happens when an outsized set of genes are deleted, duplicated or rearranged causing structural changes within the chromosome. Structural abnormalities can be due to:
1) Deletion: A portion of the chromosome is lost during cell division. A portion of chromosome without the centromere lags during anaphase movement and are lost from reorganising nuclei or digested by nucleases. The resulting chromosome lacks certain genes that get inherited to offspring. This condition is typically lethal thanks to missing genes.
Deletion are often terminal, where a terminal portion of a chromosome breaks leading to one break
Intercalary deletion, where an intermediate portion is lost resulting from two breaks, which results in 3 pieces. The middle piece is lost and the other two parts rejoin
Example of Disorder Due to Deletion:
Cri du Chat (Cry of the Cat): There occurs deletion of a small portion of 5th chromosome. Children with this disease have a small head with unusual facial features, severe mental retardation and make a sound like a cat while crying.
2) Duplication - The presence of part of a chromosome in excess is known as duplication. If the duplication is present only in one of the homologous pairs of a chromosome, the duplicated part makes a loop to maximise juxtaposition of homologous regions during pairing. The extra segment are often arranged in many ways:
Tandem duplication, where the duplicated region is present side by side (ABCDEF→ABCDEF)
Reverse tandem, here duplicated region is simply reverse of the traditional sequence (ABCDEF→ABCDEF)
Displaced duplication, here duplicated region isn't situated adjacent to the traditional sequence.
Transposed duplication is a way in which the duplicated part becomes attached to a non-homologous chromosome.
Extra-chromosomal duplication, here duplicated part acts as an independent chromosome within the presence of centromere
Fragile X: Affects 1:1500 males and 1:2500 females. This is the most common form of mental retardation. Many people have around 29 repeats at the tip of the X chromosome.
3. Inversion - inversion results from breakage and reunion of a part of the chromosome rotating by 180° on its own axis. So there occurs a rearrangement of genes. Its effects are not as severe as in other structural defects.
4. Translocation - The shifting or transfer of a set of genes or part of a chromosome to a non-homologous one is known as translocation. There is no addition or loss of genes, only the rearrangement occurs. This rearrangement may lead to phenotype changes pertaining to the new environment. It can cause difficulties in the development of egg, sperm or zygote. These often result in miscarriages and children born with disabilities.
Reciprocal translocation, in this segment of two chromosome gets interchanged
Robertsonian translocation, here a whole chromosome attaches to a different chromosome.
Acute Myelogenous Leukemia: during this sort of cancer, bone marrow and cells derived from it show the presence of a brief chromosome named as “Philadelphia (Ph1) chromosome”. The 22nd chromosome loses a neighborhood of its arm which gets translocated to the distal end of the 9th chromosome. It is not transmitted to the offspring.
Q1. Define Chromosomal Disorders in Humans.
Ans - Chromosomes are thread-like structures present within the nucleus that carry hereditary information within the sort of genes which are passed from parents to offspring. Every species features a characteristic structure and number of chromosomes present. Due to certain irregularities at the time of cell division, alteration in the structure or number of chromosomes may happen. Even the slightest alteration can lead to various abnormalities. Changes in one chromosome parts, whole chromosome or chromosomal sets are known as “chromosomal aberrations”.
Q2.What is Translocation?
Ans - The shifting or transfer of a set of genes or part of a chromosome to a non-homologous one is known as translocation. There is no addition or loss of genes, only the rearrangement occurs.