An Introduction to Chromosomal Disorders
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”.
Chromosomal Disorders in Humans
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.
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.
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 sets of chromosomes, e.g. ‘3n (triploid)’, ‘6n (hexaploid)’ etc.
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:
Deletion:
A portion of the chromosome is lost during cell division. A portion of the 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 by offspring. This condition is typically lethal thanks to missing genes.
Deletions 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 the 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.
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 the juxtaposition of homologous regions during pairing. The extra segment is often arranged in many ways:
Tandem duplication, where the duplicated region is present side by side (ABCDEF→ABCDEF)
The 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
Example of Disorder Due to Duplication:
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.
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.
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 eggs, 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.
Example of Disorder Due to Translocation:
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 neighbourhood of its arm which gets translocated to the distal end of the 9th chromosome. It is not transmitted to the offspring.
The article discusses all the important points related to chromic disorders in humans. After going through the article, students will get good information that would be helpful for them for their exams as well as for general awareness.
FAQs on Chromosomal Disorders in Humans
1. What is a chromosomal disorder?
A chromosomal disorder is a medical condition caused by an alteration in the number or structure of a person's chromosomes. Humans normally have 46 chromosomes in each cell, arranged in 23 pairs. Any deviation from this standard set, such as having an extra chromosome, a missing chromosome, or a structurally damaged chromosome, can disrupt normal development and bodily functions.
2. What are the main types of chromosomal abnormalities in humans?
Chromosomal abnormalities are primarily classified into two main groups:
- Numerical Abnormalities: This involves a change in the number of chromosomes. It includes aneuploidy (the gain or loss of one or more chromosomes, like trisomy or monosomy) and polyploidy (the gain of one or more complete sets of chromosomes).
- Structural Abnormalities: This involves a change in the structure of a chromosome. Key examples include deletion (loss of a chromosomal segment), duplication (presence of an extra segment), inversion (a segment is reversed), and translocation (a segment moves to a non-homologous chromosome).
3. How does an error like nondisjunction lead to chromosomal disorders?
Nondisjunction is the failure of paired chromosomes or sister chromatids to separate during cell division (meiosis). This error results in gametes (egg or sperm cells) with an incorrect number of chromosomes. If a gamete with an extra chromosome (n+1) or a missing chromosome (n-1) is involved in fertilisation, the resulting embryo will have an abnormal chromosome count in every cell, leading to conditions like Down's Syndrome (Trisomy 21) or Turner's Syndrome (Monosomy X).
4. What are the three main chromosomal disorders studied in the CBSE Class 12 syllabus?
The three key chromosomal disorders specified in the CBSE Class 12 curriculum for the 2025-26 session are:
- Down's Syndrome: Caused by the presence of an extra copy of chromosome 21 (Trisomy 21). Individuals typically have short stature, a small round head, a furrowed tongue, and varying degrees of cognitive developmental delay.
- Klinefelter's Syndrome: A genetic condition in males caused by an extra X chromosome, resulting in an XXY karyotype. Affected individuals are sterile and often exhibit some feminine characteristics, such as the development of breasts (gynaecomastia).
- Turner's Syndrome: A condition in females caused by the absence of one of the X chromosomes, resulting in an XO karyotype. Affected females are sterile as their ovaries are rudimentary and lack secondary sexual characters.
5. What happens if a human has 47 chromosomes instead of 46?
Having 47 chromosomes instead of the usual 46 is a condition known as trisomy, meaning there are three copies of a particular chromosome instead of the usual two. The effects depend on which chromosome is extra. For instance, Trisomy 21 results in Down's Syndrome. An extra sex chromosome, as in Klinefelter's Syndrome (XXY), also leads to a total of 47 chromosomes and causes specific developmental and reproductive issues in males.
6. What is the difference between aneuploidy and polyploidy?
The main difference lies in the scale of the chromosomal change. Aneuploidy refers to the gain or loss of one or a few individual chromosomes, leading to a chromosome number that is not an exact multiple of the haploid set (e.g., 2n+1 or 2n-1). This is the cause of disorders like Down's and Turner's Syndromes. In contrast, polyploidy involves the addition of one or more entire sets of chromosomes (e.g., 3n or 4n). While common in plants, polyploidy is typically lethal in humans.
7. Why are most autosomal trisomies lethal, while sex chromosome trisomies are often viable?
Autosomes (chromosomes 1-22) contain a large number of genes essential for basic development and survival. An extra autosome creates a severe gene dosage imbalance that disrupts development so profoundly that it is usually lethal. However, sex chromosome trisomies (like XXX or XXY) are often viable due to a natural mechanism called X-inactivation, where any extra X chromosomes are largely inactivated in each cell. This compensates for the extra dose of X-linked genes, mitigating the effects.
8. Can chromosomal disorders be inherited from a parent?
Most chromosomal disorders, such as those caused by nondisjunction, are typically random, spontaneous events that occur during the formation of reproductive cells and are not inherited. However, a parent can be a carrier for certain types of disorders. For example, a person with a balanced translocation (where chromosome segments are rearranged but no genetic material is lost) is healthy but can produce gametes with an unbalanced amount of genetic material. This increases the risk of having a child with a chromosomal disorder.
