Chromosomes are thread-like structures located within the nucleus that contain hereditary information in the form of genes, which is passed from parents to their offspring. Each species typically has a distinct structure and number of chromosomes. However, due to irregularities during cell division, alterations to the structure or number of chromosomes can occur. Even the slightest of changes can lead to various abnormalities.

“Chromosomal aberrations” refer to changes in one chromosome part, whole chromosome, or chromosomal sets.

Table of Contents

Chromosomal Disorders Due to Numerical Abnormalities

Chromosomal Disorders Due to Structural Abnormalities

[Frequently Asked Questions](#Frequently Asked Questions)

Chromosomal Disorders in Humans

Each human cell contains 46 (2n) chromosomes, which are present as 23 pairs (n pairs). Of these, 22 pairs are autosomes and 1 pair is a pair of sex chromosomes.

Chromosomal disorders result from structural changes or numerical changes in chromosomes.

A. Numerical Abnormalities Resulting in Chromosomal Disorders

Chromosomal disorders are due to a change in the number of chromosomes present. These can be categorised into various types:

Aneuploidy: Loss or gain of a chromosome, which occurs when chromatids fail to separate during cell division and results in one gamete having two copies of one chromosome, and the other having no chromosome.

1. Trisomy: The cell has three copies of a particular chromosome instead of the usual two (3n)

2. Monosomy: The cell has one fewer chromosome than normal (2n-1).

Aneuploidy can be caused by nondisjunction of autosomes (chromosomes 1-22) or sex chromosomes.

Fig. 1: Karyotype of Down’s Syndrome

Fig. 2: Klinefelter’s Syndrome

Fig. 3 - Turner’s Syndrome

Chromosomal Disorders Caused by Aneuploidy: These are the primary source of most inherited genetic disorders and miscarriages during pregnancy.

Genetic Disorder	Defect in	Genotype	No. of Chromosomes	Phenotypic Effect

Syndrome	Chromosomes Involved	Chromosome Affected	Chromosome Count	Occurrence
Down’s Syndrome	Autosomes	Trisomy of 21st Chromosome	47	1:700 Live Births

Short stature with a round head

Furrowed Tongue with Partially Open Mouth

Broad palm with palm crease

Called as “Mongolism” or “Mongolian Idiocy”

Prone to heart disease and respiratory issues

Shorter Life Span

Physical, psychomotor, and mental development is delayed indicates a table

Name	Age
John	25

Name	Age
John	25

Condition	Chromosome Involved	Chromosome Count	Prevalence
Patau Syndrome	Trisomy of 13th	47	1:5000 live births

Rarely live longer than a few months

Sloping forehead, hair clip, and cleft palate

Serious defects in the eyes, brain, kidney, or circulatory system

Intellectually or developmentally challenged indicates the table

Name	Age
John	22

Name	Age
John	22

Syndrome	Chromosome	Occurrences	Live Births
Edward’s Syndrome	Trisomy of 18th Chromosome	47	1:10000

Multiple malformations such as those in the lower jaw, fingers, skull, face, and feet

Cardiac Malformations

I can’t survive for more than 3-4 months.

This is bold text.

This is bold text.

| Klinefelter’s Syndrome | Sex Chromosome | XXY | 47 | These males have tall and masculine stature with feminine characteristics

Development of Breasts (Gynecomastia)

Sterile

Small testicles, high-pitched voice, and sparse body hairs

Mental Retardation represents a table

Name	Age
John	25

Name	Age
John	25

Syndrome	Chromosome Abnormality	Chromosome Number	Frequency
Turner’s Syndrome	Monosomy-XO	45	1:5000 Live Births

Only Viable Monosomy in Humans

Sterile Females with Short Stature

Breasts and ovaries are underdeveloped, with reduced thickness and fewer pubic hairs. indicates a table

Name	Age
John	25

Name	Age
John	25

Syndrome	Abbreviation	Chromosome	Occurrence
Triple X Syndrome	XXX	47	1:1000 live births

They are referred to as “super females”

Mild Development Delays and Menstrual Irregularities

This	is	a	table
1	2	3	4

Syndrome	Abbreviation	Chromosomes	Additional Information
XYY Syndrome	XYY	47	*Males with an unusual height

Severe Acne

Below Average Intelligence

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Header 1	Header 2
Data 1	Data 2

Table: Example of Organizing Data

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Euploidy: Loss or gain of an entire set of chromosomes. This is more common in plants.

Haploid: Possessing only a single set of chromosomes, i.e. ’n’ number of chromosomes.

Polyploid: The addition of one or more sets of chromosomes, such as 3n (triploid) or 6n (hexaploid), etc.

B. Chromosomal Disorders due to Structural Abnormalities

Structural abnormalities can be caused by:

• Deleting a large set of genes
• Duplicating a large set of genes
• Rearranging a large set of genes

1) Deletion: During cell division, a portion of the chromosome can be lost when the centromere is absent. This leads to the chromosome lagging behind during anaphase movement, which can result in the chromosome being either reorganized in the nuclei or digested by nucleases. The chromosome that is inherited by the offspring lacks certain genes, and this condition is typically lethal due to the missing genes.

Deletion can be terminal, where a terminal portion of a chromosome breaks, resulting in one break.

Intercalary deletion occurs when an intermediate portion is lost due to two breaks, resulting in three pieces. The middle piece is then lost and the remaining two parts rejoin.

⇒ Example of a disorder caused by deletion:

Cri du Chat (Cry of the Cat): Deletion of a small portion of the 5th chromosome can lead to Cri du Chat Syndrome. Children with this disorder typically have a small head with unusual facial features, severe mental retardation, and make a sound similar to a cat while crying.

2) Duplication: Duplication of a part of a chromosome occurs when there is an excess of that part. If the duplication is present only in one of the homologous pair of a chromosome, the duplicated part forms a loop to bring the homologous regions together during pairing. The extra segment can be arranged in various ways.

Tandem duplication, where two copies of a region of DNA are present side by side (ABCDEF→ABCDEDEF)

Reverse Tandem, here the duplicated region is the reverse of the normal sequence (FEDCBA → FEDCBAEDCBA).

Displaced duplication, where a duplicated region is not located next to the original sequence.

Transposed Duplication: Duplicated part becomes attached to a non-homologous chromosome.

Extra-chromosomal Duplication: In this case, the duplicated part acts as an independent chromosome, with the presence of a centromere.

⇒ Examples of disorders caused by gene duplication:

Fragile X Syndrome: Affects 1 in 1,500 males and 1 in 2,500 females. It is the most common form of inherited mental retardation, caused by the expansion of a CGG trinucleotide repeat sequence beyond 200 copies.

Inversion: A rearrangement of genes occurs when a part of the chromosome breaks and then reunites, having rotated 180° on its own axis. The effects of this type of structural defect are not as severe as those of other structural defects.

Translocation: The process of transferring a set of genes or a portion of a chromosome to a non-homologous chromosome is known as translocation. This does not involve the addition or loss of genes, but rather a rearrangement of them. This rearrangement can lead to changes in the phenotype due to the new environment. It can also cause problems in the development of the egg, sperm, or zygote, leading to miscarriages and children born with disabilities.

Reciprocal Translocation: Here, segments of two chromosomes are exchanged.

Robertsonian translocation, here an entire chromosome attaches to another chromosome.

⇒ An example of a disorder due to translocation is Down Syndrome

Acute Myelogenous Leukemia: In this type of cancer, bone marrow and cells derived from it show the presence of a short chromosome known as the Philadelphia (Ph1) chromosome. This occurs when the 22nd chromosome loses a part of its arm, which is then translocated to the distal end of the 9th chromosome. This mutation is not inherited by offspring.

Frequently Asked Questions

What is Tay-Sachs Syndrome?

Tay-Sachs Syndrome is a rare, inherited disorder that results in the progressive destruction of nerve cells in the brain and spinal cord. It is caused by a genetic mutation that prevents the body from producing an enzyme that is necessary for the normal breakdown of fatty substances in the body. Symptoms typically begin in early childhood and can include an exaggerated startle response, progressive loss of muscle control, and eventually, paralysis and death.

Tay-Sachs is a genetically inherited disease that primarily affects infants. This illness is caused by a lack of enzymes which would normally break down fatty acids. As a result, these substances accumulate in the brain and spinal cord, leading to impaired neural function.

Is Autism a Chromosomal Disorder?

Autism is a neurodevelopmental disorder that can be either genetically inherited or caused by de novo changes in chromosomes.

What is Deletion 4p Syndrome?

Deletion 4p Syndrome, also referred to as Wolf-Hirschhorn Syndrome, is a genetic condition characterized by a deletion in the short arm of chromosome 4. This condition can lead to intellectual disability and a distinct craniofacial phenotype.