Chromosome and Chromosomal Disorders

What is a chromosome?

The genes are arranged in precise arrays to form a linear unit known as chromosome
It consists of DNA and proteins called Chromatin.
Heterochromatin refers to highly coiled regions where genes aren’t expressed.
Euchromatin refers to loosely coiled regions where genes can be expressed.

Chromosomes contain the genetic information necessary to direct the development and functioning of all cells and systems in the body. They pass on hereditary traits from parents to child (like eye color) and determine whether the child will be male or female.

What is the number of chromosome in normal cells?

The normal human somatic or body cells (cells other than gametes or sex cells i. e. sperm and egg cells) contain 23 pairs of different chromosomes, for a total of 46. One member of each pair is derived from individual’s father and the other member is derived from the mother.

Normal gametes or sex cells contain 23 chromosomes

What are autosomes and sex chromosomes?

Out of 23 pairs of chromosomes in somatic cells, one of the pairs consists of sex chromosomes that carry certain genetic characteristics linked to sex (male or female). In normal male, the sex chromosomes are a Y chromosome transmitted from father and an X chromosome from mother. Two X chromosomes are found in female, one inherited from each parent. Other 22 pairs of chromosomes are called autosomes that carry rest of the genetic hereditary information.

In gamete cells, single copies of each autosomes and sex chromosome are present. A normal male gamete cell consists either X or Y chromosome whereas female gamete has only X chromosome.

What are homologs or homologous chromosomes?

The member of each pair of autosomes are said to be homologs or homologous as their DNA is very similar. The X and Y chromosomes are not homologous of one another.

Normal somatic cells are called to have diploid (2n) number of chromosome; normal gametes are haploid or n.

How is the chromosomes number maintained?

The diploid number of chromosomes is maintained in successive generation of somatic cells by the process of cell division called mitosis whereas the haploid number is obtained through a process known as meiosis.

What is the basic structure of a chromosome?

Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.”

Classification of Chromosome

A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come in contact. It is involved in cell division as the point of mitotic spindle. According to the position of centromere, the chromosome can be classified as metacentric if the centromere occurs near the middle of the chromosomes, acrocentric if the chromosome has its centromere near the tip and submetacentric if centromere is somewhere between middle and tip.

The study of chromosomes and their abnormalities is called cytogenetics.

What is a chromosomal disorder?

Chromosomal disorders result from chromosomal aberration or abnormality in which either the number of chromosomes (numerical aberration) or structure of chromosome (structural aberration) can be altered.

Chromosomal disorders occur in approximately 1 of every 150 live births and are leading known cause of both mental retardation and pregnancy loss.

Numerical aberration can be:

Aneuploidy when an extra chromosome is present (trisomy) like trisomy 21or Down syndrome, trisomy 18 or Edward syndrome, Klinfelter syndrome (47,XXY) or when one is missing (monosomy) like Turner syndrome (45,X)

(Please note that Trisomy 18 and 21 are related with autosomes whereas Klinfelter and Turner syndrome are related with sex chromosomes.)

Polyploidy when there is presence of a complete set of extra chromosomes e g triploidy (3n) and tetraploidy (4n).

Aneuploidy results from nondisjunction during meiosis or mitosis and may involve the autosomes or sex chromosomes.

Polyploidy is caused due to abnormal cell division where the homologous pair of chromosome during meiosis I or sister chromatid during meiosis II or mitosis fail to separate leading to formation of diploid gametes (results 3n or 4n)
or can cause all of the duplicated chromosomes migrate to one of the two daughter cells (cells formed after the cell division) during mitotic cell division in early embryo
or due to dispermy ie fertilization of ovum (n) by two sperms (result 3n)
or due to fusion of ovum and a polar body, each containing 23 chromosomes, and subsequent fertilization by a sperm cells (result 3n)
or due to fusion of two diploid zygotes (result-4n)

Triploidy or tetraploidy in humans are not viable condition or those survived dies soon after the birth.

Structural aberration occurs when the chromosome's structure is altered. This can take several forms:


A portion of the chromosome is missing or deleted. Known disorders in humans include Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4; and Jacobsen syndrome, also called the terminal 11q deletion disorder.


A portion of the chromosome is duplicated, resulting in extra genetic material. Known human disorders include Charcot-Marie-Tooth disease type 1A which may be caused by duplication of the gene encoding peripheral myelin protein 22 (PMP22) on chromosome 17.


When a portion of one chromosome is transferred to another chromosome. There are two main types of translocations. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the Centromere - in humans these only occur with chromosomes 13, 14, 15, 21 and 22.


A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.


A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.


Formed by the mirror image copy of a chromosome segment including the centromere.


Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):
Thomson and Thomson Genetics in Medicine, 7th Edition
Emery's Elements of Medical Genetics, 14th Edition