Chromosomal Disorders: Principles of Inheritance And Variation Class12

CBSE Class-12 Principles Of Inheritance And Variation – Chromosomal Disorders: The chromosomes are thread-like structures that are mainly present in the nucleus which carries the hereditary information of genes that are passed from the parents to the offspring. Due to some irregularities of cell division, alteration in the number or structure of chromosomes may happen.

The disorders which are caused due to the absence or abnormal arrangements of one or more chromosomes are known as Chromosomal Disorders. The changes in one chromosome part of whole chromosomal sets are known as chromosomal aberrations.

Chromosomal Disorders in Humans

The human cell consists of 46 chromosomes as 23 pairs out of which 22 pairs are the autosomes and 1 pair is the sex chromosome. The change in the structure or numerical values in chromosomes is called Chromosomal Disorder.

Chromosomal Disorders Due to Numerical Abnormalities

Aneuploidy

This kind of condition occurs due to the nondisjunction of chromatids when the chromatids fail to separate during cell division. In this, there is loss or gain of chromosomes occurs. In this one gamete consists of two copies of one chromosome and the other consists of no chromosomes.

• Trisomy: The cell consists of one extra chromosome (2n+1).
• Monosomy: The cell consists of one less chromosome (2n-1).

They occur due to the nondisjunction of autosomes therefore sex chromosomes or chromosome1-22.

Chromosomal Disorders Due to Aneuploidy

They are genetically inherited disorders caused by abortion during pregnancy.

• Down’s Syndrome: This is a genetic disorder that is caused due to the presence of an additional copy of chromosomes of number 21. This disorder was described by Langdon Down. The individual who is affected is short-statured with a small round head, partially open mouth, and furrowed tongue, the palm is broad. The physical, mental, and psychomotor development is retarded.
• Turner’s Syndrome: This disorder is caused due to the absence of one of the X chromosomes therefore 45 with X0. This type of female is sterile as ovaries are rudimentary. It lacks other secondary sexual characters.
• Klinefelter’s Syndrome: This is a genetic disorder caused due to the presence of X chromosomes which results in a karyotype of 47, XXY. The individuals who are affected have masculine development. These individuals are sterile.
• Edward’s Syndrome: The symptoms consist of multiple malformations in fingers, lower jaw, skull, face, and fingers. They can’t live more than 3-4 months.
• Patau Syndrome: The symptoms of this syndrome consist of the individual can rarely live for more than a few months, individuals are mentally retarded, and serious defects in the eyes, brain, or kidney.
• Triple X Syndrome: These individuals are known as super females. Mild development delays and menstrual irregularities are the symptoms of this syndrome.
• XYY Syndrome: Males with this syndrome have an unusual height. The symptoms consist of severe acne.

Euploidy

In euploidy there a loss or gain of a whole set of chromosome occurs. It majorly occurs in plants.

• Haploid: There is a loss of one set of chromosomes therefore n number of chromosomes.
• Polyploid: There is an addition of one or more sets of chromosomes. For example – 3n is triploid.

Chromosomal disorders due to Structural Abnormalities

When a large set of genes are deleted, duplicated, or rearranged it causes structural changes in the chromosome. Some of the structural abnormalities can be due to:

Deletion

In this, the chromosome is lost during the cell division. Part of the chromosome without the centromere lags during anaphase movement which is lost from reorganizing nuclei by nucleases. The resulting chromosome gets inherited by offspring. This condition is lethal because of missing genes.

• Deletion can be terminal.
• Intercalary deletion is where an intermediate portion is lost resulting from two breaks, which results in three 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): Deletion of a small portion of the fifth chromosome. Children with this disease generally have small head with unusual facial features and make a sound like a cat while crying.

Duplication

When a part of a chromosome is present in excess it is called duplication. If duplication is present only in one of the homologous pairs of a chromosome, the duplicated part makes a loop to maximize the juxtaposition of homologous regions during pairing.

Example of disorder due to duplication:

Fragile X: It affects 1:1500 males and 1:2500 females. It is the most common form of mental retardation where the CGS segment is repeated more than 200 times.

Inversion

It results from the breakage and reunion of a part of the chromosome which is rotating by 180° on its own axis. Therefore, it occurs as a rearrangement of genes. The effects are not as severe.

Translocation

The transfer of a set of genes of a chromosome to a non-homologous one is called translocation. There is no addition or loss of genes, only the rearrangement occurs. This rearrangement leads to phenotype changes in the new environment. It can cause difficulties in the development of the egg, zygote, or sperm. This results in children born with disabilities and miscarriages.

• Reciprocal translocation: In this, the segment of two chromosomes gets interchanged.
• Robertsonian translocation: In this, there is an entire chromosome attached to another chromosome.

Example of disorder due to translocation

Acute Myelogenous Leukemia: A type of bone marrow, cancer and cells derived from it show the presence of a short chromosome named as “Philadelphia (Ph1) chromosome”.

FAQs on Chromosomal Disorders

Q1: What do you mean by chromosomal disorders?

Answer:

Chromosomal disorders are disorders that are caused due to the absence or excess or abnormal arrangements of one or more chromosomes.

Q2: Define Aneuploidy.

Answer:

Aneuploidy is a condition that occurs due to the nondisjunction of chromatids when the chromatids fail to separate during the cell division.

Q3: Name the types of chromosomal disorders due to aneuploidy.

Answer:

The following are the chromosomal disorders due to aneuploidy :

• Down’s Syndrome
• Turner’s Syndrome
• Klinefelter’s Syndrome
• Edward’s Syndrome

Q4: Name the Chromosomal disorders due to structural abnormalities.

Answer:

The Chromosomal disorders due to structural abnormalities are:

• Deletion
• Duplication
• Inversion
• Translocation