Question:

A disease caused by an autosomal primary non- disjunction is:

Updated On: Jun 12, 2024

Klinefelter's Syndrome
Turner's Syndrome
Sickel Cell Anemia
Down's Syndrome

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The Correct Option is D

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Down's syndrome is caused by non-disjunction of $21^{\text {st }}$ chromosome.

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Explanation:

Down’s Syndrome- is a genetic disorder caused by trisomy of 21 chromosomes also called non disjunction. In other words, down’s syndrome is a disorder where an individual may inherit an extra part of chromosome 21 or an entire copy of chromosome 21.

An embryo with nondisjunction has three copies of chromosome 21 rather than the normal two. A pair of 21st chromosomes in the sperm or the egg does not split apart before or at fertilization.

Klinfelter syndrome- It is a genetic disorder that causes males to be born with an extra copy of X chromosome. Males with this syndrome may not have any symptoms or may have symptoms like breast growth, less facial and body hair.

Turners Syndrome- It is a genetic disorder which affects only females. It is caused due to the aneuploidy of sex chromosome, when one X chromosome is missing or partially missing. It results in short height and failure of ovary development.

Sickle Cell Anaemia- Sickle cell anemia is an autosomal recessive disorder. It is a disorder of red blood cells. The normal RBCs are disc shaped whereas a person with sickle cell disease has an abnormal shape like that of a sickle making it sticky and difficult to travel through the blood vessels resulting in tissue damage and pain.

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Concepts Used:

Principles of Inheritance and Variation - Mutation

A Mutation is a change in the sequence of our DNA base pairs caused by numerous environmental stimuli such as UV light or mistakes during DNA replication. Germline mutations take place in the eggs and sperm and can be passed onto offspring, whereas somatic mutations take place in body cells and are not passed on.

Types of Mutations

There are three types of mutations, which are as follows:

Silent mutation

It refers to any change in DNA sequence that has no effect on the amino acid sequence in a protein or the functions that a protein performs. There is no phenotypic indication that a mutation has occurred.

Nonsense mutation

When there is a change in the sequence of base pairs due to a point mutation, that results in a stop codon. This leads to a protein that is either shortened or non-functional.

Missense mutation

A missense mutation occurs when a point mutation causes a change in the codon, which then codes for another amino acid.

The mutation is caused by the following factors:

Internal Causes

When DNA copies incorrectly, the majority of mutations occur. Evolution occurs as a result of all of these mutations. DNA makes a copy of itself during cell division. When a copy of DNA isn't flawless, it's called a mutation since it differs somewhat from the original DNA.

External Causes

When certain chemicals or radiations are used to break down DNA, it causes the DNA to break down. The thymine dimers are broken by UV radiation, resulting in altered DNA.