MUTATIONS

What is mutations? Mutations are sudden changes in the genotype that are inherited. Mutations are rare in nature and mutated genes are usually recessive to the normal (wild type) genes. Most mutations are generally harmful and some are lethal.

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A somatic mutation is a genetic change in somatic cells. Somatic mutations are only inherited if asexual reproduction takes place e.g. as in plants and unicellular animals. A gene mutation is a change in genes of reproductive cells and is always inherited. The resultant individual is called a mutant.

The mutant has different characteristics from the rest of the population. Types of Mutations Chromosomal mutations – are changes in number or structure of chromosomes. Gene mutations – also called point mutations – are changes in the chemical nature of the gene.

Mutagens:

These are agents that cause mutations.

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They include ultra-violet light, Gamma rays, x-rays and cosmic rays.
Certain chemicals e.g. mustard gas and colchicine also induce mutations.

Causes and consequences of chromosomal mutations

There are three main types of chromosomal mutations.
Changes in the diploid number of chromosomes (allopolyploidy).
The diploid number changes to 3n (triploid) or 4n (tetraploid) and so on.
This results from the doubling of the chromosome number in the gamete (2n).
This is due to failure of the chromosome sets to separate during meiosis.
The phenomenon is known as polyploidy.
It is common in plants and has been employed artificially to produce varieties of crops with hybrid vigour e.g. bread wheat is hexaploid (6n). This is allopolyploidy).
Change in the total number of chromosomes involving the addition or loss of individual chromosomes (autopolyploidy).
This is due to failure of individual chromosomes to separate during meiosis.
One gamete gains an extra chromosome while the other loses a chromosome.
The term non-¬disjunction is used to describe the failure of chromosomes to separate. Non-disjunction results in several disorders in humans:

Down’s syndrome

The individual has 47 chromosomes due to non-disjunction of chromosome 21.
It is also known as trisomy 21.
The individual has slanted eyes with flat and rounded face, mental retardation and large tongue and weak muscles.

Turner’s Syndrome

This brings about to a sterile and abnormally short female.
It is due to loss of one of the sex chromosomes
e. the individual has one X chromosome (44 + X) instead of two (44 + XX).

Klinefelter’s Syndrome

This results in a sterile male who may be mentally retarded.
It is due to an additional X chromosome
e. the individual i.e. 47 chromosomes (44 + XXY) instead of 46 (44 + XY).

Changes in the structure of a chromosome during meiosis.

A portion of a chromosome may break off and fail to unite again or it may be joined in the wrong way or to the wrong chromosome.

These mutations are described as follows:

Deletion:

This is the loss of a portion of a chromosome,
Deletion results in individuals born with missing body parts.
g. limbs in the extreme of cases.

Inversion:

A portion may break from a chromosome and then rejoin to it after turning though an angle-of 1800.

Translocation:

This is when a portion is joined to a non-¬homologous chromosome.

Duplication:

A certain section of an intact chromosome replicates such that the genes are repeated.

Gene Mutations

A gene mutation is a change in the structure of a gene.
It may involve only a change in one base, e.g. adenine in place of thyamine yet the effect on the individual is profound e.g. sickle cell anemia.
There are two main type of gene mutations:
Due to insertion or deletion of one or more (base) pairs.
Substitution of base pairs e.g. purine for pyrimidine.

Genetically inherited disorders in humans

Albinism is a mutation that alters the gene responsible for synthesis of skin pigment (melanin).
The gene for albinism is recessive.
Sickle cell anemia is a common condition in Kenya.
Individuals with the sickle-cell gene produce abnormal haemoglobin.
It is due to gene mutation caused by substitution of the base adenine for thymine.
The result is the inclusion of the amino acid valine (in place of glutamic acid) in the haemoglobin synthesised.
As a result the red blood cells become sickle shaped when oxygen concentration becomes low i.e. inside tissues.
This leads to blockage of capillaries.
Tissues do not get sufficient oxygen.
Homozygous individuals are seriously anaemic and die in early childhood.
Heterozygous individuals have a mixed population of normal and sickled red blood cells.
They are not seriously anaemic and can lead fairly normal lives.
Haemophila (bleeder’s diseases) is due to lack of gene for production of proteins responsible for blood clotting.