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.
  • 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.


These are agents that cause mutations.

  • 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:
See also  ENZYMES

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:


  • 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.


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


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


  • 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.

See also






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