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Mutation is an unpredictable, random and spontaneously changes in genetic material. Different alleles of a gene contain slightly different sequence of base. It is caused by several environmental factors:

  1. Ionizing radiation (α, β and γ radiation)
  2. Ultraviolet radiation
  3. Chemical (mustard gas, DDT, CCl4 etc)

Mutation is classified into

  1. Gene mutation
  2. Chromosome mutation


Gene Mutation

gene, mutation, base, addition, deletion, substituition, frameshift

Gene Mutation

Sickle cell anemia

  • Base substitution
  • Inherited blood disorder sickle cell anemia.
  • Gene codes for the β chain has amino acid sequence produced as

Val-His-Leu-Thr-Pro-Glu-Glu-Lys compared with

Val-His-Leu-Thr-Pro-Val-Glu-Lys in sickle cell anemic.

  • Base CTT is replaced by CAT.
  • This unusual β chain cause hemoglobin become less soluble and stick to each other forming long fibre
  • Red blood cells’ shaped are distorted and become half moon shaped or sickle shape.
  • It affects the oxygen transporting capability of red blood cells. Thus, red blood cells stuck in small capillary and prevent normal red blood cells from passing through.
  • Patient may suffer from severe anemia and death.
Sickle cell 01

Image credit: Wikimedia Commons


  • Base substitution
  • Affects gene codes for enzyme phenylalanine hydroxylase.
  • Functions of phenylalanine hydroxylase: catalyse the conversion of amino acid phenylalanine to tyrosine and then melanin.
  • Melanin is brown pigment in skin and hair.
  • If no phenylalanine hydroxylase, no tyrosine and no melanin. — > lighter skin and hair colour.
  • Accumulation of phenylalanine in blood and tissues causes brain damage in young children.
  • Mentally retarded if untreated.

Chromosome mutation

  • Two types of chromosome mutation:
  1. Changes of chromosome structure
  2. Changes of chromosome number
Chromosome Structure Disorder, mutation

Chromosome Structure Disorder

Down’s syndrome

  • Example of autosomal chromosome disorder.
  • Mutation in the changes of chromosome number is caused by non-disjunction in meiosis where the chromosomes are not separate normally during anaphase I or anaphase II. It causes the abnormal number of chromosomes formed in the gametes.

Down Syndrome Karyotype

Down Syndrome Karyotype
Image credit: Wikimedia Commons

Boy with Down Syndrome

Boy with Down Syndrome
Image credit: Wikimedia Commons