Gene Mutations: Causes, Examples, and Types

The term ‘mutation’ was introduced by Hugo De Vries, a Dutch Botanist and also rediscovered of Mendel’s laws of heredity.

Mutation is a sudden, hereditary change in the genetic make up of an organism. Mutation is of two types gene mutations or point mutations and chromosomal mutations.

Gene mutations include changes in the structure or composition of genes whereas chromosomal mutations or chromosomal aberrations involve changes in the structure or number of chromosomes about which discussions have been made in the preceding paragraphs.

Since gene consists of few segments of DNA, gene mutations include changes in the number or arrangement of nucleotides. Thus, gene mutations alter or modify the expressions of a particular gene. Sickle cell anemia, chlorophyll deficiencies in plants and albinism (loss of pigment) are caused by gene mutations. Naturally occurring mutations are known as spontaneous mutations. In 1910, Morgan found few white eyed Drosophila in a population of no^– mal red eyed Drosophila. In Drosophila many mutant forms such as white eye, black body, vestigial wings arose through spontaneous mutations.

Mutations caused by mutagenic agents like X-ray, Ultra-violet rays, mustard gas, formaldehyde, caffeine, phenol etc. are known as induced mutations. In contrast to spontaneous mutations, the frequency of induced mutations is high.

Causes of Gene Mutations:

Since gene mutations or point mutations involve changes in the number of nucleotides in a DNA segment or cistron, it is otherwise known as the frame shift mutations. Addition or insertion of one or more nucleotides or deletion of one or more nucleotides changes the sequence of amino acids during protein synthesis. If a nucleotide pair (= nitrogenous base pair) is substituted (substitution mutations) by another pair it will also produce gene mutations.

These substitutions are caused either as:

(i) Transitions or as

(ii) Trans versions.

In transition, a purine is replaced by another purine and a pyrimidine is replaced by another pyrimidine i.e., A = T is replaced by G = C or vice-versa. In trans version, a purine is replaced by a pyrimidine or a pyrimidine by a purine, i.e., C = G is replaced by G = C or A = I is replaced by T = A.

Examples of Gene Mutations:

The earliest record of gene mutations dates back to 1791, when Seth Wright observed a lamb with unusually short legs in his flock of sheep. This short legged or Ancon sheep could not gel over the low stone fence and damage the crop in the nearby fields. Wright thought that it would be worthwhile to have a whole flock of these short legged sheep for this reason.

In the successive generations, this character was transferred and a line was developed where all sheep had short legs. This trait resulted from a recessive mutation and the short legged individuals were homozygous recessive. This gene mutation was discovered at a time when the science of genetics did not even have its birth.

The scientific study of mutations started in 1910, when T.H. Morgan started his work on fruitfully, Drosophila melanogaster, and reported white eyed male individuals among normal red eyed females. Later it was found that the gene for this character was found on sex chromosome.

The human blood disease sickle cell anemia is another example of point mutation. It is caused due to abnormal haemoglobin S which is an insufficient oxygen carrier. It has been observed that the abnormal haemoglobin differs from the normal one only in its two P polypeptide chains (haemoglobin 2 alpha and 2 beta chains) which contain amino acid valine instead of glutamic acid at the position sixth.

Minor change involving two nucleotides in DNA brings substitution in amino acid and thus producing profound change preventing synthesis of normal haemoglobin. Thalassemia, Phenyl ketonuria, alcaptonuria and many other human diseases are caused by simple base substitution in the nucleotide that prevents synthesis of normal protein. Gene mutations although cause minute change in the base pairing, its impact is largely felt by the organisms bearing such mutant gene.

Types of Mutations:

Generally mutations are harmful or deleterious and do not produce visible effects. Less than 20% mutations are lethal. The mutant genes when present in homozygous condition cause death of the organism. Mutant genes are mostly recessive to the normal gene. These genes produce their effects only in homozygous condition hence remain undetected for a period of time. It means that the mutation rate is actually much higher than the frequency of visible or detectable mutations.

(i) Forward and Reverse Mutations:

A mutation from wild type (original type) to a new type is known as forward mutation. The mutated gene may mutate back to the wild type. It is known as reverse mutation.

(ii) Somatic and Gametic Mutations:

Mutations occurring in somatic or non-reproductive cells are called somatic mutations, these are not heritable and are lost with ‘he death of the mutant organism. Mutations occurring in germ cells or gametes produce gametic mutations which are heritable.

(iii) Spontaneous and Induced Mutations.

Spontaneous mutations occur in natural conditions and have a very low frequency. Under experimental or artificial conditions induced mutations are caused. Any physical or chemical agent which introduces mutation in an organism is a mutagen or mutagenic agent. H.J. Muller (1927) induced mutations in Drosophila by X-rays and observed that the frequency of mutations is directly proportional to the amount of X-ray.

Role of Mutation in Evolution:

Hugo De Vries (1901) of Netherland propounded the mutation theory of evolution. According to him new species evolve from earlier species, not by natural selection and accumulation of small, continuous variations through generations, but by sudden heritable changes in the characteristics of the individuals. Later, the mutation theory was widely criticized on the point that “new species arose only by mutation”.

At present mutations are considered to be raw material for evolution. Other forces of evolution like natural selection, isolation, genetic drift etc. operate on mutations to bring divergence in the naturally inter breeding populations. Though, majority of mutations are harmful or disadvantageous to their possessors, but some may be harmless and a few advantageous.

Usefulness of Mutation:

(i) In Plant Breeding:

Methods for inducing mutations are being widely used all over the world in the improvement of crop plants including food, fodder, horticulture, medicinal or industrial commodity plants. This is done with the help of some mutagens such as some chemicals and radiations. Nitrous oxide, ethylene, colchicine, mustard gas etc. are used as chemical mutagens.

Irradiations by X-rays, gamma rays etc. are also used to induce mutations. Many high yielding, high protein and high vitamin containing crops have been developed by irradiation. Sugar contents of sugarcane, oil contents of oil seed, fibre contents of many fibre yielding plants have been improved with the help of artificial mutation. Some insect pests are sterilized through artificial mutation which is an important attempt towards pest control.

(ii) In Animal Breeding:

Breeding of useful animals through mutation may lead to more healthy and disease resistant animal varieties. However, induced mutants have rarely been tried for this purpose although a number of mutant varieties of animals have also been found beneficial.

Short legged Acorn breed of sheep described in the earlier paragraph belongs lo this category. Now certain mutant varieties of cattle, horses, pigeon and cats have been selected and interbred to maintain their races. Some new breeds of sheep have been developed in U.S.A., Australia and New Zealand.

(iii) In Microorganisms:

It is easy to induce mutations in microorganisms like bread mould- Neurospora (a unicellular fungus) and intestinal bacterium Escherichia coli than the higher plants and animals. Haploid micro organisms have just one copy of each gene. In them, each and every mutation is expressed in the same generation and thus easy to locate mutation. Development of better varieties of yeast, Penicillium and some other microorganisms may increase commercial production of alcohol, antibiotics, acids and solvents.