Variation and Mutation.pdf

Prepared by Dr. Sushil Neupane (B.V.Sc. and A.H, IAAS, TU)
UNIT 3
VARIATION AND MUTATION
Genetic variation
Genetic variation refers to differences in the genetic makeup of individuals in a population. Genes
are inherited segments of DNA that contain codes for the production of proteins. Genes exist in
alternate versions, or alleles, that determine distinct traits that can be passed on from parents to
offspring.
Genetic variation is important to the processes of natural selection and biological evolution. The
genetic variations that arise in a population happen by chance, but the process of natural selection
does not. Natural selection is the result of the interactions between genetic variations in a
population and the environment. The environment determines which genetic variations are more
favorable or better suited for survival. As organisms with these environmentally selected genes
survive and reproduce, more favorable traits are passed on to the population as a whole.
Causes of Genetic Variation
Genetic variation occurs mainly through DNA mutation, gene flow (movement of genes from one
population to another), and sexual reproduction. Due to the fact that environments are unstable,
populations that are genetically variable will be able to adapt to changing situations better than
those that do not contain genetic variation.
a. DNA Mutation: A mutation is a change in the DNA sequence. These variations in gene
sequences can sometimes be advantageous to an organism. Most mutations that result in
genetic variation produce traits that confer neither an advantage nor disadvantage.
Mutations lead to genetic variation by altering genes and alleles in a population. They may
impact an individual gene or an entire chromosome. Although mutations change an
organism's genotype (genetic makeup), they may not necessarily change an organism's
phenotype.
b. Gene Flow: Also called gene migration, gene flow introduces new genes into a population
as organisms migrate into a new environment. New gene combinations are made possible

by the availability of new alleles in the gene pool. Gene frequencies may also be altered by
the emigration of organisms out of a population. The immigration of new organisms into a
population may help organisms better adapt to changing environmental conditions. The
migration of organisms out of a population could result in a lack of genetic diversity.
c. Sexual Reproduction: Sexual reproduction promotes genetic variation by producing
different gene combinations. Meiosis is the process by which sex cells or gametes are
created. Genetic variation occurs as alleles in gametes are separated and randomly united
upon fertilization. The genetic recombination of genes also occurs during crossing over or
the swapping of gene segments in homologous chromosomes during meiosis.
Examples of Genetic Variation
 A person's skin color, hair color, dimples, and blood type are all examples of genetic
variations that can occur in a human population.
 Examples of genetic variation in plants include the modified leaves of carnivorous plants
and the development of flowers that resemble insects to lure plant pollinators.
 Examples of genetic variation in animals include albinism, cheetahs with stripes, snakes
that fly, animals that play dead, and animals that mimic leaves. These variations enable the
animals to better adapt to conditions in their environments.
Mutation
The source of inherited variation is mutation, which may be defined as an unpredictable change
in the genetic material of an organism. Such a change may be in the structure of a DNA molecule
(a gene mutation), or in the structure or number of chromosomes in the cells of the organism (a
chromosome mutation).
Gene mutations
Gene mutations are the source of the different alleles of a gene. Each is a change in the sequence
of base pairs in a part of a DNA molecule coding for a polypeptide. Such a change may be:
 Substitution of one or more base pairs by others;
 Addition of one or more base pairs;
 Deletion of one or more base pairs.

These changes in the sequence of base pairs of DNA may, or may not affect the sequence of amino
acids in the polypeptide coded by the gene. Any change in the sequence of amino acids may affect
the three-dimensional structure of the polypeptide and hence alter its effect in the organism. A
substitution of one base pair for another does not always alter the amino acid sequence. For
example, changing the triplet of bases on the sense strand of DNA from C–T–T to C–T–C does
not alter the sense of the triplet. Both triplets code for the amino acid glutamic acid (glutamate).
This is a silent mutation. However, changing the DNA triplet C–T–T to C–A–T alters the amino
acid coded for from glutamic acid to valine. Just such a change in the gene for the β-globin chain
of haemoglobin, substituting valine for glutamic acid, gives sickle cell haemoglobin and the
symptoms of sickle cell anaemia.
Chromosome mutations
Chromosome mutations include changes in chromosome structure or in the number of
chromosomes. Changes in chromosome structure can happen during division, when pieces of
chromosome may:
 Be duplicated;
 Break off and be lost;
 Break off and rejoin with the sequence of genes inverted;
 Break off and attach to another chromosome.
The transfer of a portion of one chromosome to another is called translocation. The inherited form
of Down’s syndrome results from the end of the long arm of chromosome 21 joining another
chromosome. Changes in chromosome number result from unequal separation of chromosomes
between daughter cells during division. After meiosis, one daughter cell may lack one chromosome
and the other have one chromosome too many. This is called non-disjunction. A fertilisation which
joins a normal gamete with a gamete containing one chromosome too many results in three copies,
or trisomy, of that chromosome in the zygote. Trisomy gives rise to various genetic diseases
depending on which chromosome is involved. The usual form of Down’s syndrome is caused by
trisomy 21 or three copies of chromosome 21.

Discontinuous and continuous variation
The total appearance of an organism is called its phenotype. Phenotypic differences between you
and your friends include qualitative differences, such as different blood groups, and quantitative
differences such as height and mass. Qualitative characteristics fall into clearly distinguishable
categories, with no intermediates. You are either male or female, and also have only one of four
possible ABO blood groups: A, B, AB or O. This is discontinuous variation. In contrast, the
quantitative differences between individual heights or masses may be small and difficult to
distinguish. When the height of a large number of people is measured, there are no distinguishable
height classes. Instead there is a range of heights between two extremes. This is continuous
variation.

Variance
The variation shown by a quantitative character can be given by the variance, which is a measure
of how much spread there is about the mean (average) value for the character. Figure 1.2 shows
two distribution curves with different variances. Different variances in phenotype result from
differences in both an organism’s genotype (an organism’s genetic make-up) and the effects of its
environment.
The genetic basis of discontinuous and continuous variation
Both qualitative and quantitative differences in phenotype may be inherited via genes. Both may
involve several different gene loci. However, there are important differences between them.
In discontinuous (qualitative) variation:
 Different alleles at a single gene locus have large effects;
 Different gene loci have quite different effects on the character.
In continuous (quantitative) variation:
 Different alleles at a gene locus have small effects;
 Different gene loci have the same, often additive, effect on the character;
 A large number of loci may have a combined effect on a particular phenotypic character.
These are known as polygenes.

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