Sources of genetic variation
Genetic variation is a measure of the variation that
exists in the genetic makeup of the individuals within
Genetic variation is an important force in evolution as it
allows natural selection to increase or decrease
frequency of alleles already in the population.
Genetic variation can cause by mutation.
Mutation is the sudden hertiageal change of the
genetic character in the individual.
Mutation appears suddenly in the population of
Types of mutation :
In general there are following two kind of mutation, i.e.
1. Point mutation or micro mutation or gene mutation
2. Large mutation or macro mutation or chromosomal mutation
1. Point mutation or micro mutation or
It is that type of mutation in which
changes of the genetic material takes
place at the nucleotides.
It also divided into following three
It is the mutation by;
c. Frame shift mutation
One of the purine is changed into
another purine and vice versa.
The purine like adenine is changed
into the purinelike guanine or vice
Some changes occur between the
pyrimidine like cytosine into
thiamine or vice versa.
The purine like adenine is changed
into pyrimidine like thymine or vice
Same changes occur between the
cytosine and guanine.
This type of gene mutation is called
c. Frame shift mutation
The nucleotide sequence is changed
resulting new types of the message in
the codon by the frame shift
It takes by following two methods:
2. Large mutation or macro mutation or
It is kind of mutation in which the part of the chromosome is affected.
It may occur either by change in structure or number of chromosome.
It is following types:
A. Variation or change in chromosomal structure:
when there is change in structure of chromosomes in an organism, then
it is called structural changes. These changes are:
The chromosome dilates its
part by the process of
The part of the chromosome
like E is dilated causing the
mutation by dilation.
The part of the chromosome like E
is added on the another
chromosome resulting duplication
resulting duplicationof the
The gene E is duplicated in the
chromosome causing chromosomal
The chromosome consisting the
gene A,B,C,D,E undergo to inversion
at a particular part like BC resulting
new message causing chromosomal
mutation by inversion.
The part of the chromosome
undergo translocation resulting
message in the chromosome.
The two chromosome having
the genes ABCDE and FGHIJ
are resulted into ABCDHIJ and
Thus is the chromosomal
mutation by translocation.
B. Variation or changes in chromosomal
When there is addition or loss of some number of chromosomes in an
organism, then it is callednumerical changes. Numerical changes are of
The term eupolidy designates genomes containing whole
sets of chromosomes.
The euploids are those organisms which contain balanced set
or sets of chromosomes or genomes in any number, in their
The euploidy is of following type;
An individual that contains one half the normal number of
chromosomes is a monoploid and exhibits monoploidy.
Monoploids are very rare in nature because recessive lethal
mutations become unmasked, and thus they die before they are
These alleles normally are not a problem in diploids because their
effects are masked by dominant alleles in the genome.
Some species such as bees, ants and male bees are normally
monoploid because they develop from unfertilized eggs.
Consequently, these individuals will be sterile. A stage in the life
cycle of some fungal species can also be monoploid.
The diploidy is characterised by
two genomes (2n) in the somatic
cell of the diploid organisms.
Most of animals and plants are
The diploidy is related with
fertility, balanced growth,great
vigorosity, adapatability and
survivality of the diploid
• The prefix‘poly’meansmanyand the term‘ploidy’ indicatessets of chromosomes.
• Therefore,adirect translationis‘manysets of chromosomes’.
• Ploidy levelrefers to the numberof sets of chromosomes in a somaticcell.Somaticcells includeall of
the cells in your body,excludingthesex cells.
• Ploidy number(n)refersto the haploidnumberof chromosome for thatparticularorganism.
• Organisms,like bacteria,can be classified as monoploid (n),humansare classifiedas diploid(2n).
• Polyploidyorganismscan be classifiedas triploid(3n),tetraploid(4n),hexaploid(6n)andoctoploid(8n).
Humans are diploid creatures(2n).This means that for
everychromosome in the body
,there is another one to
match it.However, there are organisms that have more
than two sets of chromosomes.
• Polyploidyis the heritablecondition ofpossessing more thantwo completesets of chromosomes.
• It is the condition in whicha normally diploid cell or organism acquiresone or more additionalsets of
chromosomes. In other words, the polyploidy cell or organism has three or more times the haploid
• Polyploidyarises as the resultof total non disjunctionofchromosomes duringmitosis or meiosis.
• Non disjunctionhappenswhen membersof pair of homologouschromosome failto separateduring
• Polyploidyis common amongplantsandhas been a major source of speciation(formation ofnew and
distinct speciesin the course of evolution)in the angiosperm.
• Polyploidy is fairly common in plant kingdom.Roughly 50% of all angiosperms are polyploidy.
• Certain organisms including angiosperm plants, some salamanders, frogs and leeches are
polyploidy. Due to the high frequency of polyploids in these species,these organisms are well
adapted for their environment and have unique advantages.
• Particularly important is allopolyploidy,which involves the doubling of chromosomes in a
• Normally a hybrid is sterile because it does not have the required homologous pairs of
chromosomes for successful gamete formation during meiosis.
• Some cells have an abnormalnumberof chromosomes thatis not a wholr multipleof the haploid
,thiscondition iscalled aneuploidy
• Aneuploidyis often caused by some error resultingin an unequaldistributionof chromosomes to the
• Organism in which aneuploidyoccurs may deviatenoticeablyfrom the norm in appearanceandbehavior.
• Polyploidycan be artificially inducedthrough interspecific hybridization,in vitro endosperm cultureor
somaticcell doublingthrough colchicine.
• There are two differentwaysto inducepolyploidyartificially:meiotic (sexual)or mitotic (somatic).
Importantpolyploidyplantsused for humanfoodinclude:
In human,polyploidy cellsare found in criticaltissues,such as liver and placenta.
Polyploidy is a condition in which
the cells of an organism have more than
one pair of (homologous) chromosomes.
Most species whose cells
have nuclei (eukaryotes) are diploid,
meaning they have two sets of
chromosomes,where each set contains
one or more chromosomes and comes
from each oftwo parents,resulting in pairs
of homologous chromosomes between
Polyploidy is especially common in plants.
Most eukaryotes have diploid somatic cells,
but produce haploid gametes (eggs and
sperm) by meiosis.
◾ Polyploid types are labeled according to the number
of chromosome sets in the nucleus.The letter x is
used to represent the number of chromosomes in a
i. Haploid (one set;1x)
i i . Diploid (two sets;2x)
iii. Triploid (three sets;3x),for examplesterile saffroncrocus,
or seedless watermelons.
iv. Tetraploid (four sets;4x),for examplecotton Gossypium
v. Pentaploid(five sets;5x),for examplekenai birch(Betula
vi. Hexaploid(six sets;6x),for examplesome species of wheat,
vii. Heptaploid or Septaploid (seven sets;7x)
v i i i . O ctaploid or O ctoploid,(eight sets;8x),for example
i x . Decaploid (ten sets;10x),for examplecertain Strawberries
x . Dodecaploid or duodecaploid (twelvesets;12x),for
examplethe plantsCelosiaargentea and Spartina anglica
of euploidywherein the additionalset
of chromosomesis derived from a
multiplechromosomesets derived from
Most eukaryoticorganismsare diploid
(2n)for most of their life cycles,
possessing two sets of chromosomes.
, whole sets of
meiosis or mitosis, leading to polyploidy,
the presence of more than two genomic
sets of chromosomes.
Polyploids which originated by doubling the
chromosomes number of a diploid species,or a
hybrid between races of the same species,
resulting in two pairs of chromosomes is called
Autopolyploid,and the condition is referredto as
Autopolyploidy is due to accidents of meiosis or
mitosis that produce extrasets of chromosomes,
all derived from asingle species.
Autopolyploids have three(triploid),
seven(heptaploids),eight(octaploids) or more
copies of same genome.
An individual or strain whose
chromosome complement consists of more
than two complete copies of the genome of
a single ancestral species is known as
Most instances of autopolyploidy result from
the fusion of unreduced (2n) gametes,which
results in either triploid (n + 2n = 3n)
or tetraploid (2n + 2n = 4n) offspring.
Triploid offspring are typically sterile,but in
some cases they may produce high
proportions ofunreduced gametes and thus
aid the formation of tetraploids.
This pathway to tetraploidy is referred to as
the triploid bridge.Triploids may also persist
through asexual reproduction.
In agricultural setting,autopolyploidy
(particularly,autotriploidy) is applied in
producing seedlessness in watermelon and
An autotriploid would be one that has three
copies of the basic chromosomal set.
O ne way to produce an auto triploid is by
mating a diploid with haploid (n) gamete and
a tetraploid with 2n gamete.
The offspring would have an
unbalanced gametes and therefore possibly
sterile.Thus,auto triploids are mostly
Other ways of producing auto polyploids are
by the union of two diploid gametes,by
somatic doubling,by fertilizing an egg with
two sperms,and crossing a tetraploid with a
Autopolyploids are produced through chromosome doubling of a species, chromosome doubling may occur in
somatic cells giving rise to tetraploid buds.
In solanaceae, 6-36% of shoots are reported to the tetraploids. Some chemicals such as acenapthane, 8-hydroxy
quinoline and nitrous oxide induce chromosome doubling.
The most efficient method of chromosome doubling is the treatment of seeds, seedlings or shoot tips
Colchicine(C22H25O6N) is an alkoloid extracted from the bulbs of Colchicum autumnale.
It interferes with the development of spindle apparatus as a consequence of which the sister chromatids of
chromosomes are unable to migrate to the opposite poles during anaphase. Therefore, all the chromatids(=4) are
included in the same restitution nucleus leading to chromosome doubling.
Colchicine is generally applied as 0.2% aqueous solution or lanolin paste. The duration may vary from 3-24 hrs in case
of seeds and seedlings.
The chromosome doubling effect of colchicine was first described by Blakesle andAvery and by Nebel in 1937.
OF AUTO PO L
YPLO ID S
For example, if a diploid species has two similar sets of
chromosomes or genomes (AA), an autotriploid will have
three similar genomes (AAA), and an autotetraploid, will
have four such genomes (AAAA). Since an autotriploid
remains sterile and cannot produce seeds, therefore, it has
great commercial value in producing seedless varieties of
In Japan, H. Kihara produced seedless watermelons, which
Common "doob" grass of U.P
.and Bihar is an autotriploid.
Other common seedless autopolyploids are grapes,
In O. lamarckiana, the giant mutant described by de Vries
was later on discovered to be an autotetraploid. Further,
whenever autopolyploids, originate in nature, these would
be eliminated due to natural selection.
In many species, autopolyploids show an increase in general vigour and size; the phenomenon is known as
In general,leaves of autopolyploids are larger and thicker,and their flowers,fruits and seeds are larger.
Cells, pollen grains and stomata of autopolyploids are relativelylarger than are those of diploids.
All autopolyploids show variable sterility.
Their growth rate is generally lower than that of normal palnts,and they are later in flowering.
Autopolyploids have a relativelyhigher water content than that of normal plants hence the fresh weight is more.
HOW CAN WE IDENTIFY AUTOPOLYPLOIDS?
Autopolyploids typically have multivalent pairing-chromosomes are more or less identical (polysomic inheritance).
ROLE OF AUTOPOLYPLOIDS IN EVOLUTION
Autopolyploids has contributed to a limited extent in evolution of plant species.Some of our crops are
autopolyploids. E.g.potato(4x), peanut(4x),coffee(4x),lucern(4x),banana(3x),and sweet potato(6x).
Autotetraploids appear to have been more successful as crops than other forms of autopolyploidy.
In addition,many of forage grasses and severalornamental species are autopolyploids.
APPLICATIONS OF CROP IMPROVEMENT
Triploid water melons are produced by crossing
riploid sugarbeets produce larger roots and more
sugar per unitarea than do diploids.
Autotetraploid cabbage and turnip are larger in size and
water contentthan diploids.
In ornamental tetralpoids, the flowers are larger and
longer floweringdurationthando diploids.
LIMITATIONS OF AUTOPOLYPLOIDY
The larger size of autopolyploids is generally accompanied with ahigher water content.As aresult, autopolyploids
of the crop species grown for vegetativeparts do not always produce more dry matter than the respective
Triploids cannot be maintained except through clonal propagation.
New polyploids (raw polyploids) are alwayscharacterized by afew or more undesirable features, e.g.poor
strength of stem in grapes, irregular fruit size in watermelons, etc.Thus new polyploids can rarely be used directly
in crop production.
A polyploid containing genetically different
chromosome sets from two or more species is
known as allopolyploid.
The prefix “allo” indicates the involvement of
non-homologous sets of chromosomes.
ORIGIN OF ALLOPOLYPLOIDS
Natural allopolyploids must likely originate
through chromosome doubling of F1 hybrid
produced by chance through natural
hybridization between two distinct species of the
same genus or from different genera.
Experimental production of allopolyploids is
achieved through chromosome doubling of F1
hybrid with the help of colchicine.
Such allopolyploids are often called synthetic
allopolyploids. The synthesis of allopolyploids
involves two steps ;
Production of F1 hybrid by crossing two
Chromosome doubling of such F1 hybrids.
The man made Triticum aestivum is an
example of synthetic allopolyploid.
Application in crop improvements
Allopolyploidy is much more common in nature
than autopolyploidy. About 80% of all land plants
may be allopolyploids.
The common bread wheat (T. aestivum) is a
Nicotiana tobacam and N. rustica are
Several Brassica species such as B. juncea,
B.napus, B.carniata are allotetraploids. Raphano
brassica is a promising synthetic allopolyploids.
Significance of Polyploidy and its Roles
The condition in an organism in which the cells have more than two pairs of
homologous chromosomes are know as polyploidy and the organisms are called
as polyploidy organisms.
• Due to the presence of an extra set of chromosomes, the polyploidy plants
are regarded as more advanced over others. The polyploidy is very important
in development of varieties and useful crops.
• These species show vigorously in nature and are more resistant to diseases
as compared to normal.
• These are much useful in economical aspects as they show increased
production and larger disease resistant fruits.
• They are better adjusted to the environmental conditions and can withstand
climate fluctuation. Like resistant from cold and drought.
• For example; Triticum, Gossypium, Avena sativa and other Brassica species.
• Polyploidy are considered as the source of origin of new species and
varieties due to the presence of an extra chromosomes or chromosomes
that are present in addition to the actual two pairs of homologous
Larger Strawberry, than normal Seedless Banana
Causes or Origin of Polyploidy
Polyploids might originate by one of the several methods given below:
• Doubling of the chromosomes during early stage of development of
• Union or fusion of the gametes, one or both of which due to some reason
may be unreduced in chromosome number.
• Two male gametes occasionally or sometimes unite with a single egg cel
thus forming a triploid.
• Failure of formation of cell wall during mitosis under cytokinesis after the
• Failure of spindle mechanism in anaphase to separate the sister
chromatids, where consequently the chromosome number is doubled.
• Formation of restitution nucleus (Anucleus) which contain a the
chromosome present in cell i.e. failure of cell to undergo complete
Advantages of Chromosomal mutation
Some mutation can help the organisms to survive better than others.
Like; Lactose tolerance became an advantage to have over others when
numerous populations depended on cows and goats as sources of food.
Chromosomal mutation are often what make us unique as living
organisms. Like; Ficus religiosa species having natural probability of
different leaf sizes and long tips making it distinguishable from others.
Similarly, in humans their faces and its parts differ.
The Disadvantages of Chromosomal Mutations
• Genetic Disorder:
Small genetic issues that may barely affect the individual or larger issues
that may bring major concerns to the individual.
• Trisomy 21: Down Syndrome
Extra 21 chromosome causing characteristic facial features, menta
retardation and other effects.
• Klinefelter syndrome:
A genetic condition in a male characterized by the presence of at least
one extra X chromosome. This condition may not be detected early stages
rather after puberty such as: less body hair, weaker muscles, enlarged
• Triple X syndrome:
A genetic condition in which a female has and extra X
chromosome. This condition shows poor muscle tone,
low-pitched heavy voice, etc.
• Other Diseases like diabetes, cancer even asthma are
mutation related. Huntington’s disease, Parkinson’s
Results of Mutations
Mutation generally results in loss of function of genes such as:
• Mutations change the sequence of nitrogenous bases in a gene that
causes the change in the codons and then altered the sequence of amino
acids. Therefore, the mutate polypeptide chain may have modified or
reduced functional activity.
• Mutation may lead to the formation of a termination codon among the
codons for a particular polypeptide chain. This will result in the
termination of an incomplete polypeptide. This leads to the loss of
normal functioning of a gene.
• Mutation can be induced by several techniques such as physical,
chemical, and insertion mutagen treatments that are called induced
mutagenesis playing significant role in increasing genetic variability for
desired traits in various food crops.