Changes In Number And Structure Of Chromosomes SMG

Dr. Saji Mariam George
Associate Professor (Retired)
Assumption College Autonomous
Changanacherry
Changes In Number And Structure Of
Chromosomes

Changes In Number And Structure Of
Chromosomes
• Generally, the number and structure of
chromosomes of a species are constant. But, there
may be changes in the number as well as structure
of chromosomes of a species.
• Such chromosomal aberrations cause many changes
in the characteristics of the organisms.

I. Changes In Number Of Chromosomes
• Generally the chromosome number of a species is constant.
• Chromosome numbers vary from species to species.
• Generally , the somatic cell of an organism contain 2 sets (2x) of
chromosomes – diploid .
• The gametes contain a single set (x) of chromosomes – haploid.
• Sometimes the chromosomes may change in number.
• Individuals carrying chromosome numbers other than the diploid
number – heteroploids (Heteroploidy).

Heteroploidy
Euploidy Aneuploidy

1. Euploidy
• Involves one or more complete genomes.
(Genome – a single complete set of
chromosomes).
Euploidy
Haploidy Polyploidy

Changes In Chromosome Number….
Image:https://en.wikipedia.org

Haploidy (Monoploidy)
• A condition where some plants and animals have
only one set of chromosomes.
• Common in plants but rare in animals.
• Lower organisms like bacteria, some algae, fungi etc.
are haploids for most of their life and become diploid
or partially diploid only for a brief period.
• Occur spontaneously or can be induced by in vitro
cultures of anther or ovule in suitable culture media.

Polyploidy
• A condition where an organism has more
than 2 haploid sets of chromosomes .
• Polyploids are common in plants – 1/3 of
Angiosperms are polyploids.
• Polyploids have larger cell size than diploids.
• Guard cells of stomata are larger and number
of stomata is lower in polyploids than in
diploids.

• Polyploids have larger cell size than the diploids.
• Guard cells of stomata are larger and number of
stomata is lower in polyploids than in diploids.
• Polyploids have larger and thicker leaves, larger
flowers and fruits.
• Generally, there is increase in vigour and
vegetative growth.

Polyploidy…
Image:https://www.slideshare.net/cldrumm/dna-extraction-lab

Polyploidy…
Bread wheat -Hexaploid

Image: https://sustainable-secure-food-blog.com

Origin Of Polyploids
• Arise spontaneously by the
reduplication of
chromosome number in
somatic tissue with the
suppression of cytokinesis
(Endomitosis)
• By the formation of gametes
with an unreduced number
of chromosomes.
Image:https://ib.bioninja.com.au

Induction Of Polyploidy
Polyploidy can be induced by treatment with -
• Physical agents – heat or cold treatment
• Chemical agents – Colchicine, Acenaphthene,
8- Hydroxyquinoline etc.
• These treatments inhibit the formation of spindle and
thus the cell division is not completed.
• After sometime, the cell recover the normal activity ,
but it has double number of chromosomes.

Depending on the level of ploidy , polyploids may be
• Triploids (3x)
• Tetraploids (4x)
• Pentaploids (5x)
• Hexaploids (6x)
• Heptaploids (7x)
• Octaploids (8x)
• Nonaploids (9x)
• Decaploids (10x) etc.

Image:https://www.slideshare.net/sylviapuglisi/nondisjunctio
n-and-polyploidy

Highest chromosome
number
• Reported in a fern, the polyploid
species of Ophioglossum
reticulatum,
2n = 1262, (n = 631- had repeated
cycles of polyploidy) by Abraham
and Ninan (1954).
Abraham, A. and Ninan,C.A. 1954.
Chromosomes of Ophioglossum
reticulatum L. Curr. Sci. 23: 213-214.
Dr. C. A Ninan,
Professor of Genetics (Rtd),
Head, Department of Botany,
University of Kerala,
Thiruvanthapuram, Kerala,
India

Ophioglossum reticulatum ,
2n = 1262
Image:http://www.plantsystematics.org/imgs/robbin/r/Ophioglossaceae_Ophiogloss
um_reticulatum_7545.html

Ophioglossum reticulatum adder's-tongue ferns
Ophioglossum comes from the Greek, and means
"snake-tongue".
Image:https://en.wikipedia.org
http://www.plantsystematics.org/imgs/robbin/
r/Ophioglossaceae_Ophioglossum_reticulatum
_2932.html

Types Of Polyploids
• All the genomes present in a polyploid species are
identical – Auto triploids (AAA), Autotetraploid
(AAAA) , Autopentaploid (AAAAA), Autohexaploid
(AAAAAA) etc.
(A – stands for one set of genome; AAA means 3 sets of
identical genomes- an autotriploid).
e.g.
Potato – Autotetraploid (4x)
Coffee - Autotetraploid (4x)
Banana – Autotriploid (3x)
Sweet potato – Autohexaploid (6x)
i) Autopolyploids

ii) Allopolyploids
• Have genomes from two or more species (e.g. AABB).
• Most likely produced by chromosome doubling in F1
hybrids between two distinct species belonging to the
same genus (Interspecific or intrageneric hybridization) or
two different genera (Intergeneric hybridization ) .
• Majority of the polyploids are allopolyploids.
e.g. Cultivated Wheat, Triticum aestivum – Allohexaploid –
6x.
• First synthetic allopolyploid, Triticale (Wheat , Triticum
turgidum x Rye, Secale cereale - produced by Wilhelm
Rimpau, 1888 – His results were published in 1891.)

2. Aneuploidy
• Aneuploidy is the loss or gain of one or a few
chromosomes . Chromosome number of an
individual is not an exact multiple of the basic
chromosome number of the species.
• When there is an excess of chromosomes, it is
known as hyperploidy and when one or more
chromosomes are lacking , it is hypoploidy .

• Mostly, aneuploidy
is due to non-
disjunction (the
failure of separation
of chromosomes
during meiosis).
• Generally, plants
tolerate aneuploidy
better than animals.
Non - disjunction
Image: https://www.biology.iupui.edu

Types Of Aneuploidy
Monosomy
• Loss of one member of a pair of chromosomes (2n
– 1).
• Such organisms are called monosomics.
• A monosomic individual with the genetic formula
2n – 1 forms two types of gametes (n) and (n -1).
• The (n -1 ) gametes do not survive in plants.

Monosomics In Cotton.
Some examples of morphology of Cotton monosomic plants
compared to original parental line:
(a) Parental line L-458 (b) Mo50(c) Mo31(d) Mo76
“Reduced” stigma , a new phenotypic marker for Cotton monosomics, which makes it
possible to distinguish cytotypes without cytological analyses.

Double Monosomy
• Loss of two non – homologous chromosomes
(2n -1-1).
• Organisms are called double monosomics.
Nullisomy
• Loss of a homologous pair , (2n – 2).
• Organisms are called nullisomics.
• A nullisomic individual with the genomic
formula , 2n – 2 does not survive.
• Nullisomic polyploid (e.g. hexaploid wheat,
6x – 2 ) may survive, but exhibit reduced vigour
and fertility.

Nullisomics In Wheat
Mutants of the Ear in Bread Wheat(Triticum aestivum)
Ear shape of the wild type (WT) and of several mutants lacking single pairs of
chromosomes (Nullisomics). Due to hexaploidy (6x), the lack of a pair of
chromosomes is tolerated. The effects are usually different though stunted growth is
usually one of them. (E. R. SEARS, 1953)
http://www1.biologie.uni-hamburg.de/b-online/e12/12d.htm

Trisomy
• Addition of a third homologue to a pair or three copies of a
particular chromosome, (2n + 1).
• The organisms are called trisomics.
• In trisomics with 2n + 1 , a trivalent structure may be formed
during meiosis.
• If two chromosomes of the trivalent go to one pole and the third
goes to the opposite pole, then the gametes will be (n + 1) and (n)
respectively .
• Trisomy can produce different phenotypes, depending upon which
chromosome of the complement is present in triplicate form.

Trisomics in plants
• Reported in Tomato,
maize, Thorn apple
(Datura stramonium) ,
Arabidopsis thaliana etc.
Images:https://www.esc.nsw.gov.au
https://www.iewf.org
Datura stramonium

Trisomy in Datura (n = 12) -Addition of a copy of every single
chromosome of the set leads to abnormal changes in the shape
of the fruit that are typical for the respective chromosome.
(A. F. Blakeslee, 1921, 1934).
Image: https://www.slideshare.net/ar_shad/variation-in-chromosome-structure-
and-number-chapter-8

Trisomy In Humans
• Trisomy has been reported in man and the affected
person has three copies of one of the chromosomes
instead of two and possesses 47 chromosomes
instead of the normal 46 (23 pairs of homologous
chromosomes - 22 pairs of autosomes + 1 pair of
allosomes or sex chromosomes , either XX (female)
or XY (male).
• Trisomy 21(Down syndrome), Trisomy 13 (Patau
syndrome) and Trisomy 18 (Edwards syndrome)
are the most common forms of trisomy in man.

i).Trisomy 21 (Down Syndrome or Mangolism or
Mangoloid Idiocy )
• Has 3 copies of the 21st chromosome and hence the name trisomy
21.
• First described by a British Physician Langdon Down(in 1866) and
hence the name Down syndrome.
• 2n = 47 - In females, 2n = 44 Autosomes + XX + 1 extra copy of
21st chromosome.
In males, 2n = 44 Autosomes + XY + 1 extra copy of 21st
chromosome.
• Can be caused by chromosome non- disjunction in one of the
meiotic divisions , more likely in females.
• The frequency of non-disjunction increases with maternal age.

Karyotype of a normal human male (2n = 46 ,
22 pairs of Autosomes + XY chromosomes)
Image :Human Genome Centre, Universiti Sains Malaysia, Malaysia
https://www.researchgate.net

Karyotype of a human male with Trisomy 21
2n = 47, 44 Autosomes + XY + 1 extra copy of
21st chromosome.
Credit: Wessex Reg. Genetics Centre. Attribution 4.0 International (CC BY 4.0)
https://wellcomecollection.org

Karyotype of a normal human female (2n = 46 ,
22 pairs of Autosomes + XX chromosomes)
https://slideplayer.com/slide/5984761

Karyotype of a human female with Trisomy 21
2n = 47, 2n = 44 Autosomes + XX + 1 extra copy of
21st chromosome.
Karyotype prepared by Fatma Soylemez
https://www.intechopen.com

Symptoms Of Trisomy 21 (Down Syndrome)
Trisomy 21 (Down Syndrome) causes physical as well as mental
developmental delays and disabilities. The degree of
developmental and intellectual problems may vary and may be
mild, moderate or severe. Some of the most common features
include
• Short stature
• Broad skull
• Flattened face
• Unusually shaped or small ears
• Upward slanting eye lids
• Wide nostrils

• Large , protruding tongue with a distinctive furrowing
• Heart and gastrointestinal disorders
• Poor muscle tone
• Loose joints
• Broad, short hands with a single crease in the palm,
short fingers
• Intellectual disability etc.

Down Syndrome
https://www.savedownsyndrome.com/blog/theneweducationalgraphics-csyye

Down Syndrome
Image:https://favpng.com

Trisomy 13 (Patau Syndrome)
• Affected individuals have three copies of 13th
chromosome instead of the normal two copies.
First observed by Thomas Bartholin (1657), but the
chromosomal condition associated with this syndrome
was discovered by Dr. Klaus Patau (1960) and hence the
name Patau syndrome. It is also known as Bartholin -
Patau syndrome.
• 2n = 47 - In females, 2n = 44 Autosomes + XX + 1 extra
copy of 13th chromosome.
In males, 2n = 44 Autosomes + XY + 1 extra

Karyotype of human with Trisomy 13
2n = 47 , 2n = 44 Autosomes + XX + 1 extra copy of
13th chromosome (female) ; 2n = 44 Autosomes + XY
+ 1 extra copy of 13th chromosome (male).
.
https://ghr.nlm.nih.gov/condition/trisomy-13

Symptoms Of Trisomy 13 (Patau Syndrome)
• This genetic disorder is characterized by several
physical and mental problems.
Children with trisomy 13 (Patau Syndrome )often have
• Heart defects.
• Brain or spinal cord abnormalities.
• Poorly developed eyes .
• Extra fingers or toes (Polydactyly) and clenched
hands (with outer fingers on top of the inner fingers).

• Cleft lip
• Cleft palate .
• Weak muscle tone.
• Severe intellectual disability etc.
• Infants with trisomy 13 have only a short life span - may
die within a few days or weeks after birth .
• Five or ten per cent of the affected children may live for
a few years.

Trisomy 18 (Edwards Syndrome)
• Caused by the presence of an extra 18th chromosome. Affected
individuals have three copies of 18 th chromosome instead of the
normal two.
• First described by John H. Edwards ( 1960) and hence the name
Edwards syndrome.
• 2n = 47 - In females, 2n = 44 Autosomes + XX + 1 extra
In males, 2n = 44 Autosomes + XY + 1 extra

Karyotype of human with Trisomy 18
2n = 47 , 2n = 44 Autosomes + XX + 1 extra copy of
18th chromosome (female) ; 2n = 44 Autosomes + XY
+ 1 extra copy of 18th chromosome (male).

Symptoms Of Trisomy 18 (Edwards Syndrome)
Trisomy 18 (Edwards
Syndrome) is characterized
by several abnormalities
such as
• Delay in development -
intrauterine growth
retardation and a low birth
weight.
• Small head
• Heart defects
• Clenched fists with
overlapping fingers.
• Kidney abnormalities
https://www.pmfias.com/bio-medical-waste-rules-2016

• Clubfeet with toes that may be fused or webbed.
• Intellectual disability etc.
• Trisomy 18 may cause death before birth or after
birth – within the first month. 5% to 10 % of the
affected children may survive beyond a year.

Double Trisomy
• Two different chromosomes are present in triplicate
and has the genomic formula 2n + 1 + 1
• Double trisomy causes great genetic imbalance.
Tetrasomy
• Addition of two homologues to a pair or four copies
of a particular chromosome (2n + 2 ).
Double Tetrasomy
• Addition of two homologues to a pair and two
homologues to another pair. (2n + 2 + 2).

• In addition, to variation in whole chromosome numbers,
genetic stocks have been developed, especially in plants,
where parts of chromosomes are retained.
• One example is telocentrics which are chromosomes that
have a terminal centromere. These structures represent
chromosomes that are missing the genetic material
beyond that centromere. Stocks containing these types of
chromosomes are called monotelosomics ( monotelos ).
• Another type of structure is the isochromosome which is
a chromosome that contains the same genetic material on
both arms. Genetic stocks which contain these
chromosomes are called monoisosomics (monoisos).

Isochromosomes
https://alchetron.com/Isochromosome

II. Changes In The Structure Of Chromosomes
• Involve a change in the molecular organization of
chromosomes.
• Caused due to accidental, natural or induced
breakage of chromosomes.
• May remain confined to a single chromosome or
may extend to both of the members of a pair or
may involve two or more pairs of chromosomes.

Types
1.Deletion (Deficiency)
2. Duplication
3.Inversion
4. Translocation

1. Deletion (Deficiency)
• The loss of a chromosomal segment , either
interstitial or terminal.
• Only one break is necessary for terminal deletion.
• Two breaks are essential for interstitial deletion.
• The parts of the chromosome lacking the
centromere are generally lost.

• Deletion may be heterozygous (one chromosome
normal) or homozygous (both the chromosomes are
deficient) and the latter generally do not survive.
• Depending on the length of the lost segment , the
genes lost may vary from one to many.
• A loss of any considerable portion of chromosome
is usually lethal to a diploid organism because of
genetic imbalance.

Deletion (Deficiency)
Image: Modern Genetic Analysis edited by Sarah Tenney

https://www.slideshare.net/Ali_Qatrawi/principles-of-clinical-cytogenetics

Peter J. Russel,Genetics Copyright Pearson Education, Inc.publishing as Benjamin Cummings
http://www.csun.edu

eg. Cri – du – chat
syndrome in humans
• Caused by a deletion in
the short arm of
chromosome 5.
• Physically as well as
mentally impaired
• Cat like crying during
infancy.
https://criduchat-syndrome.weebly.com

Cri – du – chat syndrome in humans
https://www.shutterstock.com

2. Duplication
• Occurs when a segment of the chromosome is
represented two or more times in the chromosome.
• May be a free segment with a centromere .
• A chromosomal segment of the normal
complement.
• If the fragment includes the centromere, it may be
incorporated as a small chromosome
(Extrachromosome).

Duplication
http://www.csun.edu

• The phenotypic effects on the organisms depends
on the action and interaction of the duplicated
genes.
• If the genes is of additive nature, a duplication
would intensify the character.
e.g. Numerous variations in the wing and eye of
Drosophila are due to duplications – duplication of
Bar locus of Drosophila – if the Bar region
undergoes duplication , the size of the eye is
reduced.

• Duplications of chromosomes are not deleterious to
the organism like deficiency and they usually protect
the organism from the effect of a deleterious
recessive gene.
• However, large duplications may be lethal due to
disturbance in the genic balance.
• Duplication is useful in the evolution of new genetic
material and in altering the phenotype due to re-
allocation of chromosomal material (Position effect –
Phenotypic changes which are the result of
differences in the position of genes).

Types Of Duplication
i) Tandem Duplication
• The duplicated segment lies by the side of the same
genes and in the same gene order in the
chromosome.
eg. If the sequence of genes in a normal chromosome
is abcde . fghi (the dot in the centre represents the
centromere of the chromosome) , in a tandem
duplication, the section de is repeated immediately
after its normal position. i.e., abcdede.fghi (Note-
The duplicated genes de in bold - de)

Tandem Duplication

Tandem Duplication- Example : Bar locus in Drosophila.
• In 1930’s C . B. Bridges analyzed X chromosome
carrying the Bar mutation and found that the 16 A
region, which contain a gene for eye shape, had been
tandemly duplicated. This dominant X – linked
mutation alters the size and shape of the compound
eyes, transforming them from large, spherical
structures into narrow bars.
• Tandem triplications of 16 A were also observed and
in these cases, the compound eye was extremely
small – double bar . The severity of the mutant eye
phenotype is related to the number of copies of the
16 A region of the X chromosome.

Tandem Duplication
Bar Locus Of Drosophila
Image:https://www.chegg.com

ii) Reverse Tandem Duplication
• The order of genes in the duplicated segment is the
reverse of the original.
e.g.
abcde . fghi
abcdeed . fghi

Reverse Tandem Duplication

iii) Displaced Duplication
• A segment of the chromosome is repeated some
distance away from its normal location. The
repetition may take place on the same arm of the
chromosome – homobrachial displacement
abcde . fghi
e.g. adebcde.fghi
or on the other arm – heterobrachial displacement
e.g. abcde.fghdei

iv) Transposed Duplication (Transposition)
• The segment of a chromosome is repeated on a
non – homologous chromosome .
e.g.
abcde . fghi
klmn . opqderst

v) Extrachromosomal Duplication
• The duplicated segment may have its own
centromere, in which case it becomes an
extrachromosome .
abcde . fghi
.de (. represents centromere)

3. Inversion
• Two breaks in the
chromosome and
subsequent reinsertion
of the segment
between the breaks
after the rotation of
the segment by 180°.
• Thus the order of genes
in the segment is
reversed.

• Inversions are called
pericentric when the
segment includes the
centromere.

Pericentric Inversion

• Inversions are called
paracentric when the
segment excludes the
centromere. That is the
centromere is located
outside the segment.

• Pericentric inversion may change the relative lengths
of the two arms of the chromosomes where as a
paracentric inversion has no such effect.
• An individual in which one chromosome is inverted,
but its homologue is not is an inversion heterozygote.
• During meiosis, the inverted and non – inverted
chromosomes pair point-for- point along their length.
• However, because of inversion, the chromosome
must form a loop for pairing in the region where their
genes are in reversed order – either the inverted or
non – inverted chromosome can form the loop to
maximize pairing between them.

Loop formation during synapsis ( during Meiosis) in
Inversion heterozygote

• Genetic consequence of inversion heterozygosity is
the suppression of recombination because crossing
over is usually inhibited near the breakpoints of a
rearrangement in heterozygous condition, probably
due to the rearrangements which disrupts
chromosome pairing.

4. Translocation
• Discovered by Bridges (1923) in Drosophila melanogaster.
• Transfer of a segment of a chromosome to a different part
of the same chromosome or to a different chromosome.
• Translocation change only the arrangement of the genes
in the chromosome , not the quality or quantity of the
genes.
• Individuals carrying such rearrangements should be
phenotypically entirely normal unless the relations of a
gene or genes to adjacent genes affect the phenotypic
expression (Position effect).

Types Of Translocation
1. Simple – A segment of one chromosome is transferred to
a different part of the same chromosome or to another
chromosome .
2. Shift – involves the insertion of an interstitial piece of a
chromosome into a different portion of the same
chromosome or into an interstitial portion of a non-
homologous chromosome – requires a minimum of three
breaks in the chromosome.
3. Reciprocal – Exchange of chromosome segments
between two non – homologous chromosomes – may be
homozygous or heterozygous.

Simple, Shift and Reciprocal Translocations

• In translocation homozygotes, breaks occur in both
chromosomes of two different chromosome pairs and
there is mutual interchange of segments.
• The homozygotes do not have any cytological
peculiarities and undergo regular pairing during
meiosis.
• In translocation heterozygotes, breaks occur on only
one of the chromosomes of a homologous pair –
complications arise during meiosis and produce a
cross – shaped pairing configuration. Instead of
bivalents, quadrivalents will appear at diakinesis and
metaphase I as a ring or circle of four chromosomes.

Pairing During Meiosis In A
Translocation Heterozygote
Image:http://web.pdx.edu

Robertsonian Translocation
(Centric Fusion)
• Named after the Cytologist F. W. Robertson.
• A special type of translocation in which the long arms of
two acrocentric chromosomes fuse to form a metacentric
chromosome.
• It is a process that establishes a new type of chromosome
and reduces the chromosome number of the species.
• Centric fusion occurred during the phylogeny of
Drosophila, Grass hoppers, Reptiles etc.

Robertsonian Translocation
(Centric Fusion)

Changes In Number And Structure Of Chromosomes SMG

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