Chromosomal Abbe Rations

8/6/2019 Chromosomal Abbe Rations

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CHROMOSOMAL ABBERATIONS

A SEMINAR REPORT

SUBMITTED TO

DEPARTMENT OF BIOTECHNOLOGY

ICG

IIS UNIVERSITY

FOR PARTIAL FULFILMENT OF DEGREE OF

MASTER OF SCIENCE IN BIOTECHNOLOGY (2010-11)

SUBMITTED BY

POOJA PANDEY

UNDER GUIDANCE OF

MRS. CHARU SHARMA


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INDEX:-1) Definition and importance of chromosomal aberration.

2) Variation in chromosome structure

a) Deletion

b) Duplicationc) Translocation

d) Inversion

3) variation inchromosome number

a) Euploidy

i. Monoploidy or haploidy

ii. Polyploidy

Autopolyploids

Allopolyploids

Example-1)Raphanobrassica

2)evolution of wheat

b) Aneuploidy

Hyperploidy

Hypoploidy

Trisomy

Tetrasomy

Monosomy

nullisomy

c) origin of aneuploids

4)genetic disorders

a) Downs syndrome

b) Turner syndrome

c) Klinefelter syndrome


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CHROMOSOMAL ABERRATIONS

DEFINITION:-They are the visible changes in the structure of chromosomes, involving either in total

number of genes loci in a chromosome or their rearrangement or change in the

chromosome number. These are called as Chromosomal rearrangements or

Chromosomal aberrations.

It is of 2 types:-

1) Variation in chromosome structure

2) Variation in chromosome number

Many chromosome mutation lead to abnormalities in cell and organisal function. There ar

2 basic reasons for it :-

1.Chromosomal mutation can result in abnormal gene number or position.

2.If chromosome mutation includes chromosome breakage, then break may occur in

middle of genes thereby distrupting its function.

Importance of chromosomal mutation:-1.In research, they provide ways of designing special arrangements of genes, uniquely

suited to answer certain biological questions.2.they are important at applied level, especially in medicinal and in plant and animal

breeding.

1) Variation in chromosome structure:-A) Changes involving the amount of DNA on a chromosome

a. Deficiency or Deletion

b. Duplication

B) Change in arrangement of gene loci

a. Translocation

b. Inversion

VARIATION IN CHROMOSOME STRUCTURE

Ii is studied a lot from study ofpolytene chromosome-it is special kinds of chromosome

found in certain tissue (such as salivary gland in larval stages of insects of order Diptera

(Drosophila).


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Polytene chromosome consists of chromatid bundles resulting from respected cycles of

chromosome duplication with out nuclear or cell division, a process called as

endoreduplication.

1)DEFICIENCY OR DELETIONDefinition:- The deficiency is the deletion of a chromosomal segment resulting in theloss of genes. Depending upon the length of the lost segment, the genes lost may vary from

a single gene to a block containing several genes.

Nature:- The break in the chromosome may be induced by several agents such aschemicals, drugs and radiations. The break may occur at any time (spontaneous) during

cell cycles either in somatic or germ cells. The breakage usually occurs at random and

either in both the chromatids of a chromosome (chromosome break) or only in one

chromatid (chromatid break).The loss of chromosomal segment occurs when a portion of chromosome gets

detached due to certain reasons and the lost segment does not survive, because it lacks the

centromere. The portion of the chromosomal carrying the centromere functions as a

genetically deficient chromosome.

Types of deletion:-

1) Terminal deletion:-It refers to the loss of a segment from one or the other end othe chromosome. The terminal acentric part of the chromosome is unable to survive

and causese terminal deletion. It is caused by a single break in the chromosome.

2) Intercalary or interstitial deletion:-It involves the loss of an intercalarysegment of the chromosome with the reunion of terminal segments. Therefore, the

intercalary deletion is caused by two breaks and reunion of terminal parts.


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Although, deficiency and deletion are synonymous, but a distinction is often made. The

loss of terminal segment of chromosomes is described as Deficiency, whereas the loss in

intermediary position of chromosomes is designated by Deletion.

DETACTION OF DEFICIENCY:-1) By phenotypic effects shown in organisms having deleted chromosomes like notched

margin of wings in drosophila, therefore known as notch character.

2) By cytological studies of homologously paired chromosome at meiotic prophase- Th

normal chromosome when pairs with deletion chromosome during prophase of

meiosis, its part homologous to deleted segment buckles out.

3) By cytological study of polytene chromosomes.

GENETIC PROPERTIES:-1) Failure of chromosome to survive as a homozygote; however this sffect could also be

produced by any lethal mutations.

2) Chromosomes with deletion can never revert to a normal condition.


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3) Criterion for inferring sometimes unmasks recessive alleles present on the other

homolog leading to their unexpected expression this is called as Pseudodominance It

is helpful in Deletion Mapping.

OCCURRENCE OF DEFICIENCY:-Deficiencies have been observed in both plants and animals. In Drosophila, a notch

character is found, a number of deficiency characters are found in man, such as

a) Cri-du-Chat orcat cry syndrome caused by deletion in the short arm of

chromosome 5.

b) Philadelphia chromosome caused in long arm of 22nd chromosome.

2)DUPLICATION

DEFINITION:- The presence of same block of genes more than once in a haploidcomplement is known as duplication and the additional block of genes is called a repeat

TYPES OF DUPLICATION:-Three types of duplications have been recognized onbasis of attachment of chromosme segment.

1) Tandem duplication:-In tandem duplication the added segment has the samegenetic sequence as is present in the original state in the chromosome. Moreover the

added segment lies in close association with original segment.2) REVERSE TANDEM DUPLICATION:- In such a duplication the sequence

of genes aligned in the attached chromosome piece is just the reverse of the original

alignment.

3) DISPLACED DUPLICATION:- In it the chromosomal segment gets attachedto some nonhomologous chromosome.


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EXAMPLE:-Anexample f tandem duplication inDrosophila is the Bar character of

the eye. Bar character is a dominant X linked character. It is controlled by 7 bands in

16A region of X chromosome. In normal female (heterozygous wild type) 16A regionis represented once in both the X chromosome .In heterozygous double bar (BB/+) the

16A locus is represented three times in one X and only once in the other X chromosom

. In homozygous ultrabar the 16A locus is represented three times in both the X

chromosomes.

ORIGIN OF DUPLICATION:-Duplication can arise by any one of the 2 methods-a) By the attachment of a deleted from one chromosome to other.

b) By unequal crossing over in the homologous chromosome.

SIGNIFICANCE:-They are more frequent and less deleterious. These do not lower thviability, but do produce abnormality of structure and function.

Duplication play significant role in evolution , because dupli


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cation increases number of genes in the chromosome complement.

Duplications may overcome the effect of deletion, thus reducing

their effect to nonlethal.

3)TRANSLOCATION

DEFINITION:-It is a chromosomal mutation in which there is a change in position ofchromosome segments and the gene sequences they contain to a different location in the

genome. No gain or loss of genetic material is involved in a translocation.

ORIGIN OF TRANSLOCATION:- Translocation occurs as a result of interchange chromosome segments in nonhomologous chromosomes.

TYPES OF TRANSLOCATION:- Depending upon which part or parts ofnonhomologus chromosomes become detached and reunited the translocation can be:-

a)SIMPLE TRANSLOCATION:-In such cases a small segment of a chromosome iadded to the end of other nonhomologous chromosomes. This type of translocation is ver


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rare in nature and is caused by a single break in nature and is caused by a single break in

one chromosome only.

b)SHIFT TRANSLOCATION:-In it an interstitial segment of one chromosome isbroker off and is inserted within the break in another nonhomologous chromosome. Thus

it involves 3 breaks: two in one chromosome and one in nonhomologous chromosome.c)RECIPROCAL TRANSLOCATION:-It is the exchange of parts betweennonhomologous chromosome. It is of 2 types:-

i. HOMOZYGOUS RECIPROCAL:-In which both the homologous chromosomes

exchange parts with the 2 nonhomologous of another pair.

ii. HETEROZYGOUS TRANSLOCATION:- In which only one number of each of

the 2 homologous pairs exchange part.

d)MULTIPLE TRANSLOCATION:-In it more than two pairs of nonhomologouschromosomes exchange parts.


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BEHAVIOUR OF TRANSLOCATION ZYGOTES:-

a) HOMOZYGOUS TRANSLOCATIONS:- They can not be identified cytologically

because these form regular bivelents at pachytene stage during meiosis.

b) In Reciprocal translocation heterozygote the 2 normal and 2 interchangedchromosomes pair to form a cross shaped configuration at pachytene of meiosis, whereas

normal chromosomes forms a bivalent.

The anaphasic movement of chromosomes towards the poles takes place in an

one of the three different ways given below:-

i. ALTERNATE SEGREGATION:- Thealternate chromosomes of the ring

go to the same pole; the normal chromosomes on one pole and the

translocation chromosomes on the other pole. Therefore, all the gametes

receive full complement and are fully viable.

ii. ADJACENT-1-SEGREGATION:-In open ring configuration, if one

normal and one translocated chromosome reach one pole and aretranslocated and one normal chromosome to the other pole, it is called

adjacent-1-segregati

iii. ADJACENT-2-SEGREGATION:-In some open ring congiguration the 2

homologous chromosomes( one normal and other translocated )go to one

pole and the other two homologous go to other pole. This is called adjacen

2-segregation.


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OCCURRENCE OF TRANSLOCATION:-It played an important role in introduction of genetic polymorphism in the population and orig

of new species. Translocation have been studied inDrosophila, Oenothera, Maize .In mantranslocation between 15 and 21 chromosomes results in 21 trisomy and causes Downs

Syndrome.

SIGNIFICANCE OF TRANSLOCATION:-

1) Translocation may cause change in the morphology or appearance of chromosomes by

centric fusion between two acrocentric chromosome segments. This may lead to the change in

the number of chromosomes.

2) Translocation introduce genetic polymorphism in the populations of a species and play a

significant role in formation of new varieties and new species.

INVERSIONDEFINITION:- Sometimes the number of genes in a chromosome is changed but thesequence of genes is altered by the rotation of gene block within a chromosome by 180 degree

TYPES OF INVERSION:- It is of 2 types:-

a)

Paracentric inversion:-When both the breaks in the chromosome during inversion

occur on the same side of the centromere, means do not involve thecentromere the

inversion is known as paracentric.

b) Pericentric inversion:- In pericentric inversion the inverted segmaent contains thecentromere, i.e. it involves one break on either side of the centromere.

Inversion occurring in a single chromosome is called chromosomal, whereas

that occurring in both the members of a homologous pair is called allelosomal. Allelosomal

inversions occurring in the homologous arms of the 2 chromosomes are called allelobranchia

but those occurring in nonhomologous chromosomes are called heterosomal.


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BEHAVIOUR OF INVERTED CHROMOSOMES:-The chromosome of a pair having identical inversions undergo normal meiotic pairing

and are distributed normally during meiosis (homozygous). Even when heterozygous, the

inversion has no detectable phenotypic effect but at the time of gamete formation, the normal

pairing is disrupted and abnormal cytological configurations are produced.

The paracentric heterozygous inversion with one chismata in the region of inversion resulin the formation of an acentric chromatid lacking centromere, and a dicentric chromatid

connecting the two chromosomes. Thus at the end of meiosis there are 2 non-crossover

chromatids, one acentric chromatid and one dicentric chromatid. Both chromatids are inviable

so only the noncrossover chromatids form the viable products of meiosis in paracentric

inversions of heterozygotes.


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Crossing over in a pericentric heterozygous inversions result in the formation of the

chromosomes which have duplications for certain genes and deficiency for others. The gamet

receiving chromosomes with deficiency are rarely viable. Therefore, the recombination is

lowered down in the heterozygous inversions.


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for example, a chromosome with median centromere (V-shaped), may change into a

hook shaped or rod shaped form. Conversely, in pericentric inversion a hook shaped

chromosome may change into V shaped chromosome.

CONSEQUENCES OF INVERSIONS:-Since the number of genes in an invertedchromosome remains the same, the effects of inversion are not so drastic as are the effects ofdeletion and duplication

i. Effects of inversion on phenotype may be produced because ofposition effect. A gene

in one location on a chromosome does not necessarily have the same action that it could

have in another position.

ii. Inversions suppress crossing over and tend to retain the original combination of genes

help in preserving the superior arrangement of genes.

iii. If 2 groups of organisms are separated for a long period, they might accumulate so

many inversions of different types that these may lead to the formation of new species b

the splitting of parental stock.

SIGNIFICANCE OF INVERSION:-i. Inversions help in the origin of new species.

ii. Inversions provide proof for occurrence of crossins over and support the vies that only

of the 4 chromatids crossover.

iii. Inversions, similar to translocation , help in establishing species relation inDrosophilaiv. As a result of inversions chances of crossing over are reduced and, therefore, the

recombination of genetic loci is restricted.

VARIATION IN CHROMOSOME NUMBEREach species of animals or plants has a fixed number of chromosomes. These chromosomes are

represented once in haploid gametes and spores and twice in diploid body cells. Sometimes due

to misdivision numerical changes in the number of chromosomes may occur. These can occur

spontanelously by certain external or internal factors or by radiations. The phenomenon of

variations in the number of chromosomes is called heteroploidy. It is of 2 types a :-

1) EUPLOIDY 2)ANEUPLOIDY


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EUPLOIDYIn euploidy, an organism either loses a complete set of chromosomes or acquires one or

more additional sets of chromosomes over and above the 2 sets of diploid complement.

Therefore, euploidy can be 2 types: monoploidy (n) and polyploidy.


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1) MONOPLOIDY AND HAPLOIDY :- Monoploids have only one set or single

basic set of chromosome.

Monoploids is rarely seen in normally adult diploid organism because of the presence of

recessive lethal mutations( which are usually counteracted by dominant alleles in

heterozygous individuals) in the chromosomes of many diploid eukaryotic organisms, many

monoploids probably not survive.

HAPLOIDS have half the somatic chromosome number. In diploid

organism monoploids and haploid are identical while in a tetra or hexaploid with 4n or 6n

chromosomes the haploids will posses 2n or 3n chromosomes whereas its monoploid will

posses only one set(n) of chromosomes. Haploids of polyploids are called euploids, while

those of aneuploids are called aneuhaploids.

ORIGIN OF HAPLOIDS:-In honeybee (Hymenoptera) haploid males (drones) are found

as a routine. These arise parthenogenetically from the unfertilized eggs of dploid organism.

Similarily haploids may originate spontaneously in flowering plants dur to parthenogeneticdevelopment of unfertilized egg. Such haploids have been obtained in tomatoes and cotton.

Haploids can be artificially induced by any one of the following methods:-

a) by X-ray treatment

b) by delayed pollination

c) by temperature shocks

d) by colchicines treatment

e) by culturing pollen graibs

f) interspecific or intergeneric hybridization.

MORPHOLOGY OF HAPLOIDS:-Haploids are smaller in size in comparison to diploids.

Their leaves, flowers, fruits and seeds are also comparatively small. These are also less

resistant and comparatively weak.

CYTOLOGY OF HAPLOIDS;-The haploids have just univalent without any homologues

to pair. These univalent are distributed at random during anaphase 1 or meiosis producing

monosomics. Thus haploids are sterile.

USES OF HAPLOIDS:- Haploids are used to study the role of individual

chromosomes. Homozygous diploids are obtained from such haploids by

doubling the chromosomes with colchicine treatment

POLYPLOIDYOrganisms with more than 2 sets of chromosomes are known as polyploids i.e. these

possess one or more haploids sets of chromosomes in addition to the original diploid

configuration. Individuals with three sets of chromosomes (with additional one set) are

known as triploids(3n), those with four sets of chromosomes (with additional 2 sets) are


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tetraploids (4n), those with 5 sets are pentaploids(5n), with six sets of chromosomes are

hexaploids(6n) and so on. This phenomenon of change in the number of sets of the

chromosome is known as polyploidy.

Polyploids may arise spontaneously or be induced experimentally. They often occur

as a result of a breakdown of the spindle apparatus in one or more meiotic divisions or in

mitotic divisions. Almost all plants and animals probably have some polyploidy tissues. For

example, the endosperm of plants is triploid, the liver of mammals and perhaps often

vertebrates is polyploidy, and the giant abdominal neuron ofsea hareAplysia has about75000 copies of the genome. Plants that are completely polyploidy include wheat, which is

hexaploid(6n) and strawberry, which is octaploid(8n).

There are 2 general classes of polyploids: those with an even number of

chromosomes sets and those with an odd number of sets. Polyploids with an even number of

chromosome sets have a better chance of being at least partially fertile, because there is the

potential for homologs to be segregated equally during meiosis. Polyploids with an odd

number of chromosomes sets always have an unpaired chromosome for each chromosometype, so the probability of producing a balanced gamete is extremely low; such organisms

usually are sterile or have an increased of abortion of zygotes.

In humans , the most common type f polyploidy is triploidy and it is always

lethal. Triploidy is seen in 15 to 20% of spontaneous abortions and about 1 in 10000 live

births, but most affected infants die within 1 month.

Polyploidy is less consequential to plants. One reason is that many plants

undergo self fertilization , so if a plant is produced with an even polyploid number of

chromosome sets it can still produce fertile gametes and reproduce.

Depending upon the source of additional chromosome sets, the polyploids are

of 2 types-autopolyploids and allopolyploids

1) AUTOPOLYPLOIDS:-In it the chromosome set or sets of the same species is

added to its diploid genome. If a diploid cell has AA genome an autotriploid will have AAA

and an autotetraploid AAAA.

Origin of autopolyploids:-The autoploid could arise in any of the following

ways:-

By the union of diploid gametes produced in absence of meiosis or due

to abnormal meiosis (autotetraploid).

By somatic doubling of the chromosomes in a diploid zygote de to

abnormal mitosis-(autotetraploid). By the union of a haploid gamete with the diploid gamete-autotriploid

By fertilization of an egg with 2 sperms- autotriploid

By a cross between a tetraploid and a diploid parent-autipentaploid

They are relatively rare in nature. These can be produced artificially. Example

of natural autoploids are doob grass(cynodon dactylon)- an autotriploid, rye(secale cereale)


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an autotetraploid. They are induced by treatment ofcolchicines in banana, datura, potato,

coffee etc.

Characteristics of autoployploids:-They exhibit following characteristics-

The autopolyploids are larger than the diploids due to increased cell size.

Their leaves, flowers, fruits and seeds all are large.

These show vigorous growth.

Flowering is delayed but flowers are comparatively large.

Seeds are few but large.

Growth rate is slow due to slow rate of cell division.

With increase in cell size, water content is increased causing reduced osmotic pressure

and loss of resistance against frost.

Polyploids above tetraploid level produces abnormalities like wrinkled leaves, dwarfing

or weakling. Autooctaploids or higher have very low survival value.

Cytological behaviour ofautopolyploids:-Autopolyploids possess more than 2 sets of

homologous chromosomes . This leads to the formation of multivalents (trivalents,

quardivalents etc). Meiosis in triploids and pentaploids is abnormal. The thirs homologue

either fails to pair forming a univalent(1:1) or is randomly distributed to the gametes(2:1). The

number of chromosomes in the gametes of triploid organism may form trisomics or

tetrasomics.

2) ALLOPOLYPLOIDS:-Allopolyploids are formed by multiplication of

chromosome sets of a hybrid of 2 diploid species. This termed was coined by

G.Karpencheko in 1928. usually the hybrids between 2 species are sterile because unrelatedchromosomes do not pair during meiosis. Very rarely both the sets of chromosomes of the

hybrid enter into one gamete cell producing diploid gametes. Such diploid gametes on

fertilization produce allotetraploids. In some cases the diploid cell of the hybrid , on the

onset of mitotic division undergoes chromosomes doubling but does not divide. Such a cell

is tetraploid if it happen in the zygote an allotetraploid organisms develop.


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Example:-1)Raphanobrassica the first known tetraploid reported by Russian geneticistkarpenchenko.

It is hybrid ofRaphanus sativus andBrassica oleracea, developed by the

fertilization of diploid gametes of F1 hybrid.

Both radish and cabbage are diploid each with 18 chromosomes. Their gametes

have 9chromosomes.The union of these gametes produce F1 hybrid with 18 chromosomes.

These hybrids are sterile because the chromosomes of radish did not pair with

thechromosomes of cabbage. Sometimes viable pollen and ovules having 18 chromosomes

(diploid gamete) were formed.

The fusion of these diploid gametes produced a allotetraploid (4n=36, n=9) with

18 chromosomes of radish and 18 chromosomes of cabbage. Such tetraploids produce viable

gametes with 9 chromosomes of radish and 9 chromosomes of cabbage.


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2)EVOLUTION OF WHEAT:-

The common cultivated wheat is also an allopolyploid. Three different chromosomes

complement series are found in different species of wheat genera-Triticum. The series is

represented by 3 groups:-

a) Einkorn wheat group is represented by 2n=14 (the primitive diploid species).These are Triticum monococcum and T.Aegilopoids. The wheat is not suitable to

human consumption because the grain is tightly enclosed in the glumes.

T.aegilopoides * A.speltoides

(wild einkorn) (goat grass)2n=14 2n=14

AA BB


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HYBRID

(sterile)

AB

Triticum diocoides(ABBB)

(2N=28)

Evolved into

Triticum dicoccum

(AABB)

b)Emmer wheat group includes 7 allotetraploid species with chromosome complement

2n=28. The 2 most important species of tetraploid wheat are Triticum dicoccum and T.

durum. The emmer wheat has evolved through hybridization between einkorn wheat and

a wild speciesAegilops (goat grass).

c)vulgare wheat group consists of 5 species of allohaxaploid wheat(2n=42).

Triticum dicoccum * Aegilops squarrosa

( Tetraploid wheat) (goat grass)2n=28 2n=14

AABB DD

Triploid hybrid

ABD

alloploidy

Triticum aestivu (allohexaploid)

AABBDD

SIGNIGICANCE OF POLYPLOID:- Autopolyploidy not only increases the size and vigour of the plant but also changes the

size of fruits and pollens.

It increases the osmotic concentration. Vitamins, alkloids and sugar contents of beet are

also found to be increased.

Polyploids also lead to changes in the season of blooming and fruiting and changes the

annual plants into perennial plants.

Thus autopolyploidy provides a method by which a type may become adapted to new and

specially to less favourable conditions.


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The allopolyploids suggest how new species would have arisen by crosses between 2

species.

Polyploids can be used in propogating crops which are valued for vegetative organs.

New varieties of plants for the improvement of crops in terms of resistance to diseases,

higher yield, better quality can be obtained by interbreeding and polyploidy.

OCCURANCE OF POLYPLOIDS:-The polyploids occur frequently in plants but rarely in animals, because the plants can propogate

vegetatively through grafting , by making cutting and by rooting them. Thus the sterile triploid,

pentaploids etc. are maintained from generation to generation. Animals usually reproduce

sexually through gamete formation. The gamete formation is not normal in polyploids due to

abnormal meiosis or failure of chromosome pairing. The polyploids can be maintained and are

seen in animals which reproduce exclusively by diploid parthenogenesis.

ARTIFICIAL INDUCTION OF POLYPLOIDY:-A number of chemicals have been found to induce polyploidy in plants, the most efficient and

widely used chemical being colchicine, it inhibits the formation of spindle fibres during mitosis

but not the doubling of chromosome.Maize and some other plants respond to changes in the

temperature. So when kept in high or low temperature inhibit spindle formation and thus

tetraploid cells are formed.

ANEUPLOIDYAneuploidy is the addition or loss of one or more chromosomes to the complete diploid

chromosome complement of an organism. The organism with such chromosome complement are

known as aneuploids.

Aneuploiy can be of 2 types

1) Hyperploidy 2) Hypoploidy


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1)HYPERPLOIDY:- It is the addition of one more chromosome to the diploid gamete of an

organism. It may be called:-

a) Trisomy:-(2n+1) when only one more chromosome is added to the diploid genome or

b) Tetrasomy(2n+2) whem both the chromosomes of a homologous pair are added to the

diploid genome.

2)HYPOLPLOIDY:-It is the loss of one or more chromosomes from the diploid genome. It isknown as

a)Monosomy-(2n-1) when there loss of 1 chromosome from the complete diploid set and

b)nullisomy(2n-2) when there is loss of both the chromosomes of a homologous pair.

TRISOMY:- Individuals having 1 chromosome extra in the haploid genome are called

trisomics. Inthem 1 chromosome is represented 3 times. The extra chromosome may belong to

any one of the chromosomes of a haploid set. Example- trisomic in man is called as

Mongolism( downs syndrome).

Production of trisomics:-

i. Trisomics may develop spontaneously due to nondisjunction of chromosomes

during meiosis at the time of gamete formation. This produces gametes with n+1

chromosomes.

ii. These may be produced by selfing triploids

iii. By crossing triploids females with diploids males


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Significance of trisomy:- They are used for locating genes on specific chromosomes.

TETRASOMY:-In it individuals particular chromosome of haploid set is represented 4 times

in a diploid chromosomal complement. The general chromosomal formula for tetrasomics is

2n+2.

MONOSOMY:-In it organism lack 1 chromosome(2n-1) if 1 more chromosome of some other

pair is also lost then it is also called as double monosomics(2n-1-1)

Some monosomics lack 1 complete chromosome, they are genetically imbalanced and,

therefore, they are either lethal of reduced viability. Monosomics can easily be produced in

polyploids, because they possess more than 2 chromosomes of each type and loss of one or

more chromosomes could easily be located. The number of possible monosomics is equal to the

haploid chromosome number.

NULLISOMY:-It lack both the chromosomes of a homologous pair of chromosomes. This is

represented by 2n-2.

Origin of aneuploids:- aneuploids could arise by nondisjunction occurring during first meiotic

division. Translocation also lead to the formation of trisomics and monosomics.

GENETIC DISORDERSDowns syndrome:-

trisomy 21, is achromosomal condition caused by the presence of all or partof an extra 21st chromosome. It is named afterJohn Langdon Down,the Britishphysician who described the syndrome in 1866. The condition was identifiedas a chromosome 21 trisomy by Jrme Lejeune in 1959 Down syndrome in a fetuscan be identified with amniocentesis (with risks of fetal injury and/or miscarriage)during pregnancy, or in a baby at birth.Down syndrome is a chromosomal condition characterized by the presence of an extra

copy of genetic material on the 21st chromosome, either in whole (trisomy 21) or part(such as due totranslocations)Many of the common physical features of Down syndrome may also appear in peoplewith a standard set of chromosomes, includingmicrogenia (an abnormally small chin)an unusually round face, macroglossia protrudingor oversized tongue), an almondshape to the eyes caused by an epicanthic fold of the eyelid, upslanting palpebralfissures (the separation between the upper and lower eyelids), shorter limbs, a single
http://en.wikipedia.org/wiki/Chromosomal_conditionhttp://en.wikipedia.org/wiki/Chromosome_21_(human)http://en.wikipedia.org/wiki/John_Langdon_Downhttp://en.wikipedia.org/wiki/United_Kingdomhttp://en.wikipedia.org/wiki/Physicianhttp://en.wikipedia.org/wiki/Syndromehttp://en.wikipedia.org/wiki/Trisomyhttp://en.wikipedia.org/wiki/J%C3%A9r%C3%B4me_Lejeunehttp://en.wikipedia.org/wiki/Amniocentesishttp://en.wikipedia.org/wiki/Chromosome_21_(human)http://en.wikipedia.org/wiki/Trisomyhttp://en.wikipedia.org/wiki/Chromosomal_translocationhttp://en.wikipedia.org/wiki/Microgeniahttp://en.wikipedia.org/wiki/Macroglossiahttp://en.wikipedia.org/wiki/Epicanthic_foldhttp://en.wikipedia.org/wiki/Palpebral_fissurehttp://en.wikipedia.org/wiki/Palpebral_fissurehttp://en.wikipedia.org/wiki/Single_transverse_palmar_creasehttp://en.wikipedia.org/wiki/Chromosomal_conditionhttp://en.wikipedia.org/wiki/Chromosome_21_(human)http://en.wikipedia.org/wiki/John_Langdon_Downhttp://en.wikipedia.org/wiki/United_Kingdomhttp://en.wikipedia.org/wiki/Physicianhttp://en.wikipedia.org/wiki/Syndromehttp://en.wikipedia.org/wiki/Trisomyhttp://en.wikipedia.org/wiki/J%C3%A9r%C3%B4me_Lejeunehttp://en.wikipedia.org/wiki/Amniocentesishttp://en.wikipedia.org/wiki/Chromosome_21_(human)http://en.wikipedia.org/wiki/Trisomyhttp://en.wikipedia.org/wiki/Chromosomal_translocationhttp://en.wikipedia.org/wiki/Microgeniahttp://en.wikipedia.org/wiki/Macroglossiahttp://en.wikipedia.org/wiki/Epicanthic_foldhttp://en.wikipedia.org/wiki/Palpebral_fissurehttp://en.wikipedia.org/wiki/Palpebral_fissurehttp://en.wikipedia.org/wiki/Single_transverse_palmar_crease


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transverse palmar crease (a single instead of a double crease across one or bothpalms), poor muscle tone, and a larger than normal space between the big and secondtoes. Health concerns for individuals with Down syndrome include a higher riskforcongenital heart defects, gastroesophageal reflux disease, recurrent earinfections, obstructive sleep apnea, and thyroid dysfunctions.

Trisomy 21

Trisomy 21 (47,XX,+21) is caused by a meioticnondisjunction event. With

nondisjunction, agamete (i.e., a sperm or egg cell) is produced with an extra copy of

chromosome 21; the gamete thus has 24 chromosomes. When combined with a
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normal gamete from the other parent, theembryo now has 47 chromosomes, with

three copies of chromosome 21. Trisomy 21 is the cause of approximately 95% of

observed Down syndromes, with 88% coming from nondisjunction in the maternal

gamete and 8% coming from nondisjunction in the paternal gamete

Mosaicism

Trisomy 21 is usually caused by nondisjunction in the gametes prior to conception,

and all cells in the body are affected. However, when some of the cells in the body

are normal and other cells have trisomy 21, it is called mosaic Down syndrome

(46,XX/47,XX,+21). This can occur in one of two ways: a nondisjunction event during

an early cell division in a normal embryo leads to a fraction of the cells with trisomy

21; or a Down syndrome embryo undergoes nondisjunction and some of the cells in

the embryo revert to the normal chromosomal arrangement. There is considerable

variability in the fraction of trisomy 21, both as a whole and among tissues. This is thecause of 12% of the observed Down syndromes.]

Robertsonian translocation

The extra chromosome 21 material that causes Down syndrome may be due to

a Robertsonian translocation in the karyotype of one of the parents. In this case, the

long arm of chromosome 21 is attached to another chromosome, often chromosome

14 [45,XX,der(14;21)(q10;q10)]. A person with such a translocation is phenotypically

normal. During reproduction, normal disjunctions leading to gametes have a

significant chance of creating a gamete with an extra chromosome 21, producing achild with Down syndrome. Translocation Down syndrome is often referred to

as familial Down syndrome. It is the cause of 23% of observed cases of Down

syndrome. It does not show the maternal age effect, and is just as likely to have come

from fathers as mothers.

Duplication of a portion of chromosome 21

Rarely, a region of chromosome 21 will undergo a duplication event. This will lead to

extra copies of some, but not all, of the genes on chromosome 21 (46,XX,

dup(21q)). If the duplicated region has genes that are responsible for Down syndromephysical and mental characteristics, such individuals will show those characteristics.

This cause is rare and no rate estimates are available.
http://en.wikipedia.org/wiki/Embryohttp://en.wikipedia.org/wiki/Mosaic_(genetics)http://en.wikipedia.org/wiki/Down_syndrome#cite_note-occurrence-35http://en.wikipedia.org/wiki/Robertsonian_translocationhttp://en.wikipedia.org/wiki/Chromosome_14_(human)http://en.wikipedia.org/wiki/Chromosome_14_(human)http://en.wikipedia.org/wiki/Meiosis#Nondisjunctionhttp://en.wikipedia.org/wiki/Embryohttp://en.wikipedia.org/wiki/Mosaic_(genetics)http://en.wikipedia.org/wiki/Down_syndrome#cite_note-occurrence-35http://en.wikipedia.org/wiki/Robertsonian_translocationhttp://en.wikipedia.org/wiki/Chromosome_14_(human)http://en.wikipedia.org/wiki/Chromosome_14_(human)http://en.wikipedia.org/wiki/Meiosis#Nondisjunction


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2)TURNERS SYNDROME:-

The persons with Turners syndrome are phenotypic females but their interphase nucleus is

without a barr body.

These are sterile females with poorly developed ovaries and underdeveloped breasts. These

exhibit webbed neck low set ears and broad chest. Their intelligence is also below average. In

such women, instead of normal overies only ridges of whitish tissue are present which are

known as streak gonads.Many scientists use the term gonadial dysgenesis to replace Turners

syndrome. They have 45chromosomes instead of 46. since they possess no barr body, it means

they possess only one X chromosome(XO).

They are retarded physically as well as mentally. They average about 4ft

10inches .

Cause

Risk factors for Turner syndrome are not well known. Genetic mosaicism (46XX/45XO) is

most often implicated, alongside nondisjunction(45XO) andpartial monosomy (46XX).

Nondisjunctions increase with maternal age, such as forDown syndrome, but that effect is notclear for Turner syndrome. It is also unknown if there is a genetic predisposition present that

causes the abnormality, though most researchers and doctors treating Turners women agree that

this is highly unlikely. In 75% of cases inactivated X chromosome is paternal origin. There is

currently no known cause for Turner syndrome, though there are several theories surrounding

the subject. The only solid fact that is known today is that during conception part or all of the
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second sex chromosome is not transferred to the fetus. In other words, these females do not

have Barr bodies, which are those X chromosomes inactivated by the cell.

3)KLINEFELTER SYNDROME:-

Klinefelter discovered some XXY individuals which are phenotypic males. These have

poorly developed male sex organs. Their face and hair are somewhat feminine abd these have

somewhat enlarged breasts . They are , therefore, sexually sterile. Such abnormal males

exhibit klinefelters syndrome. They possess 47 chromosomes. From these observations, it

appears that Y chromosomes possess some genes which promote the emergence of male

characteristics and the X chromosome produces female charavters or inhibits male

characteristics.

Cause

The extra X chromosome is retained because of a nondisjunction event during meiosis

I(gametogenesis). Nondisjunction occurs with when homologous chromosomes, in the case the

X and Y sex chromosomes, fail to separate, producing a sperm with an X and a Y chromosome.

Fertilizing a normal (X) egg produces an XXY offspring. The XXY chromosome arrangement
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is one of the most common genetic variations from the XY karyotype, occurring in about 1 in

500 live male births

Another mechanism for retaining the extra X chromosome is through a nondisjunction event

duringmeiosis II in the female. Nondisjunction will occur when sister chromatids on the sex

chromosome, in this case an X and an X, fail to separate. An XX egg is produced which, when

fertilized with a Y sperm, yields XXY offspring.

In mammals with more than one X chromosome, the genes on all but one X chromosome are

not expressed; this is known as X inactivation. This happens in XXY males as well as normal

XX females.[16] However, in XXY males, a few genes located in thepseudoautosomal regions of

their X chromosomes, have corresponding genes on their Y chromosome and are capable of

being expressed These triploid genes in XXY malesmay be responsible for symptoms

associated with Klinefelter syndrome.

Thus these are some examples of genetic diseases which are caused due to

changes in variation in chromosome number and chromosome structure. The branch of

genetics which deal with the inheritance of characters in man is termed as human genetics.

REFRENCES:-

1)A Text book of Genetics by Dr. Veer Bala Rastogi.

2)Genetics by Russel
http://en.wikipedia.org/wiki/Karyotypehttp://en.wikipedia.org/wiki/Meiosis_IIhttp://en.wikipedia.org/wiki/Mammalhttp://en.wikipedia.org/wiki/Genehttp://en.wikipedia.org/wiki/X_inactivationhttp://en.wikipedia.org/wiki/Klinefelter's_syndrome#cite_note-arghg2005-15http://en.wikipedia.org/wiki/Pseudoautosomal_regionhttp://en.wikipedia.org/wiki/Triploidhttp://en.wikipedia.org/wiki/Karyotypehttp://en.wikipedia.org/wiki/Meiosis_IIhttp://en.wikipedia.org/wiki/Mammalhttp://en.wikipedia.org/wiki/Genehttp://en.wikipedia.org/wiki/X_inactivationhttp://en.wikipedia.org/wiki/Klinefelter's_syndrome#cite_note-arghg2005-15http://en.wikipedia.org/wiki/Pseudoautosomal_regionhttp://en.wikipedia.org/wiki/Triploid


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3)An introduction to genetic analysis by Anthony J.F.Griffith

4)Principle of genetics by E.J.Gardner.

5)en.wikipedia.org/wiki/Down-syndrome.

6) en.wikipedia.org/wiki/turner syndrome

7) en.wikipedia.org/wiki/klinefelter syndrome.

Chromosomal Abbe Rations

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