whole knowledge about Duplication structural chromosome aberrations.

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“Duplication: Production and Uses”
Department of Plant Breeding and Genetics
Jawaharlal Nehru Krishi Vishwa Vidyalaya,
Jabalpur (M.P.)
(2017-18)
Assignment
on
Nitesh Kumar Panwar
Roll No. 170134002

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The term duplication was coined by Bridges in 1919, and the first
duplications were described in Drosophila melanogaster. The
presence of an additional segment, as compared to that normally
present in a nucleus is known as duplication. In a diploid
organism, presence of a chromosome segment in more than two
copies or occurrence of a segment twice in the same
chromosome is called duplication.
There will be discussed on some point here:-
1. Types of Duplications
2. Origin of Duplications
3. Chromosome Pairing
4. Phenotypic Effects of Duplications
5. Uses of Duplications
6. Duplications in Human

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1. Types of Duplications:
Broadly, duplications are divided into two types which are further
subdivided into different subtypes.
(A). Inter-Chromosomal duplication:
The duplicated segment of a chromosome is present in another
chromosome of the genome. It is of two types (Fig.1).
(a) The duplicated segment of a chromosome is incorporated into
a non-homologous chromosome (Translocation duplication).
(b) The duplicated segment is present as a separate
chromosome. Clearly, it must have a centromere to be able to
survive (Centric fragment).

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(B). Intra-Chromosomal duplication:
The duplicated segment remains in the same chromosome. It may
be present at different locations (Fig.1).
(a) In the other arm (Reverse displaced).
(b) In the same arm but removed from the original segment
(Displaced).
(c) In the same arm and next to the original segment. This type of
duplication is called tandem duplication which is further subdivided
into the following two types. (Fig.1).
(i) Direct tandem:
Gene order of the duplicated segment is the same as that of the
original segment.
(ii) Reverse tandem:
Gene order of the duplicated segment is inverted.

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2. Origin of Duplications:
Duplications may originate in the following four ways:
1. Primary structural change of chromosomes
2. Disturbances in the crossing over process
(unequal crossing over)
3. Crossing over in inversion heterozygotes
4. Crossing over in translocation heterozygotes and
segregation
1. Primary structural change:
A broken segment of a chromosome becomes inserted into its
homologue or in a non-homologous chromosome. In 1950,
McClintock described the Dissociation-Activator (Ds-AC) system
in maize, which is a very remarkable case of genetically governed
production of aberrations.

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The cytological effects produced by this system include various
kinds of chromosomal aberrations, such as, deficiencies,
duplications, translocations, inversions and ring chromosomes.
The Ds and Ac both are capable of transposition to any
chromosome or within the same chromosome. The standard
location of Ds is proximal to Wx on chromosome 9 in maize.
When the transposition of Ds takes place, a break occurs at the
location it was inserted earlier. Other than this, chromosome
breakage occurs either spontaneously or could be induced
artificially.
Various kinds of ionizing radiations, such as, X-rays, y-rays, fast
and thermal neutrons, and chemical mutagens such as, EMS
(ethyl-methane sulfonate), MMS (methyl-methane sulfonate), dES
(di-ethyl-sulphate) and EI (ethylene imine) etc. have been used to
produce different kinds of chromosomal aberrations.

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2. Unequal crossing over:
Deviations from normal chromosome pairing and crossing over
processes may occur in specific cases, particularly in
heterochromatic regions. This kind of pairing is called
heterochromatic fusion or nonspecific pairing, and it may lead to
unequal crossing over Sturtevant observed the occurrence of
unequal crossing over in D. melanogaster ; it involved the Bar eye
locus (Fig.2).

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3. Crossing over within inversion:
Crossing over within an inversion produces chromosomes
showing deficiency-duplication.
4. Translocations:
Deficiency-duplication gametes are produced by translocation
heterozygotes, but these gametes are sterile. Hagberg in 1965
produced duplications in barley using translocations.
3. Chromosome Pairing:
The duplicated segment forms a loop during pachytene in
duplication heterozygotes (Fig.3). Unequal crossing over may
occur in duplication heterozygotes leading further duplications of
the concerned segment. For example, Bar eye locus of Drosophila
gives rise to the double-Bar (ultra bar) eye following unequal
crossing over; conversely, double-Bar may revert to Bar due to
unequal crossing over (Fig.2).

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Reverse tandem duplications may form a loop to pair with the
normal chromosome. A crossing over within the loop produces a
dicentric chromatid bridge at AI (Fig. 13.4). In some cases, the
duplicated segment of the chromosome folds back to pair with the
original segment present in the same chromosome.
A crossing over within this paired segment produces a loop at AI
which gives rise to a dicentric chromatid bridge in one cell of the
dyad at All (Fig. 13.5). In 1941, McClintock obtained a reverse
tandem duplication in the short arm of chromosome 9 of maize;
this segment included the genes for colourless aleurone (c),
shrunken endosperm (sh) and waxy pollen (wx). As a result of
chromosome pairing, dicentric chromatids were produced which
formed dicentric bridge at AI (Fig. 13.4). The AI bridge was
broken, and chromosomes having smaller or larger deficiencies
and duplications were produced depending on the position of the
break.

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During the following interphase, when the chromosomes replicate,
the broken ends of the sister chromatids may unite and form
dicentric bridge at the subsequent anaphase. This will lead to a
“breakage-fusion-bridge” cycle. In corn, the breakage-fusion-
bridge cycle continues through the successive cell divisions in the
gametophyte as well as in the endosperm but not in the embryo.
The duplicated segment forms a loop during pachytene in
duplication heterozygotes (Fig.3).

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4. Phenotypic Effects of Duplications:
(1) Duplications may produce specific effects when the phenotype
is affected due to a change in the position of a gene; it is called
position effect.
The position effects are of two types:
(i) Stable type or S-type (cis-trans type), and
(ii) Variegated type or V-type.
An example of the stable type of position effect is the “Bar-eye”
phenotype of Drosophila. The Bar eye phenotype is the result of a
duplication of the 16A region of the X chromosome (Fig. 13.6).
The 16A region contains 5 bands, two of which are doublets. In
the case of the Bar eye phenotype; the number of facets in the
compound eye of the adult fly is reduced from the normal 779 to
only 358 in case of heterozygous bar (BB+
).

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But in homozygous bar flies (BB) the average number of facets is
further reduced to 68. Three repeats of the 16A region produces
the ultra-Bar or double-Bar phenotype, in which the number of
facets is greatly reduced; it is reduced to only 45 in the case of
heterozygous double-Bar, and to only 25 in the case of
homozygous double-Bar. The type of position effect is related to
the euchromatic regions of chromosomes.
The V-type position effects are confined to the genes present in
the heterozygous state. It is the result of a partial repression
through hetero-chromatinization when the functional allele of the
gene is brought close to heterochromatin. The wild type allele
expresses like a mutant allele due to the hetero-chromatinization.
However, the normal allele may escape repression due to
heterochromatin in many cases and a variegated phenotype (a
mixture of wild type and mutant type sectors) is produced. Such
type of position effects is produced by structural changes like

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(2) Duplication may lead to a more intense effect of the
duplicated gene. In the breakage- fusion-bridge cycle, the gene
C (for coloured endosperm) becomes duplicated in some cells,
while some other cells lose this gene. The latter cells produce
colourless sectors, while the former give rise to coloured sectors
(twin sectors). Further, bridge-break-fusions produce spots in the
endosperm with different colour intensities.
(3) The activity of certain enzymes is increased by a duplication
of the concerned gene. Hagberg, in 1965, obtained a duplication
for a short segment of the chromosome 6 of barley; the plant
having this duplication showed the doubled activity of the
enzyme a-amylase.

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Other Effects of Duplications:
In Drosophila, deficiency of the band 3C7 results in the notch
phenotype, but its duplication produces the “abruptex” (Ax)
phenotype. Abruptex is characterized by short, thin and arched
wings: veins not reaching the margin, presence of fewer hairs on
thorax and head, and bald patches. In Drosophila, other example
exists where duplications produce dominant phenotypic effects.
Hairy wing (Hw) is due to a tandem repeat of two bands, band
lB1.2, of the X chromosome. Hairy wing males have extra hairs
and bristles along the wing veins on the head and on their thorax;
this effect is more pronounced in females. Another phenotype
“confluence” (Co) is associated with tandem duplication of the
bands 3C5 to 3D6; this phenotype is characterized by thickened
and delta like ends of the wing veins.

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Duplications are believed to have played an important role in
evolution. In Drosophila where salivary gland polytene
chromosomes can be analysed accurately, numerous duplicated
segments have been identified. Similarly, in many plants, many
duplicated loci have been investigated. The duplicated segments
may be large or very small. Duplications are proposed to have
given rise to new gene functions.
The duplicate and polymeric factors are considered to represent
duplications. The several types of haemoglobins in man and
animals are believed to have originated through duplication of a
common ancestral gene. In 1970, it was suggested by Ohno that
the duplications are responsible for effective evolution by increase
in DNA content per cell; in’ conjunction with mutation, it gives rise
to new genes.

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The latter is achieved because duplication provides additional
copies of genes which can go on accumulating mutations (in the
duplicated copies of the genes) without any deleterious effect on
the organisms, since a fully functional copy of the concerned
genes is always available. In due course of time, the accumulation
of mutations changes the duplicate copies to the extent that they
may ultimately assume new functions.
5. Uses of Duplications:
(1) Duplications can be used to study the chromosome behaviour
during meiosis, such as, chromosome pairing, crossing over and
their consequences.

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(2) Duplications offer a number of possibilities in plant breeding.
They can be used to increase the dosage of certain desirable
genes for increasing disease or pest resistance, enzymatic activity
or other characteristics. For example, the activity of the enzyme
alpha-amylase in grains of barley is greatly increased due to a
duplication in the short arm of chromosome 6.
Duplication has an advantage over polyploidy because the genetic
dis-balance due to the duplication of chromosomal segments is
lesser as compared to polyploidy where the whole genome is
duplicated.
(3) In case where genes for resistance to diseases or pests are
linked to some undesirable genes or the genes for resistance to
various races are allelic, a combination of resistance to different
races can be obtained through duplication.

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(4) True breeding heterosis may be established in self-fertilized
crops by the technique of duplication breeding. Homozygous
duplications in the heterozygous condition for the heterotic loci will
give rise to permanent hybrid vigour.
(5) Duplication may be used to study the dosage effect of the
nucleolar organizer.
(6) Duplications may be useful in the study of the position effects.
(7) New genes can be produced only through duplications; thus it
is believed to have played an important role in evolution.
6. Duplications in Human:
In human, duplication-deletion syndrome has been reported by
some workers. Duplication has been reported to be produced to a
crossing over in a pericentric inversion involving the chromosome
3. The main features of this duplication were facial dismorphy and
congenital anomalies.

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