Download Phenotypic effects and variations in the genetic material (part 1)

Genetics (B 252)
Lecture 4
2015-2016
Phenotypic effects and variations in the genetic material
(part 1)
"Phenotype" is an organism's actual observed characters, such
as morphology, development, or behavior.
Changes in the genetic material that affect the phenotype of any
eukaryotic organisms may be classified into:
I.
Chromosomal Aberrations (abnormalities or anomaly)
They include the variation at the Chromosomal level. These
changes
may
be
as
missing,
extra,
or
irregular
segment
of chromosome. It can also be from an atypical number of
chromosomes or a structural abnormality in one (within the
chromosome)
or
more
chromosomes
(between
chromosomes).
Chromosome anomalies usually occur when there is an error in cell
division following meiosis or mitosis. It is the most dramatic examples
of genomic instability.
II.
Mutation (or point mutation)…..next lecture
They include variations at the DNA level. It is a permanent
change
of
the nucleotide
sequence within
a
single
gene
of
the genome of an organism, virus, or extrachromosomal DNA (DNA of
plastids and mitochondria) or other genetic elements. Mutations result
from damage to DNA which is not repaired either due to errors in the
process of replication, or in segments of DNA by mobile genetic
elements.
Mutations in genes can have either no effect, alter
the product of a gene, or prevent the gene from functioning properly or
completely.
I. Chromosomal Aberrations
A. Variation in chromosome number
The variation in the number of sets of chromosomes (ploidy) is
common in nature. Changes in chromosome number can occur by the
addition or loss of all or part of a chromosome (aneuploidy), the loss of
an entire set of chromosomes (monoploidy) or the gain of one or more
complete sets of chromosomes (euploidy). Most of these conditions is a
variation on the normal diploid (2n) number of chromosomes so, cause
drastic effects on phenotypic expression.
I.
Monoploidy:
Organisms have a single set of chromosomes i.e one set of
chromosomes (n) in the nuclei of their body cells. Meiosis cannot take
place normally in the germ cells of the monoploid, because each
chromosome lacks a pairing partner and hence they are usually sterile.
Most
micro-organisms
(e.g.,
bacteria.
fungi
and
algae);
gametophytic generation of bryophytes; sporophytic generation of some
higher angiospermic plants (e.g., Sorghum, Triticum, Hordeum, Datura,
etc.) and certain hymenopteran male insects (e.g., wasps, bees, etc.) are
monoploid organisms. Monoploid human somatic cells are lethal.
Monoploids plants are often weak, less vigorous and sterile
(seedless) eg: those produced from cells in the anthers (pollen), figure
below.
The reason of plant sterility is that the chromosomes have no
regular pairing partners (homologous chromosomes) during meiosis, and
meiotic products are deficient in one or more chromosomes. For instance,
a haploid in maize (2n=20) will have 10 chromosomes and the number of
chromosomes in a gamete can range from 0-10. Consequently,
considerable sterility will be found in a monoploid maize.
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How to create a monoploid plant?
In some plant species, monoploids can be artificially derived from
the products of meiosis in a plant's anthers (pollen grains, n) by tissue
culture. After having been moved to an agar plate medium containing
different plant hormones, haploid embryoids (a small dividing mass of
cells) will formed and grow to in vitro plantlets. These plantlets will then
planted into soil to form mature monoploid plants with roots, stems,
leaves, and flowers.
They are important in plant breeding for the selection of desired
properties as herbicide resistance (figure below).
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Difference between Haploid and Monoploid?
Monoploid refers to the number of chromosomes of the normal
cell, meaning that the cell has only one chromosome from every one of
the "n" pairs, while Haploid strictly refers to the number of chromosomes
in the germ cells (sperms and eggs), which is half of the number of
chromosomes normal cells have.
Note: (read only)
Colchicine is a toxic chemical that is often used to induce
polyploidy in somatic cells of plants or microorganisms. Basically, the
colchicine prevents the microtubule formation during cell division so,
arrest of cell division at prometaphase, thus the chromosomes do not pull
apart like they normally do in anaphase.
II.
Euploidy:
Organisms
with multiples of the monoploid number of
chromosomes are called euploid.
Diploids are normal euploidy, so, euploid types that have more
than two sets of chromosomes are called polyploid. The polyploid types
are named triploid (3n), tetraploid (4n), pentaploid (5n), hexaploid (6n),
and so forth (reported previously in Lecture 2). Polyploid is considered
aberrations because they differ from the previous normal (monoploid and
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diploid). Triploids and pentaploid are characteristically sterile because no
regular pairing partners during meiosis. For example, triploid arises
naturally or constructed by geneticists from the cross of a 4n (tetraploid)
and a 2n (diploid). The 2n and the n gametes unite to form a 3n triploid.
Many species of plants, microorganisms and animals have clearly
arisen through polyploidy, so evidently evolution can take advantage of
polyploidy when it arises.
Several species are polyploidy:
1. Haloarchaea
species:
Halobacterium
salinarum,
Haloferax
mediterranei, and Haloferax volcanii.
2. yeasts of the Saccharomyces genus
3. Oomycetes, which are non-true fungi members, such as within the
Phytophthora genus
4. Green algae Oedogonium and Cladophora
5. Potato plants
6. Amphibians and Fishes
Note:
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III.
Aneuploidy:
The abnormal conditions were one or more chromosomes of a set of
chromosomes are missing (-) or present (+) in more than their usual
number of copies. These variations happen during the formation of
gametes. Rarely, at either the 1st or 2nd meiosis, separation (disjunction) of
the chromatids of a tetrad does not occur. Instead, both members move to
the same pole during anaphase i.e. fail to disjoin. Such event is called
non-disjunction.
The different forms of aneuploidy are:
1. Hypoploidy:
These are when an organism missing one or pair of chromosomes.
a. Nullisomics – (2n-2) the loss of a pairs of homologous
chromosomes. Nullisomy is a lethal condition in diploids.
Some polyploids, however, can lose 2 homologous
chromosome of a set and still survive.
Eg:
Bread
wheat,
a hexaploid
(6n)
which
behaves
meiotically like a diploid and can tolerate nullisomy (due to
the presence of multicopy of the lost chromosomes) but
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exhibit reduced vigor and fertility = Nullosomic hexaploid
wheat (6n-2).
b. Monosomics – (n-1, 2n-1) Only one copy of a specific
chromosome is present instead of the usual two found in
its diploid cell. Monosomic chromosome complements are
generally deleterious
‫ مؤذ‬as the missing chromosome
perturbs the overall gene balance in the chromosome set.
Nondisjunction is a failure of this disjoining process, and
two chromosomes (or chromatids) go to one pole and none
to the other. In meiotic nondisjunction, the chromosomes
may fail to disjoin at either the first or second division.
Either way, n
+ 1 and n − 1 (monosomic) gametes are
produced.
If
an n − 1 gamete is
a monosomic (2 n
fertilized
by
an n gamete,
− 1) zygote is produced.
When a monosomic of a diploid plant undergoes meiosis,
haploid (n) and nullisomic (n-1) gametes are produced. In
plant the n-1 gametes rarely function.
Eg: Monosomic plants are produced in large number after
certain treatment through regeneration by tissue culture
every time we need it as it is not transmitted by diploid
organism.
Eg: In human, Turner syndrome.
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2. Hyperploidy:
These are an organism that gains extra copy of one or pair of
chromosomes.
a. Disomics (n+1) the gain of an extra copy of a chromosome.
A disomic is an aberration of a haploid organism.
Eg: In fungi, they can result from meiotic nondisjunction. In
the fungus Neurospora (a haploid), an n − 1 meiotic
product aborts and does not darken like a normal ascospore; so
we may detect MI and MII nondisjunctions by observing asci
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with 4:4 and 6:2 ratios of normal to aborted spores,
respectively, as shown below.
In these organisms, the disomic (n
+ 1) meiotic
product becomes a disomic strain directly. The abortion
patterns themselves are diagnostic for the presence of
disomics in the asci.
b. Trisomics – (2n+1) the gain of an extra copy of a
chromosome so, the individual will have 3 copies of a certain
chromosome.
Nondisjunction is a failure of this disjoining process, and
two chromosomes (or chromatids) go to one pole and none
to the other. In meiotic nondisjunction, the chromosomes
may fail to disjoin at either the first or second division.
Either way, n
+
1 (disomic) and n
− 1 (monosomic)
gametes are produced (previous figure).
If an n + 1 gamete is fertilized by an n gamete, a trisomic
(2 n + 1) zygote is produced.
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Eg: trisomics of the Jimson weed Datura stramonium
Eg: In human, Klinefelter, Down, Patu and Edwards
syndromes.
c. Double Trisomics- (2n+1+1) the gains of 2 different extra
copy of chromosome, so 2 different chromosomes are present
in triplicate.
d. Tetrasomics- (2n+2) the gain of an extra pair of homologous
chromosomes, so a chromosome are present in 4 copies.
Note:
B. Variation in chromosome structure
In these cases, the number of chromosomes remains the same but
their genetic material becomes modified through the loss, grain or
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rearrangement of particular sections. Such structural changes are caused
by breaks in the chromosome or the chromatid. Each break produces 2
ends which may join either as following:
- Remain ununited: lead to loss of chromosomal segment due to
absence of centromere.
- Immediate reunion: reunion of the same broken ends to
reconstruct the original chromosome structure.
- Exchange: one or both ends of a particular break may join those
produced by different break.
Depending upon the number of breaks, their locations, and the
pattern in which broken ends join together, a wide variety of structural
changes are possible. These variations may be either:
I. Variation in arrangement of chromosome segment
II. Variation in number of chromosomal segments
III. Variation in chromosome morphology
I. Variation in arrangement of chromosome segment
Translocation and inversion are error formed by crossing-over.
i. Translocation
It is the transfer of a section in the same chromosome from
an
arm
to
the
other
(non-reciprocal
intrachromosomal
translocation) or a section from one chromosome to a nonhomologous chromosome (interchromosomal translocation). There
are 2 types of interchromosomal translocation: reciprocal
translocations (interchanges) and Transposition (non-reciprocal =
insertion).
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a. Non-reciprocal intrachromosomal translocation
This type involves the insertion of an interstitial segment
produced by two breaks in one chromosomal arm to the single
break in the other.
b. Reciprocal translocation (interchanges)
It is the most common type in which single breaks occur in
two on-homologous chromosomes and producing an exchange
of chromosome sections between them. Eg: Leukemia
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c. Transposition (insertion= non-reciprocal)
It may involve the attachment of a fragment from one
chromosome
to
another
non-homologous
chromosome
(transposition) or it involve the insertion of an interstitial
segment produced by two breaks in one chromosome to the
single break in the other.
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ii. Inversions
When 2 breaks occur in a chromosome, the part between them may
be reattached in its original place but in a reverse order. There are 2
types: Paracentric and Pericentric.
a. Paracentric inversion
The inverted reattached segment does not include centromere.
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b. Pericentric inversion
The inverted reattached segment includes centromere.
In meiosis alignment, loop is formed
II. Variation in number of chromosomal segments
There are 2 types: Deletions (Deficiency) and Duplication (Addition)
Deletion and duplication are formed due to error of replication.
i. Deletions (Deficiency)
It means the loss of a portion of the original chromosome, these
may occur either terminal or interstitial. A loss of any considerable
portion of a chromosome is usually lethal to a diploid organism due to
the genetic unbalance.
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a. Terminal: A part at the end of the chromosome is lost as a
result of one break. Eg: cri du chat syndrom
b. Interstitial (intercalary): A chromosome break in two regions
and the middle segment is lost; these may originate both a ring
chromosome and acentric fragment.
ii. Duplication (Addition)
It happens when a segment of the chromosome is represented 2 or
more times due to an unequal crossing-over between homologous
chromosomes during meiosis. There are 3 types: Tandem Reverse
tandem and terminal tandem.
a. Tandem: Segment is repeated 2 or more times with the same
arrangement as original.
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b. Reverse tandem: Segment is repeated 2 or more times with
reverse position.
c. Terminal tandem: Segment is repeated 2 or more times at the
end of the chromosome.
Note: when join with its sister chromatid → unbalance
III.
Variation in chromosome morphology
There are 2 types: Isochromosomes and Shifts
i. Isochromosomes:
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It rises from a break or a missdivision at the centromere.
The 2 resultant telocentric chromosomes produce chromosomes
with 2 identical arms.
ii. Shifts:
It is formed when 3 breaks occur in same chromosome and the
resulted segment from the 2 break is inserted at the third single break of
the same chromosome. Eg: seen in Neurospora
A B C D E F G H I J K
Produces either:
A B C F G D E H I J K shift segment
or
A B C G F D E H I J K shift segment & reverse the other
Importance of chromosome aberration:
- They can be associated with species differences.
- They have played a very important part as indicators of genetic
damage in both clinical and cancer studies.
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So, they are useful to medical and graduate students, physicians,
molecular biologists, and cytogeneticists
Animations:
Polyploidy
https://www.youtube.com/watch?v=DieKrGX90Pc
Non-disjunction
http://www.uic.edu/classes/bios/bios100/lectures/nondisjunction.htm
http://www.sumanasinc.com/webcontent/animations/content/mistakesmei
osis/mistakesmeiosis.html
Translocation
https://www.youtube.com/watch?v=eUZYACO236c
Inversion
https://www.youtube.com/watch?v=ZcnyMMHLkAw
Chromosome abnormalities
https://www.youtube.com/watch?v=FgMKGIED4Yo
https://www.youtube.com/watch?v=WNboyqs67p4
Reference:
1. An Introduction to Genetic Analysis (2000), 7th edition, W. H.
Freeman, New York.
2. Chromosome
Engineering
in
Plants:
Genetics,
Breeding,
Evolution, (1991), P.K. Gupta and T. Tsuchiya, 656 pp.,Elsevier.
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