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Chromosomal Aberrations• The somatic (2n) and gametic (n) chromosome numbers of a
species ordinarily remain constant.
• This is due to the extremely precise mitotic and meiotic cell division.
• Somatic cells of a diploid species contain two copies of each chromosome, which are called homologous chromosome.
• Their gametes, therefore contain only one copy of each chromosome, that is they contain one chromosome complement or genome.
• Each chromosome of a genome contains a definite numbers and kinds of genes, which are arranged in a definite sequence.
• Sometime due to mutation or spontaneous (without any known
causal factors), variation in chromosomal number or structure do
arise in nature. - Chromosomal aberrations.
• Chromosomal aberration may be grouped into two broad classes:
1. Structural and 2. Numerical 2
INTRODUCTION:• Chromosome structure variations result from
chromosome breakage.
• Broken chromosomes tend to re-join; if there is more than one break, rejoining occurs at random and not necessarily with the correct ends.
• The result is structural changes in the chromosomes.
• Chromosome breakage is caused by X-rays, various chemicals, and can also occur spontaneously.
• There are four common type of structural aberrations:
1.Deletion or Deficiency,
2.Duplication or Repeat
3.Inversion, and
4.Translocation 3
DELETION OR DEFICIENCYLoss of a chromosome segment is known as deletion
or deficiency
It can be terminal deletion or interstitial or intercalary deletion.
A single break near the end of the chromosome would be expected to result in terminal deficiency.
If two breaks occur, a section may be deleted and an intercalary deficiency created.
Terminal deficiencies might seem less complicated.
But majority of deficiencies detected are intercalary type within the chromosome.
Deletion was the first structural aberration detected by Bridges in 1917 from his genetic studies on X chromosome of Drosophila. 4
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Chromosome with deletion can never revert back to a normal
condition and transmitted to next generation.
In intercalary deletion, broken acentric fragment of
chromosomes appear as small chromatin bodies in cells known
as Micronuclei.
Homozygous deletion is lethal.
Heterozygous deletions can revealed a phenomenon known as
Pseudodominance /
One copy of a gene is deleted
So the recessive allele on the other chromosome is now
expressed
Eg., sex linked lethal and dominant notch-wing etc. in
Drosophila.
Cri-du-chat (Cat cry syndrome) of babies results from the
chromosome deficiency in the short arm of chromosome 5 .
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•Deletion can be recognized during meiotic pairing of
homologus chromosome and during somatic pairing in
specialized tissue like salivary gland of Drosophila or during
pachytene in Maize.
•Due to intercalary deletion unpaired loop is formed.
DUPLICATION
The presence of an additional chromosome 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 per nucleus is called duplication.
Four types of duplication:
1. Tandem duplication
2. Reverse tandem duplication
3. Displaced duplication
4. Translocation duplication
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The extra chromosome segment
may be located immediately after
the normal segment in precisely
the same orientation forms the
tandem .
When the gene sequence in the
extra segment of a tandem in the
reverse order i.e, inverted , it is
known as reverse tandem
duplication.
In some cases, the extra segment
may be located either on the
same chromosome or on a
different one but away from the
normal segment –termed as
displaced duplication.
Later condition is also termed as
translocation duplication. 9
ORIGIN Origin of duplication involves
chromosome breakage and reunion of chromosome segment with its homologous chromosome.
As a result, one of the two homologous involved in the production of a duplication ends up with a deficiency, while the other has a duplication for the concerned segment.
Another phenomenon, known as unequal crossing over, also leads to exactly the same consequences for small chromosome segments.
For e.g., duplication of the band 16A of X chromosome of Drosophila produces Bar eye.
This duplication is believed to originate due to unequal crossing over between the two normal X chromosomes of female. 10
GENETIC EFFECTS Majority of small duplications have no phenotypic effect
However, they provide raw material for evolutionary change
Lead to the formation of gene families.
A gene family consists of two or more genes that are
similar to each other derived from a common gene
ancestor ex- globin gene family whose Genes encode
proteins that bind oxygen
Tandem duplications play a major role in evolution, because
it is easy to generate extra copies of the duplicated genes
through the process of unequal crossing over. These extra
copies can then mutate to take on altered roles in the cell, or
they can become pseudogenes, inactive forms of the gene, by
mutation. 11
INVERSION• When a segment of chromosome is oriented in the reverse
direction, such segment said to be inverted and the phenomenon is termed as inversion.
• The existence of inversion was first detected by Strutevant and Plunkett in 1926.
• Inversion occur when parts of chromosomes become detached , turn through 1800 and are reinserted in such a way that the genes are in reversed order.
• For example, a certain segment may be broken in two places, and the breaks may be in close proximity because of chance loop in the chromosome.
• When they rejoin, the wrong ends may become connected.
• The part on one side of the loop connects with broken end different from the one with which it was formerly connected.
• This leaves the other two broken ends to become attached.
• The part within the loop thus becomes turned around or invert12
Inversion may be classified into two types:
PERICENTRIC - include the centromere
PARACENTRIC - do not include the centromere
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Individuals with one copy of a normal chromosome and one copy of an inverted
chromosome
Usually phenotypically normal
Have a high probability of producing gametes that are abnormal in genetic
content
Abnormality due to crossing-over within the inversion interval
During meiosis I, homologous chromosomes synapse with each other
For the normal and inversion chromosome to synapse properly, an inversion
loop must form
If a cross-over occurs within the inversion loop, highly abnormal
chromosomes are produced
Inversions in natural populations
In natural populations, pericentric inversions are much less frequent than
paracentric inversions.
In many sp, however, pericentric inversions are relatively common, e.g., in
some Drosophila. Grasshoppers etc.
Paracentric inversions appear to be very frequent in natural populations of
Zea maize etc.
INVERSION HETEROZYGOTES
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When a paracentric inversion crosses over with a normal chromosome, the resulting chromosomes are an acentric, with no centromeres, and a dicentric, with 2 centromeres. The acentric chromosome isn't attached to the spindle, so it gets lost during cell division, and the dicentric is usually pulled apart (broken) by the spindle pulling the two centromeres in opposite directions. These conditions are lethal.Eg, Dicentric bridges formed in Maize female tissue and pollen grains are sterile. 15
Crossing Over Within Inversion Interval Generates Unequal Sets of Chromatids
PARACENTRIC INVERSION
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When a pericentric inversion crosses over with a normal chromosome, the resulting chromosomes are both duplicated for some genes and deleted for other genes. (They do have 1 centromere apiece though). The gametes resulting from these are aneuploid and do not survive. Eg, Drosophila.Thus, either kind of inversion has lethal results when it crosses over with a normal chromosome. The only offspring that survive are those that didn't have a crossover. Thus when you count the offspring you only see the non-crossovers, so it appears that crossing over has been suppressed. 17
Crossing Over Within Inversion Interval Generates Unequal Sets of Chromatids
PERICENTRIC INVERSION
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Inversions Prevent Generation of Recombinant Offspring Genotypes
• Only parental chromosomes (non-recombinants) will produce normal progeny after fertilization
PARACENTRICPERICENTRIC 19
TRANSLOCATIONIntegration of a chromosome segment into a nonhomologous
chromosome is known as translocation.
Three types:
1. simple translocation
2. shift
3. reciprocal translocation.
1. Simple translocation: In this case, terminal segment of a chromosome is integrated at one end of a non-homologous region. Simple translocations are rather rare.
2. Shift: In shift, an intercalary segment of a chromosome is integrated within a non-homologous chromosome. Such translocations are known in the populations of Drosophila, Neurospora etc.
3. Reciprocal translocation: It is produced when two non-homologous chromosomes exchange segments – i.e., segments reciprocally transferred. Translocation of this type is most common eg, Rhoeo, Oenothera, Tradescantia etc. 20
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SHIFT RECIPROCAL
In reciprocal translocations two non-homologous
chromosomes exchange genetic material
Usually generate so-called balanced translocations
Usually without phenotypic consequences
Although can result in position effect.
CYTOLOGY OF TRANSLOCATION HETEROZYGOTE
In simple translocations the transfer of genetic material
occurs in only one direction
These are also called unbalanced translocations
Unbalanced translocations are associated with phenotypic
abnormalities or even lethality
Example: Familial Down Syndrome
In this condition, the majority of chromosome 21 is attached to
chromosome 14. 22
Individuals carrying balanced translocations have
a greater risk of producing gametes with
unbalanced combinations of chromosomes.
This depends on the segregation pattern during
meiosis I
During meiosis I, homologous chromosomes
synapse with each other
For the translocated chromosome to synapse
properly, a translocation cross must form
Balanced Translocations and Gamete Production
BALANCED LETHALS AND GAMETIC
COMPLEXES
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Meiotic segregation can occur in one of three ways
1. Alternate segregation
Chromosomes on opposite sides of the translocation cross
segregate into the same cell
Leads to balanced gametes
Both contain a complete set of genes and are thus viable
2. Adjacent-1 segregation
Adjacent non-homologous chromosomes segregate into the
same cell
Leads to unbalanced gametes
Both have duplications and deletions and are thus inviable
3. Adjacent-2 segregation
Adjacent homologous chromosomes segregate into the same
cell
Leads to unbalanced gametes
Both have duplications and deletions and are thus inviable24
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