5.
Structural changes in chromosomes may be of
following types :
Deletion – Terminal, Interstitial.
Duplication – Tandem, reverse tandem & displaced.
Translocation – Simple, isochrome, reciprocal, Shift.
Inversion – Pericentric & Paracentric.
Types of structural changes in
chromosomes
6. Portions of chromosome without a centromere (acentric fragments) lag in
anaphase movement and are lost form reorganizing the nuclei or digested
by nucleases.
Such loss of a portion of chromosome and the genes associated with it is
termed deletion.
The chromosomes with deleted parts can never revert back to normal
condition.
If the deletion takes place in allosomes, then the deletion is transmitted to
the next generation as well.
When a part of chromosome of the dominant allele is deleted, the recessive
allele starts showing dominance, this phenomenon is called
“pseudo dominance”.
There are two type of deletions
Terminal
Interstitial
DELETION:-
7. 1.Terminal Deletion : This is when a part of
the chromosome is deleted at the terminal end
of the chromosome. It results by only one
break in the chromosome.
8. 2.Interstitial : This type of aberration results when a
portion is lost in the middle of the chromosome. This
is caused by two breaks in the chromosome, one at
the Starting and another at the end of the deleted
region.
9. Examples : Cri du chat syndrome
Deletion of some chromosome regions produce their own unique
phenotypes such as “cri du chat” syndrome in human babies.
• Human babies missing a portion of the short arm of chromosome 5
(autosome) have a distinctive cat-like cry, hence the name.
• They are also mentally retarded (IQ below 20), have saddle nose and
exhibit microcephaly (small head).
The karyotype of cri du
chat patient is called 5𝒑−
10.
The presence of a part of chromosome in excess of the
normal chromosome is known as duplication. Due to this,
even though the chromosome number is the same, there is
extra genetic material.
These extra genes present in the duplication may cause
those genes to not function properly, these extra
instructions leads errors while producing the phenotype.
There are three major types of duplication :
1. Tandem.
2. Reverse tandem.
3. Displaced.
DUPLICATION:-
11. 1. Tandem : In case of tandem duplication the
duplicated section of the chromosome sits right beside
the normal chromosome part which it duplicated
form.
12. 2. Reverse Tandem : In this case the duplicated
section of the chromosome sits right beside the normal
chromosome part which it duplicated form but in a
reversed sequence.
13. 3. Displaced : In Displaced duplication the duplicated
section does not sit adjacent to the section which it
duplicated from. Depending on weather the
Duplicated portion is on the same side of the
Centromere as the section which it duplicated from or
on the opposite side, displaced duplication can be
termed either homo bronchial or hetero bronchial.
14. Example : Bar eyed Drosophila.
The bar phenotype of Drosophila is characterized by narrower, oblong, bar
shaped eye with fewer number of facets.
There are few genes which are responsible to give the narrow shape to the
eye of the Drosophila, when the region of the chromosome containing
these genes are duplicates, the expression of genes will become higher
and the eye of the fly becomes narrower, like a bar.
15.
The shifting or transfer of a part of chromosome or setoff
genes to a non homologous chromosome is called
translocation.
There is no addition or deletion of genes in translocation,
only the sequence and the position of the genes are
altered.
There are four types of translocation :
1. Simple.
2. Isochrome.
3. Reciprocal.
4. Shift.
TRANSLOCATION:-
16. 1. Simple Translocation: When a terminal segment of
the chromosome breaks and re-attaches to one end of a
non-homologous chromosome then it is called simple
Translocation.
17. 2. Isochrome Translocation : This type of translocation
is caused by the simultaneous deletion and duplication
of chromosome parts. In this type of abnormality the
chromosomes arms are mirror images if each other. This
type of chromosome is also called isochromosome.
18. 3. Reciprocal Translocation : In this type of
translocation a segment of one chromosome is
exchanged with a segment from another non-
homologous chromosome. Hence, two translocations are
simultaneously achieved.
19. 4. Shift Translocation : In this type of translocation an
intercalary part of one chromosome breaks and gets
itself inserted into intercalary position of a non-
homologous chromosome.
20.
This type of chromosomal aberration involves rotation of
a part of a chromosome by 1800 on its own axis.
Just like in translocation, there is no addition or deletion
of genes but simply a re-arrangement of the gene
sequence.
It takes place in a intercalary position.
The location of the inverted segment can be detected in
meiotic nuclei by the presence of an inversion loop.
Accordingly there are two types of inversion :
1. Pericentric
2. Paracentric
INVERSION:-
21. 1. Paracentric Inversion : In this type of inversion the
segment involved is present wholly within one
chromosome, that means no centromere is involved.
22. 2. Pericentric Inversion : In this type of inversion
the break points are located on both the arms of
the chromosome are involved including the
centromere.
23.
24.
25.
loss of single chromosome
Monosomy of autosomes is lethal
Turner syndrome XO i.e. loss of sex chr.
Cause : 1. non disjunction – one gamete receives 2
copies of
homologous chr. & other will have no copy
2. loss of chr. As it move towards pole of cell
during anaphase .
MONOSOMY:-
26.
Gain of homologous chr.
Trisomy 21 (down’s ) , trisomy 18 ,
klinefelter’s syndrome (47 XXY)
TRISOMY:-
27.
Failure of bivalents to separate during meiosis 1 ,
the gametes get both
homologues of one chr. pair
Sometimes in meiosis 2 when sister chromatids fail
to separate - gets 2
copies of one of the homologues
It can also occur during early mitotic div. of zygote
- there will be
presence of 2 or more cell lines
NONDISJUNCTION:-
28.
29.
Aging effect on primary oocyte
Radiation
Delayed fertilization after ovulation
Effects – monosomy or trisomy
CAUSES:-
30.
Multiples of haploid no.
Triploid or tetraploid
Foetus does not survive
Cause –
1. retention of polar body
2. Formation of diploid sperm
3. Dispermy – fertilization by 2 sperms
POLYPLOIDY:-
34.
Trisomy 18
Described by Edward in 1960
Rare to find live born
Do not live beyond few months
Features – mental retardation , failure to
thrive , hypotonia ,
prominent occiput , low set ears , receding
jaw , short sternum ,
clenched fists, rocker bottom feet.
Edward’s syndrome
36.
X monosomy , 45 X
described by Turner in 1938
Phenotype is female
Short stature
Webbing of neck
Cubitus valgus
Low posterior hair line
Broad chest with widely spaced nipples
High arched palate
Turner’s syndrome
37. Coarctation of aorta
VSD
Horseshoe kidney
Renal hypoplasia
Streak gonads
Secondary sexual characters do not develop
turner’s syndrome
