Transcript Slide 1

Chromosomal Abnormalities
Dr. Attya Bhatti
Chromosomal Abnormalities
 Any change in the normal structure or number of
chromosomes; often results in physical or mental
Chromosome Abnormalities
Basic categories :
 Chromosome rearrangements:
 Chromosome rearrangements alter the structure of chromosomes; for
example, a piece of a chromosome might be duplicated, deleted, or
inverted.
 Aneuploids:
 the number of chromosomes is altered: one or more individual
chromosomes are added or deleted.

 Polyploids:
 one or more complete sets of chromosomes are added.
Table: Types chromosome abnormality
Numerical
Structural
Aneuploidy
Translication
Monosomy
Trisomy
Tetrasomy
Polylpoidy
Tripolody
Tetrapoildy
Different cell lines (mixoploidy)
Mosaicism
Chimerism
Reciprocal
Robertsonian
Deletions
Insertions
Inversions
Paracenyric
Pericentric
Rings
Isochromosomes
HETEROPLOIDY ABERRATIONS
 Chromosomal aberrations that changes the ploidy (one
set of chromosomes) of an organisms is called
heteroploidy.
 Euploid Aberrations
 Aneuploid Aberrations
Numerical Abnormalities
 Involves the loss or gain of one or more
chromosomes, referred as Aneuploidy
 Euploid/ Polyploidy
EUPLOID ABERRATIONS
 Euploid mutations produces organisms possessing
multiple sets of chromosomes.
 These are the changes in the number of chromosomes.
Monoploids
 One set of chromosomes (n=ABC) is present, mostly, in
the nuclei of haplotonic organisms.
 E.g. Chlamydomonas
 Neurosopora
 Also in diploid organisms, is usually in sex cells (male
bees and wasps)
Triploids
 Organisms may receives three sets of chromosomes(3n=
AAA-BBB-CCC).
 Results due to union of haploid gamete with diploid
gametes.
 These organisms are sterile and not common in Natural
populations.
Tetraploids
 Tetraploid organisms have four sets of
chromosomes(4n= AAAA-BBBB-CCCC).
 Arises in body cells by the somatic doubling of
chromosomes number.
 Produced by the union of diploid gametes.
Two groups
 Autotetraploids (auto-self):
Produced by either somatic doubling of homologous
chromosomes, or by the union of diploid gametes of the
same species.
 Parental genotype AABBCC X AABBCC
AAAABBBBCCCC
Allotetraploids
(Allo: non-homologous)
 Produced by fusion of diploid gametes of different
species,
 Reproduce true and behave as a new species.
 P.Genotyope:
AABBCC X DDEEFF
AADDBBEECCFF
 Found only in plants and called Amphi-di-ploids
Polyploids
 Polyploid organisms have more than 2n chromosomes.
 Wheat, e.g. hexaploid
 Many commercial fruits , ornamentals plants and human
liver cells are polyploidy.
 Polyploidy provide for studying dosage effect (How
many alleles interact to form phenotypes)
Polyploidy
Changes in the number of chromosome sets (polyploidy).
 Polyploids include



triploids (3n) ; Major cause is two sperm fertilization by single egg
(dispermy)
tetraploids (4n), rare and lethal, due to failiur to complete the first
zygotic division
pentaploids (5n), and
 higher numbers of chromosome sets.
 Polyploid organisms have more than 2n chromosomes.
 Many commercial fruits and ornamentals plants are polyploidy.
Polyploid cells contain multiples of the haploid number of chromosomes
such as 69, triploidy, 92, tetraploidy, Wheat, e.g. hexaploid
Polyploidy
 Polyploid cells contain multiples of the haploid number
of chromosomes such as 69, triploidy
 92 tetraploidy
Polyploidy
 Polyploid cells contain multiples of the haploid number
of chromosomes such as 69, triploidy
 92 tetraploidy
Causes
 Failure of a maturation meiotic division in an ovum or
sperm.
 By fertilization of an ovum bt two sperms, called
dispermy.
 When triploidy results from the presence of an
additional set of paternal chromosome, the placenta ia
usually swollen known as Hydatidiform changes.
Mixoploidy
1.
Mosaicism; An individual possesses two or more genetically
different cell lines all derived from a single zygote.
2. Chimerism: An individual has two or more genetically different
cell lines originating from different zygotes. (Organism derived
from more than one zygote).
ANEUPLOID ABBERATIONS
 Organism is that, which bears an irregular number
of a particular chromosomes
( addition and deletion of whole sets of
chromosomes).
 Usually caused by failure of chromosomes to
separate during meiosis (non-disjunction).
 Abnormal male (XO) and female(XXY).
Non-disjunction
 When a member of synaped homologous pair of
chromosomes, at anaphase, fail to separate and the gametes
thus formed become abnormal.
 Some gametes receives both members of homologues while
other gamete none.
 Fertilization of such abnormal gametes from zygotes that
either have an additional chromosomes(2n+1) or lack
chrmosomes(2n-1).
Origin of non-disjunction
 An error in meiosis I leads to the gamete containing
both homologs of one chromosomes pair.
 In meiosis II results in the gamete receiving two copies
of one of the homologs of the chromosomes pair.
 Can also occur during an early mitotic division in the
developing zygote, which results in mosaicism
PARENTAL ORIGIN OF MEIOTIC ERROR LEADING TO
ANEUPLOIDY
Chromosomes abnormality Parental (%)
Maternal (%)
Trisomy 13
15
85
Trisomy 18
10
90
Trisomy 21
5
95
45,X
80
20
47XXX
5
95
47,XXY
45
55
47,XYY
100
0
Causes of Non-disjunction
 An aging effect on the primary oocyte, which can remain
in a state of suspended inactivity for upto 50 years.
 Association b/w advancing maternal age and increased
incidence of down Syndrome.
Factors causing Non-disjunction
 An absence of recombination b/w homologous chromosomes in foetal
ovary
 An abnormality in spindle formation
 Radiation
 Delayed fertilization after ovulation.
Monosomy
 The absence of a single chromosome is refered to as monosomy.
 Diploid organisms that has one chromosomes less than its normal diploid
number(2n-1= AABBC).
 Monosomies on meiosis produces two types of gametes with (n) and (n-1)
chromosomes.
 In animals , loss of one chromosomes often results in genetic imbalance
which is associated with high mortality or reduce fertility.
 Human Syndromic disease like Turners syndrome (XO) is an example of
monosomic mutations.
 Results due to non-disjunction in meiosis
Monosomy
 If
one gamete receive two copies of a homologous
chromosomes, (Disomy)
 While other corresponding daughter gamete will have no copy
of the same chromosome (nullisomy)
 Also due to loss of a chromosomes as it moves to the pole of
the cell during anaphase, known as Anaphase lag.
Trisomy
Presence of an extra chromosome is refered to as trisomy.
 Down Syndrome (trisomy 21)
 Patau Syndrome (trisomy 13)
 Edward Syndrome (trisomy 18)
Trisomy
 Caused by Failure of separation of one of the pairs of
homologous chromosomes during anaphase of meiosis I.
 Can be caused by non-disjunction occurring during meiosis II
when a pair of a sister chromatids fails to separate.
Trisomy
 Diploid organisms that have one chromosome extra
(2n+1= AABBCCC) are called Trisomics.
Tetrasomics
 Diploid organisms that have one chromosome in quarduplicate (2n+
2= AABBCCCC).
Nullisomics
 An diploid organism, that has lost one chromosome pair from
its genotype is called nullisomic.
 It is lethal in diploids
 Some polyploids, however, can lose one homologous pair
without serious effects (AAAABB)
 Nullisomics of hexaploid wheat(6n-2) show reduced vigor and
fertility but can survive to maturity.
Double Trisomics
 If in a diploid organism, two different chromosomes are
present in triplicate , called double trisomic and presented as
(2n+1+1) AABBCCC
 In humans Klinefelter syndrome (XXYY)
Type
No. of chromosomes
Example
Normal diploid
2n
AABBCC
Monosomic
2n-1
AABBC
Nullisomic
2n-2
AABB
Polysomic
Extra chromosomes
a)
Trisomic
2n+1
AABBCCC
b)
Double
trisomic
2n+1+1
AABBBCCC
c) Tetra somic
2n+2
AABBCCCC
d) Pentasomic
2n+3
AABBCCCCC
Sexual Aneuploids in human and their phenotypes
Sex chromosomes
Sexual phenotype
No. of bar
bodies
XX, 46
Normal
1
XO, monosomic, 45
Turner syndrome
0
XXX, trisomic, 47
MR female
2
XY, 46
Normal
0
XXY, trisomic, 47
Klinefelter syndrome
1
XXYY, double trisomic, Klinefelter syndrome
48
1
XXXY, tetrasomic, 48
2
Female
Male
Klinefelter syndrome
Chromosome Morphology
Under the microscope chromosomes appear as thin, thread-like structures.
They all have a short arm and long arm separated by a primary constriction called the
centromere. The short arm is designated as p and the long arm as q.
 The centromere is the location of spindle attachment and is an integral part of the
chromosome. It is essential for the normal movement and segregation of chromosomes during
cell division.

Human metaphase chromosomes can be categorized according to the length of the short and long
arms and also the centromere location.
• Metacentric chromosomes have short and long arms of roughly equal length with the centromere
in the middle.
•
Submetacentric chromosomes have short and long arms of unequal length with the centromere
more towards one end.
• Acrocentric chromosomes have a centromere very near to one end and have very small short arms.
They frequently have secondary constrictions on the short arms that connect very small pieces of
DNA, called stalks and satellites, to the centromere.
The stalks contain genes which code for ribosomal RNA.
 The diagrams showing region on chromosomes, called ideograms.

Submetacentric
(Chromosome 9)
Metacentric
(Chromosome 1)
Acrocentric
(Chromosome 14)
•The ideogram is basically a "chromosome map" showing the relationship between the
short and long arms, centromere (cen), and in the case of acrocentric chromosomes the
stalks (st) and satellites (sa). Each band is numbered to aid in describing
rearrangements.
Chromosomes are identified by their size, centromere position and
banding pattern
Autosomes are numbered from largest to smallest, except that chromosome 21 is smaller
than chromosome 22.
Group Chromosomes
Description
A
1–3
Largest; 1 and 3 are metacentric but 2 is submetacentric
B
4,5
Large; submetacentric with two arms very different in size
C
6–12,X
Medium size; submetacentric
D
13–15
Medium size; acrocentric with satellites
E
16–18
Small; 16 is metacentric but 17 and 18 are submetacentric
F
19,20
Small; metacentric
G
21,22,Y
Small; acrocentric, with satellites on 21 and 22 but not on
the Y
Cytogenetics
Is the study of the structure and properties of chromosomes,
chromosomal
behaviour
during
mitosis
and
meiosis,
chromosomal influence on the phenotype and the factors that
cause chromosomal changes.
Related to disease status caused by abnormal chromosome
number and/or structure.
Methods for chromosomal analysis:
Karyotyping and banding
The collection of all the chromosomes is referred to as a
Karyotype.
The method used to analyze the chromosome constitution of an
individual, known as chromosome banding.
Chromosomes are displayed as a karyogram.
Obtaining and preparing cells for
chromosome analysis

Cell source:

Blood cells

Skin fibroblasts

Amniotic cells / chorionic villi
 Increasing the mitotic index
- proportion of cells in mitosis using colcemid

Synchronizing cells to analyze prometaphase chromosomes
Key procedure
In the case of peripheral (venous) blood

A sample is added to a small volume of nutrient medium containing
phytoheamagglutinin, which stimulates T lymphocytes to divide.

The cells are cultured under sterile conditions at 37C for about 3 days,
during which they divide, and colchicine is then added to each culture.

This drug has the extremely useful property of preventing formation of
the spindle, thereby arresting cell division during metaphase, the time
when the chromosomes are maximally condensed and therefore most
visible.

Hypotonic saline is then added, which causes the red blood cells to lyze
and results in spreading of the chromosomes, which are then fixed ,
mounted on a slide and stained ready for analysis
PREPARATION OF CHROMOSOMES
Karyotype Analysis
Following Steps are involved;
 Counting the number of cells, sometimes referred as
metaphase spread
 Analysis of the banding pattern of each individual chromosome
in selected cells.
 Total chr. Count is determined in 10-15 cells, but if mosaicism
is suspected then 30 or more cell count will be undertaken.
 Detailed analysis of the banding pattern of the individual
chromosomes is carried out in approx. 3-5 metaphase spread,
which shows high quality banding.
 The banding pattern of each chromosome is specific and shown
in the form of Idiogram.
MITOTIC CHROMOSOMAL SPREAD
Chromosome Banding
 Chromosome banding is developed based on the presence of
heterochromatin and euchromatin.
 Heterochromatin is darkly stained whereas euchromatin is
lightly stained during chromosome staining.
oEuchromatin, which undergoes the normal process of condensation and
decondensation in the cell cycle, and
oHeterochromatin,
which
remains
in
a
highly
throughout the cell cycle, even during interphase.
condensed
state
Euchromatin
 Exist in extended state, dispersed through the nucleus and staining diffusely.
 Early-replicating and GC rich region.
 In prokaryotes, euchromatin is the

only form of chromatin present.
Genes may oy may not expressed
Heterochromatin
darkly
1.
stained
two types
Constitutive ; always inactive an condensed.
Consist
Late
replicating and AT rich region
Present

of repetitive DNA
at identical positions on all chromosomes in all cell types of an organism.
Genes poorly expressed.
Human chromosomes 1, 9, 16, and the Y chromosome contain large regions of
constitutive heterochromatin.

Occurs
around the centromere and near telomeres.
2. Facultative;
Genetically active(decondensed) and inactive (condensed)
Variable in its expression. It varies with the cell type and may be manifested as
condensed, or heavily stained.
Types of chromosome banding
 G-banding
 C-banding
 Q-banding
 R-banding
 T-banding
Chromosomal Banding
 G-banding, gives dark bands

C-banding: C-banding stains the constitutive heterochromatin,
which usually lies near the centromere.

Q-banding: Q-banding is a fluorescent pattern obtained using
quinacrine for staining. The pattern of bands is very similar to that
seen in G-banding.

R-banding: reverse of G-banding (the R stands for "reverse").
 Dark regions are euchromatic (guanine-cytosine rich regions) and
the bright regions are heterochromatic (thymine-adenine rich
regions).

T-banding: Identifies a subset of the R bands which are
especially concentrated at the telomeres.
G-Banding
۩-
G-banding is obtained with Giemsa stain following digestion of
chromosomes with enzyme trypsin.
۩-
Giemsa stain, named after Gustav Giemsa, an early malariologist, is
used for the histopathological diagnosis of malaria and other parasites.
۩-
It is a mixture of methylene blue and eosin.
۩-
It is specific for the phosphate groups of DNA and attaches itself to
regions of DNA where there are high amounts of adenine-thymine
bonding.
۩-
it yields a series of lightly and darkly stained bands – the dark regions
tend to be heterochromatic, late-replicating and AT rich.
۩-
The light regions tend to be euchromatic, early-replicating
and GC rich .
G- Banding
Molecular Methods for chromosomal
analysis Molecular Cytogenetics

Fluorescent in situ Hybridization (FISH)

Chromosome painting

Comparative Genomic Hybridization (CGH)

Molecular karyotyping and Multiplex
FISH(M-FISH)

Spectral Karyotyping

Array CGH
Key points
 Cytogenetic analysis usually focuses on chromosomes in
dividing cells.
 Dyes such as Quinacrine and Giemsa create banding patterns
that’s are useful in identifying individual chromosomes within
a cell.
 A karyotype shows the photographed chromosomes of a cell
arranged for cytogenetic analysis.
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