Dr. Attya Bhatti

Chromosomal Abnormalities

Chromosomal Abnormalities

Any change in the normal structure or

number of chromosomes; often results in

physical or mental

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.

Chromosome Abnormalities

Table: Types chromosome abnormality

NumericalAneuploidy

Monosomy

Trisomy

Tetrasomy

Polylpoidy Tripolody

Tetrapoildy

Different cell lines (mixoploidy)

Mosaicism

Chimerism

StructuralTranslication

Reciprocal

Robertsonian

Deletions

Insertions

InversionsParacenyric

Pericentric

Rings

Isochromosomes

HETEROPLOIDY ABERRATIONSChromosomal aberrations that changes the ploidy

(one set of chromosomes) of an organisms is called heteroploidy.

Euploid AberrationsAneuploid Aberrations

Numerical AbnormalitiesInvolves the loss or gain of one or more

chromosomes, referred as Aneuploidy

Euploid/ Polyploidy

EUPLOID ABERRATIONSEuploid mutations produces organisms

possessing multiple sets of chromosomes.

These are the changes in the number of

chromosomes.

MonoploidsOne set of chromosomes (n=ABC) is

present, mostly, in the nuclei of haplotonic organisms.

E.g. ChlamydomonasNeurosoporaAlso in diploid organisms, is usually in sex

cells (male bees and wasps)

TriploidsOrganisms 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.

TetraploidsTetraploid 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 groupsAutotetraploids (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

PolyploidsPolyploid organisms have more than 2n

chromosomes.Wheat, e.g. hexaploidMany 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 divisionpentaploids (5n), andhigher 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

PolyploidyPolyploid cells contain multiples of the

haploid number of chromosomes such as 69, triploidy

92 tetraploidy

PolyploidyPolyploid cells contain multiples of the

haploid number of chromosomes such as 69, triploidy

92 tetraploidy

CausesFailure 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 ABBERATIONSOrganism 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-disjunctionWhen 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-disjunctionAn 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 ANEUPLOIDYChromosomes 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-disjunctionAn 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

Female

XX, 46 Normal 1

XO, monosomic, 45 Turner syndrome 0

XXX, trisomic, 47 MR female 2

Male

XY, 46 Normal 0

XXY, trisomic, 47 Klinefelter syndrome 1

XXYY, double trisomic, 48

Klinefelter syndrome 1

XXXY, tetrasomic, 48 Klinefelter syndrome 2

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.

Metacentric(Chromosome 1)

Submetacentric(Chromosome 9)

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.

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

Obtaining and preparing cells forchromosome analysis

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

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.

Karyotype Analysis

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 condensed state

throughout the cell cycle, even during interphase.

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

Heterochromatindarkly stained two types

1.Constitutive ; always inactive an condensed.

Consist of repetitive DNA

Late replicating and AT rich region

Present 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.

THANK YOU