Cytogenetics examines chromosomes microscopically to detect abnormalities in chromosome number or structure. Chromosomes are stained and have characteristic banding patterns used to identify them. Variations in chromosome structure include deficiencies/deletions, duplications, inversions, and translocations. Deficiencies occur when a chromosome fragment is lost. Duplications involve repetition of a chromosomal segment. Inversions flip a segment to the opposite orientation. Translocations exchange genetic material between non-homologous chromosomes. These structural variations can impact phenotypes but many are phenotypically normal.
2. INTRODUCTIONINTRODUCTION
Genetic variation refers to differences
between members of the same species or
those of different species
◦ Allelic variations are due to mutations in
particular genes
◦ Chromosomal aberrations are substantial
changes in chromosome structure or number.
3. Variation in ChromosomeVariation in Chromosome
StructureStructure
Cytogenetics -The field of genetics that involves
the microscopic examination of chromosomes
A cytogeneticist typically examines the
chromosomal composition of a particular cell or
organism
◦ This allows the detection of individuals with abnormal
chromosome number or structure
◦ This also provides a way to distinguish between species
4. Since different chromosomes can be the same size
and have the same centromere position,
chromosomes are treated with stains to produce
characteristic banding patterns
Example: G-banding
Chromosomes are exposed to the dye Giemsa
Some regions bind the dye heavily
Dark bands
Some regions do not bind the stain well
Light bands
In humans
300 G bands are seen in metaphase
2,000 G bands in prophase
CytogeneticsCytogenetics
5. Cytogeneticists use three main features to identify
and classify chromosomes
1. Location of the centromere
2. Size
3. Banding patterns
CytogeneticsCytogenetics
6. The banding pattern is useful in several
ways:
1. It distinguishes Individual chromosomes
from each other
2. It detects changes in chromosome structure
3. It reveals evolutionary relationships among
the chromosomes of closely-related species
CytogeneticsCytogenetics
7. There are two primary ways in which the structure
of chromosomes can be altered
1. The total amount of genetic information in the
chromosome can change
Deficiencies/Deletions
Duplications
2. The genetic material remains the same, but is
rearranged
Inversions
Translocations
Mutations Can AlterMutations Can Alter
Chromosome StructureChromosome Structure
8. Deficiency (or deletion)
◦ The loss of a chromosomal segment
Duplication
◦ The repetition of a chromosomal segment compared to
the normal parent chromosome
Inversion
◦ A change in the direction of part of the genetic material
along a single chromosome
Translocation
◦ A segment of one chromosome becomes attached to a
different chromosome
◦ Simple translocations
One way transfer
◦ Reciprocal translocations
Two way transfer
9. A chromosomal deficiency occurs when a
chromosome breaks and a fragment is lost.
DeficienciesDeficiencies
10. The phenotypic consequences of deficiencies
depends on the
1. Size of the deletion
2. Chromosomal material deleted
Causes Of Deletions
* Heat or Radiation
( especially ionization)
* Chemicals
* Viruses
* Errors in recombination
Deletions do not revert because the DNA is degraded.
DeficienciesDeficiencies
11. 2 types:
terminal deletion or intercalary deletion.
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
example, the disease cri-du-chat syndrome in humans
Caused by a deletion in the short arm of
chromosome
The deletion results in several mental retardation
and physical abnormalities , For Example ,Microcephaly
12.
13. A chromosomal duplication is usually caused by
abnormal events during recombination. And more
than one copy present.
DuplicationsDuplications
14. Types DuplicationTypes Duplication
-Tandem Duplications
are adjacent to each other.
-Reverse Tandem Duplicat-
ions
result in genes arranged
In opposite order of the
original.
-Tandem duplication at the
end of chromosome is a
Terminal tandem duplication .
15. Duplications can provide additional genes,Duplications can provide additional genes,
forming gene familiesforming gene families
The genes in a duplicated region may accumulate
mutations which alter their function
◦ After many generations, they may have similar but distinct
functions
◦ They are now members of a gene family
◦ Two or more genes derived from a common ancestor are
homologous
◦ Homologous genes within a single species are paralogs
17. The globin genes all encode subunits of proteins
that bind oxygen
Over 500-600 million years, the ancestral globin gene
has been duplicated and altered so there are now 14
paralogs in this gene family on three different
chromosomes
Different paralogs carry out similar but distinct functions
All bind oxygen
myoglobin stores oxygen in muscle cells
different globins are in the red blood cells at different
developmental stages
provide different characteristics corresponding to the oxygen
needs of the embryo, fetus and adult
18. A chromosomal inversion is a segment that has
been flipped to the opposite orientation,
InversionsInversions
19. In an inversion, the total amount of genetic information stays
the same
Therefore, the great majority of inversions have no phenotypic
consequences
In rare cases, inversions can alter the phenotype of an
individual
About 2% of the human population carries inversions that
are detectable with a light microscope
Most of these individuals are phenotypically normal
However, a few an produce offspring with genetic abnormalities
20. Individuals with one copy of a normal chromosome and one
copy of an inverted chromosome
Inversion HeterozygotesInversion Heterozygotes
Such individuals may be phenotypically normal
They also may have a high probability of producing gametes that are
abnormal in their genetic content
The abnormality is due to crossing-over in the inverted segment
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
21.
22.
23.
24. There are two main types of translocations ;
*Reciprocal (balanced) translocations
*Robertsonian(unbalanced) translocations
In reciprocal translocations two non-homologous
chromosomes exchange genetic material.
Reciprocal translocations lead to a rearrangement
of the genetic material, not a change in the total
amount
Thus, they are also called balanced translocations
TranslocationsTranslocations
25. 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
The individual would have three copies of genes found
on a large segment of chromosome 21
Therefore, they exhibit the characteristics of Down syndrome
26. Familial Down Syndrome is an example of
Robertsonian translocation
This translocation occurs as such
Breaks occur at the extreme ends of the short arms of
two non-homologous acrocentric chromosomes
The small acentric fragments are lost
The larger fragments fuse at their centromeric regions to
form a single chromosome which is metacentric or
submetacentric
This type of translocation is the most common type
of chromosomal rearrangement in humans
Approximately one in 900 births