Genetics

1. VARIATION IN
CHROMOSOME STRUCTURE
Afifa hameed Afifa hameed
11011514-03111011514-031
sec “G”sec “G”

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 Chromosome Variation 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 Alter Mutations 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

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

16. Genes derived
from a single
ancestral gene

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

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