3. Introduction
Inversion involves at least two breaks in a
chromosome; the broken segment rotates 180°
and is then reunited at the two break points.
4. Introduction
First discovered by Sturtevant in 1921 in Drosophila.
Later detected in a wide variety of plant and animal
species.
Studied extensively in different Drosophila species, such as,
D. pseudoobscura, D. persimilis and D. willistoni in their
salivary chromosomes.
D. meridiana, anopheline mosquitoes, Amphibia, and many
grasshopper sp. relatively free of inversions.
Reported in several plant species like maize, barley, broad
bean, Tradescantia etc.
The most common inversion seen in humans is
on chromosome 9, at inv(9)(p12q13).
This inversion is generally considered to have no harmful
effects, but there is some suspicion it could lead to an
increased risk for miscarriage or infertility for some affected
individuals.
5. Types of Inversion
• Based on the number of inverted segments
within a chromosome and the location of the
inversion points with respect of each other, the
inversion may be broadly classified into two
types:
1. Single inversion
2. Complex inversion
6. Single inversion
• In this case, only one segment of the
chromosome is inverted. There are two types
of single inversion:
Paracentric inversion
Pericentric inversion
7. Paracentric inversion
• The inversion is confined to a single arm of the
chromosome, i.e., both the inversion breakpoints are
located in the same arm.
8. Paracentric inversion
• More common.
• In pachytene and salivary chromosomes, an
inversion in a heterozygous condition can be
recognised by the inversion loop.
• Inversion loop is formed when all the portions
of the two chromosome synapse in a
homologous fashion.
• In a meiotic anaphase the common inversion
bridge results when a crossover takes place
within the inverted section.
9. Paracentric inversion
• The frequency of such crossing over depends
upon
The length of the inverted segment
Its location in the chromosome
Crossover characteristics of the individual
• Resulting chromatids abnormal: dicentric and
acrocentric formation.
• Acentric fragment will be lost (no centromere).
• Dicentric bridge is broken.
• These two events will lead to inviability of
gametes.
11. Paracentric inversion
• However, in Drosophila gamete lethality is not
serious.
• This can be explained as:
No crossing over in males
Lack of crossing over within an inversion or
exclusion of dicentric bridge from the egg nucleus
in females.
• Similar conditions has been seen in the
embryo sacs of maize plant only (1981) among
the higher plants.
• When two crossovers are formed within an
inverted segment, the results will depend on
the number of chromatids involved.
12. Paracentric inversion
• Two strand crossing over
will yield four normal
chromatids, two of which
were involved in crossing
over and the other two were
not.
• Three strand crossing over
yields one non-crossover
chromatid, one crossover
chromatid, and one
dicentric bridge with
acentric fragment.
• Four strand crossing over
yields two dicentric
chromatids and two acentric
fragments.
13. Pericentric inversion
The inverted segment includes the centromere, i.e.,
the two breakpoints are located in different arms of
the chromosome.
14. Pericentric inversion
• If the breaks are equidistant from the
centromere, the chromosome would appear
unchanged morphologically.
• At different distances, a shift in the centromere
position would take place.
• Crossing over within a pericentric
chromosome, when heterozygous, produces
characteristic chromatid products.
• A single crossover within the inversion loop
produces the following:
16. Pericentric inversion
• Crossover that would not result in
deletion/duplication chromatids would be the 2-
strand doubles.
• These would have their frequency determined
by the size of inversion loop.
18. Complex inversion
• Occurrence of more than one inversion in a
chromosome is called complex inversion.
• Based on the mutual relationship of the
inverted regions, it may be grouped into the
following five types:
19. Complex inversion
Independent inversions
Inversions occur in different regions of the chromosome
and they are separated from one another by un-inverted
(normal) segment.
20. Complex inversion
Direct tandem inversions
There are two or more inverted segments which are
directly adjacent to each other, i.e., the inverted regions
are not separated by normal regions.
21. Complex inversion
Reversed tandem inversions
The two inverted segments are adjacent to each other but
their positions are mutually interchanged i.e., g. the first
segment lies in place of the second and vice-versa.
22. Complex inversion
Included inversions
One inversion is confined within another inversion,
i.e., a segment within an inverted segment is inverted
again; as a result, the second inverted segment
possesses the normal gene sequence for the
concerned segment.
23. Complex inversion
Overlapping inversions
Such inversions have a common segment, i.e., a part
of an inverted chromosome segment is inverted
again together with an adjacent segment which was
not included in the first inverted segment.
24. Inversions and Evolution
• Centres largely on the Paracentric type.
• In an organism having a large number of
inversions in the same arm and in which the
salivary gland chromosomes can be studied,
their family history, as well as that of the
species, can be reconstructed by using
overlapping inversions.
• Inversions may also be selected in for natural
populations because each chromatin, due to
crossover, tend to become allelically different
compared to the blocks of chromatin due to
random mutations.
25. Inversions and Evolution
• Size of inversion is important; it must not be
too long to affect the allelic sequence, but
should be long enough to accumulate a genetic
variation which is sufficient to accommodate a
differential response to environmental
variations.
• Inversions can probably serve as a centre for
species divergence, given sufficient time to
accumulate genetic variation and formation of
barriers to prevent breeding with the parent
population.
26. Effects of Inversion
• Effect on Fertility
Fertility of inversion heterozygotes is reduced due to
the production of unbalanced gametes which carry
the deficiency-duplication chromatids obtained from
crossing over within the inversion loop. The effect on
fertility varies according to the type of inversion and
the organism carrying it.
• Recessive mutations
Damage to the DNA at the breakpoints may
sometimes result in a recessive mutation. Some of
the mutations may be recessive lethals; such
mutations are also lethal in hemizygous condition.
27. Effects of Inversion
• Position effect
Due to inversion euchromatic segment of a chromosome
may become located in the vicinity of a heterochromatic
region.
In such a condition, the euchromatic region may become
hetero-chromatinized; this would cause the suppression of
gene activity i.e., suppression of transcription of the
euchromatic segment.
Heterochromatinization is variable as a consequence of
which variegated type (V-type) position effects are
produced.
28. Effects of Inversion
• Effect of inversion on the activity of
nucleolar organizer region (NOR) of other
chromosome
Inversion in one chromosome influences the NOR
activity in the other chromosomes.
Viseras and Camacho in 1991 did not find decreased
activity of NOR in L3 does chromosome due to the
presence of a pericentric inversion in the smallest
chromosome (S11 chromosome) of grasshopper
Aiolopusstrepen.
29. References
1. “Variation: Nature and Consequences of
Altered Chromosomal
Structure.” Cytogenetics, by C. P. Swanson,
2nd ed., Prentice Hall of India., 1988.
2. “Changes in Chromosome Structure.”
Genetics, by Monroe W. Strickberger, 2nd
ed., Macmillan, 1976.
3. http://biologydiscussion.com
4. https://images.google.com/
5. https://en.wikipedia.org/wiki/Chromosomal_in
version
