Pericentric Inversions
(around the centromere)
Kinds of chromatids produced by various crossovers within the inversion in
a pericentric inversion heterozygote
C
Constitution
i i off 4 chromatids
h
id after
f C.O.
CO
C.O. event
Position of C.O.
Normal
Single C.O
CO
Inversion
dp + df
Any point
1
1
2
2 Strand
2-Strand
12
1,2
2
2
0
3-Strand
1,3 or 1,4
1
1
2
4 St d
4-Strand
15
1,5
0
0
4
Double
The numbers in column 2 refer to those in inversion figure.
g
C.O.=Crossover
Pericentric Inversions
(
(around
d the
th centromere)
t
)

The two types of inversions (para- and peri-centric) result in different
cytological events

Chromosome inversions have no effect on mitotic divisions, but do effect
meiosis

Meiosis is normal in individuals with homozygous inversions

If the inverted regions of the inversion heterozygote is large enough for
crossing-over
i
to occur within
i hi the
h inversion
i
i loop,
l
a portion
i off the
h resulting
li
gametes will be abnormal
Pericentric Inversions
(around the centromere)

Pericentric inversions result in dp + df gametes which is related to the
chromosome segments distal to the breakpoints of the inverted segment

Products of crossover event within the loop are lost
1.
2.
3.

Recombinant types are not recovered
Crossing over may not be suppressed cytologically
Only 2 strand double crossover types are recovered
No bridges or fragments produced at anaphase I or II
Paracentric Inversions
(beside the centromere)
In plants
p a ts

A homozygous inversion will produce normal pollen and seed set

A heterozygous inversion will produce partial ovule and pollen abortion

The degree
Th
d
off pollen
ll abortion
b ti is
i dependent
d
d t upon the
th amountt off crossing-over
i
within the inversion loop

To distinguish a homozygous inversion from a homozygous normal
individual, the unknown can be crossed with a homozygous normal individual
If the unknown is a homozygous inversion, the F1 will be
h t
heterozygous
for
f the
th inversion
i
i andd be
b partially
ti ll sterile
t il
Paracentric Inversions
(beside the centromere)

Crossing-over in paracentric inversions result in bridges (dicentric
chromosomes)
h
) andd fragments
f
t (acentric
(
t i chromosomes)
h
)

The size of the acentric fragment represents the length of the inverted region
plus twice the length of the distal segment

If deficiencies for the segment distal to the inversion, resulting from the loss
of the acentric fragment, cause gamete spore abortion, the pollen abortion
percentage can be predicted by cytological observation of meiosis
Paracentric Inversions
(besides the centromere)
Kinds of chromatids produced by various crossovers in a paracentric
inversion heterozygote
Constitution of 4 chromatids after C.O.
C.O. event
Position of C.O.
Normal
Single C.O
Inversion
dp + df
Any point
1
1
2 (dicentric+acentric)
2-Strand
1,2
2
2
0
3-Strand
1,3 or 1,4
1
1
2 (dicentric+acentric)
4-Strand
1,5
0
0
4 (dicentric+acentric)
IN/OUT
1 in loop +
1 in interstitial
1
1
2 (dicentric+acentric)
IN/OUT
44-strand
strand double +
1 in interstitial
0
0
4 (dicentric+acentric)
Double
Chromatid tie in Dorsophila female Oogenesis
Division
vso I
A dicentric bridge orients
crossover chromatids away
from the poles at division I
Deficiency-duplication
chromatids will occur in
intercalary cell and polar
cells will produce a fertile
ovum with intact
chromosomes
Division
v s o II

During female gamete formation in plants and animals,
animals only one
polar megaspore will function in production of the ovule

The duplication-deficiency
p
y chromosomes are oriented to the
intercalary cells by the chromatid tie and the normal chromosomes
will be included in the nuclei of the polar cells more often than the
dp-df chromosomes

The result will be nearly normal female fertility of inversion
heterozygotes, but recombination will be substantially reduced in
regions
i
involved
i l d in
i the
th inversion
i
i
Use of inversions to produce duplications without deficiencies
Intercross two inversions with one breakpoint in common:
1
2
3
4
5
6
7
8
1
3
2
4
5
6
7
8
1
X
2
1
4
3
4
5
6
7
8
3
2
5
6
7
8
4
1
3
2
5
6
7
8
1
3
2
5
6
7
8
1
4
3
2
4
1
3
2
5
6
7
8
4
5
6
7
8
Intercross overlapping inversions:
1
2
3
4
5
6
7
8
1
3
2
4
5
6
7
8
1
X
2
1
3
2
4
3
2
4
2
5
4
5
6
6
7
7
1
8
1
1
3
2
2
4
4
5
3
4
2
3
3
6
5
7
6
8
7
8
1
1
2
4
5
3
1
8
5
3
4
5
3
5
3
2
1
1
4
5
7
6
7
6
7
8
6
7
8
6
2
6
7
4
8
8
8
5
6
7
8
c
b
d
e
a
A double crossover is required for
recovery of chromatid with
recombination within the inversion
loop
c
c will have greatest amount of
recombination with a
a
b and d will have equal amount of
recombination with a, but less than c
b
d
e
Interchromosomal Translocation
(part of one chromosome is attached to another)
Types
1. Interstitial translocations (intercalary)
A segment from one chromosome is transferred to a position in another
chromosome. Requires three breaks.
2. Reciprocal translocation (interchange)
Two non-homologous chromosomes have symmetrically exchanged
segments. One break in each chromosome is sufficient. Nearly always
involves terminal end segments.
segments
I t titi l segmentt = segmentt off an interchange
Interstitial
i t h
chromosome
h
between
b t
the
th breakpoint
b k i t
and the centromere.
Interchromosomal Translocation

A cross configuration is formed at pachytene of interchange
heterozygotes. The position of the cross is a reflection of where the
breakpoint has occurred.

During diplotene and diakinesis, the chromosomes shorten, the chiasma
terminalize, and the cross configuration opens up to form a ring of 4 if
chiasma are present.
a
b
1
c
d
e
f
2
g
h
a
b
12
g
h
e
f
21
c
d
d
c
12
d
b
h
a
g
1
h
b
g
a
c
Pachytene pairing of interchange heterozygote
21
e
f
2
e
f
d
d
1
c
c
21
e
f
b
a
f
a
e
2
b
g
12
g
h
h
Interchromosomal Translocation
Observed meiotic configurations depend on the occurrence of chiasmata
No. of Arms with Chiasma
Diakinesis Configuration
4
Ring of 4 (4)
3
Chain of 4 (IV, 4 types)
2 adjacent arms
Chain of 3 + univalent (III+I, 4 types)
2 alternate arms
2 pairs (2II,
(2II 2 types)
Orientation of interchange heterozygote quadrivalent at Metaphase I
d
c
12
d
b
h
a
g
1
h
b
g
a
c
Adjacent I
21
e
f
2
e
f
Adjacent non
non-homologous
homologous
centromeres pass to the same pole
1 + 21
12 + 2
dp cd + df gh
dp gh + df cd
Orientation of interchange heterozygote quadrivalent at Metaphase I
Adjacent II
a
a
b
b
12
1
h
g
c
d
h
g
c
d
21
2
e
e
Adjacent homologous centromeres
pass to the same pole
f
f
1 +12
21+ 2
dp ab + df ef
dp ef + df ab
Orientation of interchange heterozygote quadrivalent at Metaphase I
d
c
12
d
b
h
a
g
1
h
b
g
a
c
Alternate
21
e
f
2
e
f
Alternate disjunction of non
nonhomologous centromeres
1 +2
12 + 21
Normal
Balanced translocation
Disjunction from a ring quadrivalent
Orientation of chromosomes of a ring of 4 may be either an open or a zig-zag
zig zag
configuration leading to either adjacent or alternate chromosome disjunction.
Adjacent I disjunction
Adjacent but non-homologous centromeres migrate to the same pole.
1+21
12+2
Dp
p fe +Df jjk
Dp jk +Df fe
Dp=duplication
p
p
Df=deficiency
Gametes usually abort.
Adjacent II disjunction
Occurss rarely
Occu
a e y if eve
ever.. Adjacent
djace t but homologous
o o ogous ce
centromeres
t o e es migrate
g ate to the
t e same
sa e pole.
po e.
1+12
21+2
Gametes abort.
Dp abcd +Df ghi
Dp ghi+Df abcd
Disjunction from a ring quadrivalent
Alternate disjunction
Alternate centromeres migrate to the same pole at anaphase I.
1+ 2
12+ 21
Normal chromosome complement
p
Interchange chromosome complement
Both combinations produce viable gametes.
Factors influencing orientation of a ring quadrivalent

Considering 2 normal bivalents, there is complete independence and adjacent I
and alternate disjunction will occur with equal frequency.

Adjacent II should be impossible since there is no opportunity for coorientation between non-homologous centromeres.

With production of quadrivalent co-orientation
co orientation of non-homologous
non homologous centromeres
becomes possible.

With random co-orientation:
alternate
l
disjunction
di j
i frequency
f
= adjacent
dj
disjunction
di j
i frequency
f

Even within species there is considerable genetic variation affecting the ratio of
alternate and adjacent
j
disjunction.
j

In most cases either alternate or adjacent predominates so that co-orientation is
not a reality.

Random orientation may occur in early prophase but soon forces act on
quadrivalent, changing the orientation of the quadrivalent.
Factors influencing orientation of a ring quadrivalent

Forces acting on the quadrivalent:
1. Contraction of chromosomes resulting in stiffness and torsion.
Short stiff chromosomes or those with little tendency for chiasma
terminalization do not have sufficient flexibility for alternate disjunction.
2. Centromere activity
Centromere orientation is maintained by the presence of counter-force
exerted on the centromere

Alternate orientation p
provided more stable counter forces and will not readily
y
revert to adjacent orientation.

With adjacent orientation if the pull from a single opposite centromere lapses,
both co-orienting centromeres become unstable and resume equal probabilities
to orient to either pole.

With time the alternate orientation often accumulates.

In rye interchange heterozygotes, alternate rings may occur in up to 95% of
PMCs in late metaphase.
Factors influencing orientation of a ring quadrivalent
1.
Forces acting on the quadrivalent.
2.
Length of interchange and interstitial segment.
3.
Localization and terminalization of chiasmata.
Genetic consequence of interchange

A iinterchange
An
t h
behaves
b h
like
lik a single
i l genetic
ti factor.
f t

Two reciprocal translocations that do not have a chromosome in common
segregate independently.

In the translocation homozygote, the linkage relationship will be changed.

Genes in the translocated segment fail to show linkage with genes in the
chromosome
h
where
h they
th originally
i i ll occurred.
d
Products of Alternate disjunction
a
b
1
c
d
e
f
2
g
h
a
b
12
g
h
e
f
21
c
d
Normal linkage map
A
Linkage
g map
p from pprogeny
g y of
translocation heterozygote
E
F
B
C
G
D
H
E
F
G
A
H
C
B
D
Id ifi i off chromosomes
Identification
h
involved
i l d in
i interchanges
i
h
1.
Cytology
 Pachytene
y
analysis
y of chromosomes involved in the cross configuration
g
 Karyotype analysis of somatic cells
 Unequal size of exchange segment allows identification of change
in chromosome lengths
 Banding pattern of chromosomes
 Direct observation of Dorsophila salivary gland chromosome
bands
2.
Genetic Linkage
 Genes on one chromosome become linked to those on another
 Genes known to be linked or independent suddenly change relationship
Identification of chromosomes involved in interchanges
1. Use of trisomic tester



2n 1 known trisomic tester lines are crossed with unknown interchange
2n+1
stocks
If one of the chromosomes involved in the translocation is the trisomic
chromosome, a chain of 5 is expected
If th
the ttrisomic
i
i does
d
nott involve
i l one off the
th interchange
i t h
chromosomes,
h
a
ring of 4 plus a trivalent are expected
2 Chromosome identification set
2.

Cross a series of known interchange stocks with the unknown interchange
stock and examine the F1 at meiotic metaphase I
 Two rings of 4 indicate the interchanges are independent
 A ring of 6 indicates one chromosome of the interchange is in common
with one of the tester interchange chromosomes
 An F1 from a cross between interchange
g stocks involvingg the same two
chromosomes will not produce an association larger than a ring of 4 or
may produce mostly/only pairs