Changes within chromosomes
Can occur within a chromosome or
between non-homologous
chromosomes
Types of changes
- deletions
- duplications
- inversions
- translocations
Chromosome deletions
- Loss of a piece of an arm or the lose
of an arm from a chromosome
Types:
- terminal, from the end of a
chromosome
- intercalary or intersitial, from
within the chromosome
Chromosome deletions
Origin of deletions:
- movement of transposible
elements
- radiation
- genetically induced
- product of recombination events
with heterozygous inversions
- product of meiosis with
heterozygous translocations
Movement of transposible elements
- It is speculated that when a
conservative transposible element
moves a section of the chromosome
that the element is leaving can be
lost.
- This loss can either result in a
terminal deletion or a intercalary
deletion
transposible element
reconnection does not occur
terminal piece lost
Radiation:
High energy radiation can cause a
physical break in the chromosome
resulting in a deletion
Deletions caused by radiation can
be either terminal or intercalary
Terminal deletion by radiation
Intercalary deletion by radiation
Protocol for producing deletions by
radiation
- radiate pollen of dominant genotype
- pollinate recessive genotype
- screen progeny for recessive phenotype
- plants showing recessive phenotype
could be the result of a deletion
Genetically inducing deletions
In certain interspecific crosses it was
found that the presence or absence of a
chromosome from one of the species
could induce deletions
The genes that induce the deletions are
called gametocidal genes because the
deletions may lead to non-functional
gametes
Behavior of deletions in meiosis
- reduced chromosome pairing
- as the deletion size increases
pairing frequency decreases
- result is a decrease in the amount
of recombination
- terminal deletions breed true
indicating restoration of telomeres
Determining the location of
chromosomes using deletions
- molecular
- Southern blot
- PCR
Set of deletion lines
A
A
A
a
a
a
gene
expression
Set of deletion lines
location of gene
A
A
A
a
a
a
gene
expression
Southern blot analysis
- presence or absence of a marker
would determine the chromosome
and chromosome location of the
molecular marker
- if the marker was linked to a gene
then the gene location would now be
known
Del 1DS-3
Del 1DS-2
Del 1DS-1
Telo 1DL
Telo 1DS
Nulli 1D
Nulli 1B
Nulli 1A
Disomic
Southern blot results with marker used as
a probe – what is the chromosome
location of the probe?
Del 1DS-3
Del 1DS-2
Del 1DS-1
Telo 1DL
Telo 1DS
Nulli 1D
Nulli 1B
Nulli 1A
Disomic
Gene is on the short arm of chromosome
1D in the region covered by deletion
1DS-3
PCR approach
Have primers for a marker, EST or gene
of interest
Identify chromosome, chromosome
arm and deletions by the absence of
product
PCR results with primers - what is the
chromosome location of the probe?
Del 4BL-3
Del 4BL-2
Del 4BL-1
Telo 4BL
Telo 4BS
Nulli 4D
Nulli 4B
Nulli 4A
Disomic
Del 4BL-3
Del 4BL-2
Del 4BL-1
Telo 4BL
Telo 4BS
Nulli 4D
Nulli 4B
Nulli 4A
Disomic
PCR results with primers indicate the
product is located on chromosome 4BL in
the region of deletion 2.
Chromosome duplications
Regions of a chromosome that are
repeated resulting in an increased
dosage (number) of some genes
example: isochromosome, one arm
duplicated and one arm is missing
A
Normal chromosome
B
A
Isochromosome
A
Types if duplications
- intra-arm where the duplication is
in the same arm
- inter-arm where the duplication is
in the other arm of the chromosome
- non-homologous duplication where
the duplication is on a different
chromosome
Origin of duplications
natural
- oblique synapsis (unequal
crossing-over)
- inversions
- translocations
induced
- radiation
- chemicals
Inversions
A chromosome inversion is a
change of the linear sequence of
genes on a chromosome
Two types of inversions can occur:
- paracentric where the inversion
does not involve the centromere
- pericentric where the inversion
does involve the centromere
C
B
A
D
C
A
B
D
Formation of a paracentric inversion
Formation of a pericentric inversion
An inversion in theory may not be
detrimental since no genes are gained
or lost due to the inversion
Gene function could be affected if the
break occurs in the middle of a gene or
in its promoter region
Gene function could be affected by
moving into or away from a region of
heterochromatin
normal
inverted
In individuals heterozygous for an
inversion recombination is reduced
especially for the genes close to the
breakpoints
This can play a role in speciation by
allowing genes within the inversion to
be inherited as a block of genes
The reason for the low level of
recombination of genes within the
inversion for individuals heterozygous
for the inversion are the problems
caused when crossing-over occurs
Crossing-over within the inversion can
lead to duplication and deletions
In the case of paracentric inversions
such a cross-over can result in
dicentric and acentric chromosomes in
meiosis
Prophase I – paracentric inversion
X
*showing only the chromatids involved in the cross-over
Early anaphase I
acentric
fragment
dicentric
bridge
Late anaphase I
acentric
fragment
lost
dicentric
bridge breaks
Late anaphase I
acentric
fragment
lost
dicentric
bridge breaks
The presence of chromosomes with
duplications and deletions leads to
non-viable gametes
If two of the four chromatids are
involved in the cross-over then 50% of
the gametes will be non-viable
The presence of partial sterility is one
method for identifying individuals that
are heterozygous for an inversion
To get proper pairing of the
complimentary regions a loop structure
has to form
If a cross-over occurs within the looped
area (region with the inversion) then
duplications and deletions can be
produced
• Would we get any acentric fragments in
pericentric (where centromere is
involved in the cross over) inversion ?
Prophase I – pericentric inversion
*showing only the chromatids involved in the cross-over
Late anaphase I
A B C D E
F G H I J
A B C D E
E D C B A
J I H G F
F G H I J
A B C D E
F G H I J
While there are no dicentric bridges or
acentric fragments two of the
chromosomes will have duplications
and deletions
An interesting affect of the inversion is
that genes within the inversion would
be inherited as a block of genes
because only chromatids not involved
in the cross-over would result in a
viable gamete
The inheritance of blocks of genes and
the partial sterility found in individuals
heterozygous for the inversion can lead
to speciation
For speciation to occur it would require
the presence of several inversions
With each additional inversion the
probability of viable gametes in a
hybrid between a normal by an
inversion genotype decreases while
fertility of individuals having two
normal or two inversion parents stays
at 100%
Combine the increased sterility with the
inheritance of blocks of genes and
differences between the two genotypes
becomes great enough that they can
not successfully mate and produce
fertile hybrids (i.e. become two species)
An example of this is tomato and
diploid potato
Based on molecular marker studies it
was shown that the genes for the two
species were generally on the same
chromosomes but the gene order was
inverted on several of the chromosomes
when the two species were compared