Transcript Reciprocal Translocation

Types of Chromosome Mutations
Chromosome Mutations
Deletion/
Deficiency
Duplication
Inversion
A B C
D E F
A C
D E F
A B C
D E F
A B B C
A B C
D E F
A E D
C B F
A B C
D E F
A B C
D J K
G H
J K
G H
E F
D E F
Translocation
I
I
Inversions
A
B
C
D
E
F
G
H
A
B
F
E
D
C
G
H
F
H
pericentric inversion
A
B
C
D
E
F
G
H
A
B
C
D
E
G
paracentric inversion
A heterozygote for a normal chromosome and an inversion will form an
inversion loop during meiosis.
The number of recombinant products is reduced in inversion
heterozygotes by:
1) elimination of crossing over products within the inversion loop, and
2) inhibition of pairing between homologues in the region of the
inversion.
Pairing in paracentric
inversion heterozygotes
and resulting meiotic
products.
Anaphase bridge results in
random breakage of
chromosomal material.
2 of 4 meiotic products are
not genetically balanced
and will not produce viable
gametes.
Pairing in pericentric
inversion heterozygotes
and resulting meiotic
products.
Possible effects of
inversion at the
molecular level
No disruption of any
gene.
Chromosomal
rearrangement is the
only result.
Disruption of one gene
by chromosomal
breakage.
Disruption of two
genes and fusion of
those two genes.
Chromosome Mutations
Deletion/
Deficiency
Duplication
Inversion
A B C
D E F
A C
D E F
A B C
D E F
A B B C
A B C
D E F
A E D
C B F
A B C
D E F
A B C
D J K
G H
J K
G H
E F
D E F
Translocation
I
I
Translocation
In reciprocal translocation, exchange of chromosomal segments between
two nonhomologous chromosomes establishes new linkage groups.
A B C
D E F
G H
G C
D E F
A B H
In Robertsonian translocation, long arms of two acrocentric chromosomes
are combined to form one large chromosome and one small chromosome.
If the short metacentric chromosome does not contain essential genetic
information, it could be lost without any consequence to viability.
Reciprocal Translocation
Reciprocal translocation
heterozygotes are
semisterile.
50% of gametes are
genetically unbalanced.
In plants, these gametes
are not viable.
In animals, zygotes that
are formed by these
gametes are not viable.
Adjacent segregation
produces genetically
unbalanced gametes.
Alternate segregation
produces genetically
balanced gametes.
Down Syndrome and Translocation Heterozygote
• Down syndrome is caused by trisomy 21 (3 copies of
chromosome 21).
• 95% of Down syndrome cases are associated with
nondisjunction and shows no familial recurrence.
• The other 5% (familial Down syndrome) is attributed
to Robertsonian translocation between chromosome
21 and chromosome 14.
Chromatin and Gene Expression
Heterochromatin
•
Contains methylated histones (H3)
•
Associated with heterochromatin protein-1 (HP-1)
Transcriptionally Active Euchromatin
•
Contains hyperacetylated histones
Prevention of Heterochromatin Formation
•
DNA elements (barrier insulators) promote binding of
histone acteyltransferase
Gene Silencing is Caused by the Spread of Heterochromatin
When a chromosome mutation places a gene next to
heterochromatin, the gene can become inactivated.
Inversion, deletion, duplication, and translocation
can place a gene next to heterochromatin.
Heterochromatin May Spread Farther in Some Cells Than in Others
Position-effect Variegation
A heterozygote for a gene and a
translocation can show variegated
phenotype for that gene.
Position-effect variegation is exhibited
by this w+/w heterozygote.
Wild-type allele is no longer wild-type
in its expression in some of the eye
facets.
Any chromosomal change that places
a locus next to heterochromatin can
result in inactivation of that gene.
A tissue or organ that is comprised of
a mixture of cells that express one or
the other phenotype exhibit this
variegation.