Transcript second of three for Chapter 8

Chapter 8
Human
Karyotypes and
Chromosome
Behavior
Jones and Bartlett Publishers © 2005
Changes in chromosome structure
- deletions
- duplications
- inversions
- translocations
Deletions/Deficiencies
Interstitial deficiency
Terminal deficiency
Terminal
Deficiencies
 unstable
chromosomes
Haplo-insufficient genes
• Heterozygotes for a deficiency may have a
mutant phenotype.
• If two working copies of a gene are required
for normal gene function, only havinog one
is a problem.
• ‘Cri du Chat’ syndrome in humans:
– Deficiency of a part of chromosome 5
Mapping the deletion of part of a chromosome segment
by testcrosses and uncovering of recessive genes
Mapping of genes in Drosophila using overlapping
deletions and polytene chromosomes
Duplications
• Mutations can produce an extra copy of a
part of a chromosome called a duplication.
• Tandem duplication – same sequence,
adjacent to the original copy.
• Reverse tandem duplication – opposite
orientation.
• Displaced duplication
• Small free chromosome
Unequal crossing over of misaligned repeat
sequences leads to gain or loss of repeats
Unequal crossing over involving eye
pigment genes
Inversions
Mechanism of creation of a chromosomes
with an inverted segment
Pairing of homologous chromosomes
in an inversion heterozygote
An inversion
which does not
involve the
centromere
is called a
paracentric
inversion
Synapsis between
chromosomes
bearing inversions
requires the
formation of an
Inversion Loop
If there is no
crossing over
inside the
loop 
chromosomes
disjoin
normally
Crossing over in
an inversion loop
for a pericentric
inversion 
duplications and
deletions
Crossing over in an inversion loop for a paracentric
inversion  messed-up chromosomes
Absence of recombination within an inversion
loop does not create deletions or duplications
A crossover within an inversion loop of a paracentric
inversion creates dicentric and acentric chromosomes
A crossover within an inversion loop of a
pericentric inversion creates chromosomes
with deletion and duplication
When an inversion
involves the
centromere, it is called
a pericentric inversion.
Crossing over in a
pericentric inversion
does not create
dicentric and acentric
chromosomes
Translocations
• Broken chromosomes can reattach to
different chromosomes that also were
fragmented.
• Exchanges between nonhomologous
chromosomes are called translocations.
• Reciprocal translocations – distal portions
of two chromosomes are exchanged.
• Transposition – insertion into another
chromosome.
Structure of chromosomes with a
reciprocal translocation
Pairing and segregation of chromosomes with a
reciprocal translocation during meiosis I
Robertsonian translocation
• Exchange of entire arms between
chromosomes.
• This can result in change of chromosome
number in a monoploid set.
• Can occur by fusion of two acrocentric
chromosomes, or a split in a metacentric
into two.
• Robertsonian translocations can result in
barriers, speciation.
Mechanism of creation of a
Robertsonian translocation
Pairing and segregation with a Robertsonian
translocation involving human chromosomes 14 and 21
Such a
translocation
results in a high
probability of
having a child
with Down
syndrome.
Variegation (mottling) of eye color due to positioning of
the eye color gene near centromeric heterochromatin
When the
expression of a
gene is affected by
its location on a
chromosome (even
though the gene
itself is not
changed), such a
variation is called
“position effect”
Two kinds of polyploidy
Multiplication of the entire chromosome complement is called
polyploidy. When all the genomes are the same, it is called
autopolyploidy. When two (or more) different genomes
are duplicated, it is called allopolyploidy.
Formation of a tetraploid organism
Creation of a totally homozygous diploid cell by
doubling of chromosome number in a
monoploid cell by colchicine
Monoploid cells
can only be
grown in plants.
In humans, the
only viable
monoploid cells
are the egg and
the sperm.
Monoploidy in
somatic cells is
lethal.