chapter 8 human karyotypes and chromosome behavior jones and bartlett publishers © 2005
TRANSCRIPT
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
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
A crossover within an inversion loop of a pericentric inversion creates chromosomes
with deletion and duplication
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.