Download Chromosome Mutations - Circle

Variation in
Chromosome Number
and Arrangement
Timothy G. Standish, Ph. D.
©2000 Timothy G. Standish
The Modern Synthesis
Charles Darwin recognized that variation existed
in populations and suggested natural selection as
a mechanism for choosing some variants over
others resulting in survival of the fittest and
gradual changes in populations of organisms.
Without a mechanism for generation of new
variation, populations would be selected into a
corner where only one variation would survive
and new species could never arise.
The Modern Synthesis combines the mechanism
of mutation in DNA to generate variation with
natural selection to produce new species.
©2000 Timothy G. Standish
Mutation
Mutation = Change
Biologists use the term “mutation” when talking
about any change in the genetic material. Not all
result in a change in phenotype.
There are two major types of mutations:
Macromutations - Also called macrolesions and
chromosomal aberrations. Involve changes in
large amounts of DNA.
Micromutations - Commonly called point
mutations and microlesions.
©2000 Timothy G. Standish
Macromutations
Six major types of Macromutations in two
categories are recognized:
A Change within or between chromosomes
1
2
3
Deletions - Loss of chromosome sections
Duplications - Duplication of chromosome sections
Inversions - Flipping of parts of chromosomes
4 Translocations - Movement of one part of a
chromosome to another part
B Changes in Chromosome number
5 Polyploidy - Addition of whole sets of chromosomes
6 Anuploidy - Addition or subtraction of chromosomes,
but not whole sets
©2000 Timothy G. Standish
1 Deletion
Chromosome
Centromere
Genes
A
B
C
D
E
F
A
B
C
D
G
H
G
H
E
F
©2000 Timothy G. Standish
Deletions Can Be Seen
During Pairing of Homologous
Giant Chromosomes
A
B
C
D
E
F
A
B
C
D
G
H
G
H
©2000 Timothy G. Standish
Deletions Can Be Seen
During Pairing of Homologous
Giant Chromosomes
Compensation loop
E
A
A
B
B
C
C
D
D
F
G
G
H
H
©2000 Timothy G. Standish
Production of Deletions
Two types of deletions occur:
1. Terminal deletions
A
B
C
D
E
F
A
B
C
D
E
F
G
H
G
H
2. Intercalary deletions
A
B
C
D
E
F
H
H
A
B
C
F E
D
G
A
B
C
D
G
©2000 Timothy G. Standish
Impact of Deletions
Large deletions are generally lethal
In a very general way, the larger the deletion, the larger the
impact
Deletion of genes essential for survival is always lethal
Pseudodominance, in which typically recessive alleles
behave in a dominant fashion may result from deletions
due to short stretches of chromosomes which become
hemizygous due to a deletion in one homolog
The Notch mutation, deletion of a short stretch of the
Drosophila X chromosome, provides an example of this
When the notch deletion is on one X chromosome, white,
facet and split alleles on the other X chromosome behave
in a dominant fashion
©2000 Timothy G. Standish
Cri-du-Chat Syndrome
The cry of the cat
Cri-du-chat syndrome results from deletion of
approximately 1/2 the human chromosome 5 p arm
It is one of very few human deletions known to
survive to live birth
The larger the deletion the more severe the
symptoms which include:
– Gastrointestinal and cardiac malformation
– Mental retardation
– Abnormal development of the glottis and larynx resulting
in a cry resembling that of a mewing cat
Incidence of about 1/50,000 live births
©2000 Timothy G. Standish
2 Duplication
Chromosome
Centromere
Genes
A
B
C
D
E
F
G
H
A
B
C
D
E
F
EE
FF
G
H
Duplication
©2000 Timothy G. Standish
Duplication Also Produces
Compensation Loops
A
B
C
D
E
F
G
H
A
B
C
D
E
F
E
F
G
H
©2000 Timothy G. Standish
Duplication Also Produces
Compensation Loops
A
A
B
B
C
C
D
D
E
E
F
F
E
G
G
F
H
H
Compensation loop
©2000 Timothy G. Standish
Production of Duplications
Duplications are thought to be the result of
unequal crossing over
A
B
C
A
A
B
C
A
A
B
C
A
D
B
D
B
D
B
E
C
E
C
E
C
F
G
H
D
E
F
F
G
H
D
E
F
D
G
H
F
G
H
E
F
G
H
G
H
©2000 Timothy G. Standish
Impact of Duplications
More copies of a gene results in more of
the gene product of the gene
This can result in dosage problems
Position effects may also have an
influence on the way gene duplication
impacts organisms
©2000 Timothy G. Standish
3 Inversion
Chromosome
Centromere
Genes
A
B
C
D
E
F
A
B
C
D
F
E
Inversion
G
H
G
H
©2000 Timothy G. Standish
4 Translocation
Chromosome
Centromere
A
B
C
A
B
E
Genes
D
F
E
C
F
G
H
D
G
H
©2000 Timothy G. Standish
Does Speciation Occur?
Yes
There are a number of well
documented examples of
speciation
A good example is the London
Plain Tree:
©2000 Timothy G. Standish
Normal Chromosome
Numbers In Selected Species
Organism
Chromosomes
Homo sapiens
46
Cat
38
Rat
42
Hen
78
Drosophila
8
Hydra
30
Corn
20
Peas
14
E. coli
1
©2000 Timothy G. Standish
American
Planetree
or
Sycamore
Platanus
occidentalis
©2000 Timothy G. Standish
The London Planetree
Oxford Botanical
Gardens
c1670
North
America
Platanus
occidentalis
l
l
Platanus
acerifolia
Europe
>5,000 km
50,000,000 Years
Atlantic
Platanus orientalis
Africa
Speciation in the production of P. acerifolia was the result
of hybridization, not mutation and selection
If they can interbreed, are P. occidentalis and P. orientalis
really separate species?
©2000 Timothy G. Standish
In His Own Words
"I may, of course, be egregiously
wrong; but I cannot persuade
myself that a theory which
explains several large classes of
facts can be wholly wrong...."
Charles Darwin, November 13, 1859, in letter
to L. Jenyns
©2000 Timothy G. Standish
Has Natural Selection Been Shown
To Produce New Species?
No
Darwin emphasized the slow gradual change that is
necessary if natural selection is to work on
changing a population gradually from one species
to another
“Many large groups of facts are intelligible only on
the principle that species have been evolved by
very small steps.”
– The Origin of Species Chapter VII under
“Reasons for disbelieving in great and abrupt
modifications”
©2000 Timothy G. Standish
©2000 Timothy G. Standish