Download Set 2: Mutations

Presentation MEDIA: Genetics & Evolution Series
Mutations
Set No. 2
Set 2: Mutations
Presentation MEDIA
© 1993-2001 Biozone International Ltd
ISBN 0-909031-41-X
Index to OHT Titles
OHT Title
OHT Title
1
Mutations
29
Duplication on Human Chromosome 9
2
Causes of Mutations
30
Aneuploidy
3
Effects of Mutagens
31
Down Syndrome
4
Rates of Mutation
32
Causes of Down Syndrome
5
Human Mutation Rates
33
Down Syndrome Phenotype
6
Location of Mutations
34
Patau Syndrome
7
The Effects of Mutations
35
Patau Syndrome Phenotype
8
Types of Mutations
36
Edward Syndrome
9
Single Gene Mutations
37
Edward Syndrome Phenotype
10
Point Mutations: Missense Substitution
38
Maternal Age Effect in Aneuploidy
11
Point Mutations: Nonsense Substitution
39
Causes of Maternal Age Effect
12
Point Mutations: Reading Frame Shift by Insertion
40
The Fate of Conceptions
13
Point Mutations: Partial Reading Frame Shift
41
Aneuploidy in Human Sex Chromosomes
14
Tautomerism
42
Human Sex Aneuploidy Phenotypes
15
Sickle Cell Disease
43
Faulty Sperm Production
16
Sickle Cell Mutation
44
Faulty Egg Production
17
Cystic Fibrosis
45
Klinefelter Syndrome
18
Cystic Fibrosis Mutation
46
Turner Syndrome
19
ß-Thalassaemia
47
Polyploidy
20
Huntington Disease
48
Polyploidy in Humans
21
Block Mutations
49
Autopolyploidy
22
Block Mutations: Deletion
50
Allopolyploidy
23
Deletion on Human Chromosome 1
51
The Evolution of Wheat
24
Block Mutations: Translocation
52
Mutations: Overview
25
Translocation on Human Chromosomes 9 & 22
53
Evolutionary Significance of Mutations
26
Block mutations: Inversion
27
Inversion on Human Chromosome 2
28
Block Mutations: Duplication
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Mutations
Mutations are alterations in the DNA of chromosomes.
Many mutations may be neutral or 'silent' (i.e. they have
no observable effect on the organism).
Harmful mutations become evident because they may
alter the survival capacity of the organism.
Cell
Mutations can alter
the cell’s chemistry
Chromosome
Mutation
Nucleus
This may cause an
observable change
in the organism’s:
• physiology
• anatomy
• behaviour
Set 2: Mutations
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OHT 1
Causes of Mutations
Mutations may occur randomly and spontaneously.
They may also be induced by environmental factors.
Spontaneous Mutations
– Arise from errors in replication
– Different genes mutate at different rates
Induced Mutations
Mutations can be induced by mutagens (environmental
factors that cause a change in DNA):
Examples: –
–
–
–
–
radiation (e.g. UV rays)
viruses
microorganisms
Environmental poisons and irritants
Alcohol and diet
The Effect of Mutagens on DNA
UV Light
Thymine dimer
After exposure to UV light, a potent
mutagen, adjacent thymine bases
in DNA become cross-linked to
form a 'thymine dimer'.
DNA of tumour
suppressor gene
This disrupts the normal base
pairing and throws the controlling
gene's instructions into chaos.
Set 2: Mutations
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OHT 2
Effects of Mutagens
Type of Mutagen
Effects
Nuclear and ultraviolet
radiation, X-rays and gamma
rays. Ionising radiation is
associated with the
development of cancers.
Ionising Radiation
Those Most at Risk
• Those working with
radioisotopes.
• Living near nuclear plants, waste
dumps, or testing sites.
• Fair skinned people in tropical
regions and sub tropical areas.
• Excess use of tanning beds.
Some viruses integrate into the
human chromosome, upsetting
genes and triggering cancers.
• Hepatitis B: Intravenous drug
users.
Many chemicals are
mutagenic. Synthetic and
natural examples include:
organic solvents (e.g.
benzene), asbestos, tobacco
tar, vinyl chlorides, and nitrites.
• Chemical industry workers,
including the glue, paint,
rubber, resin, and leather
industries.
Viruses and
Microorganisms
• HIV: Intravenous drug users,
those with unsafe sexual
activity (i.e. unprotected sex
with new partners).
• Smokers.
• Coal and other mining workers.
Environmental
Poisons
• Exposure to petroleum volatiles
and vehicle exhaust emissions.
Diets high in fat, slow the
passage of food through the
gut giving time for mutagenic
irritants to form. High alcohol
intake increases the risk of
some cancers.
Alcohol and Diet
• Those with a diet high in total
fats.
• There may be familial
(inherited) susceptibility.
• Risks may be compounded by
other lifestyle choices e.g.
smoking.
Set 2: Mutations
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OHT 3
Rates of Mutation
Genes mutate at known rates, but the rate varies
depending on the gene involved - some genes have
high spontaneous mutation rates.
Calculation of the average number of mutant genes in
a human:
1. There are thought to be about 100,000 genes
making up the human genome.
2. Since there are two copies of each gene (on
homologous chromosomes), each cell has a total
of 200,000 genes.
3. In higher organisms, a mutation for a specific
gene will occur in one gamete in 300,000.
4. Then each of us on average:
carries about 1 new mutant gene!
2 x 105 ÷ 3 x 10-5 = 0.67
Rates of mutation for different genes within a single
species are probably similar, but the viability of
mutations varies greatly.
Set 2: Mutations
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OHT 4
Human Mutation Rates
Examples of human genes with known mutation rates
are listed below:
mutations per
million gametes
per generation
Retinoblastoma
Dominant
15-23
Produces a tumour in the eye
Tay-Sachs disease
Recessive
11
Sex-linked
25-32
Sex-linked
43-100
Recessive
28
Produces blindness, paralysis,
mental deficiency, death, with
onset at about 6 years of age
Haemophilia
Produces uncontrollable
bleeding due to an inability
of the blood to clot
Muscular Dystrophy
Produces progressive
wasting of muscles and
eventual death
Albinism
Production of melanin
affected, resulting in lack of
pigment in skin, eyes, and hair
All genes causing death
before early adulthood:
40,000
Set 2: Mutations
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OHT 5
aa
Location of Mutations
The location of a mutation determines whether or not it
will be inherited.
Most mutations occur in somatic (body) cells and are
not inherited (not involved in reproduction).
Gametic mutations occur in the cells of gonads (sperm
and eggs) and may be inherited.
Gametic Mutations
Sperm
Somatic Mutations
Mutation
Sperm
Egg
Egg
Fertilisation
Cleavage.
Prior to implantation
Mutation
Foetus
Baby
Cells of tissues
affected by the
mutation
Gametic Mutations are inherited
and occur in the testes of males
and the ovaries of females
Somatic Mutations occur in body
cells – they are not inherited but may
affect the person during their lifetime
Set 2: Mutations
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OHT 6
The Effects of Mutations
Harmful Mutations: There are many examples of harmful
mutations that result from alterations to the DNA base sequence.
Examples include:
– Sickle-cell disease
– Cystic fibrosis
– Thalassemias
These mutations are harmful because they alter the DNA
sequence, thereby upsetting the structure and function of the
protein they code for.
Neutral Mutations: Because these often produce little or no
change in the phenotype, neutral mutations are hard to detect.
They may have little or no effect on the survival of an
organism or its ability to reproduce.
May be the result of a ‘same-sense’ mutation where a
change in the third base sequence still codes for the same
amino acid.
Beneficial Mutations: Beneficial mutations are best observed
in species with short generation times.
Examples include:
– Bacterial resistance to antibiotics.
– Insecticide (e.g. DDT) resistance in insect pests.
– Rapid mutation rates in the protein coats of viruses.
Set 2: Mutations
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OHT 7
Types of Mutations
Single Gene Mutations
Genetic change affecting
the base sequence of a
single gene.
Mutation: Substitute T instead of C
Original
DNA
May result in the
formation of a new allele.
Mutant
DNA
Chromosome Rearrangements
(Block mutations)
Chromosome
segment is lost
Change in the structure of a
chromosome involving large
pieces being rearranged.
Whole groups of genes are
affected.
C D E F
A B
G H
Break Break
M N O P Q R S T
Genes
Changes in Chromosome Number
Aneuploidy: Loss or gain of
whole chromosomes.
Polyploidy: Loss or gain of
complete sets of chromosomes.
21
Set 2: Mutations
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OHT 8
Single Gene Mutations
Point mutations change the sequence of bases in DNA
for a single gene and may produce a new allele of a gene.
Single gene mutations involving a single nucleotide are
usually called point mutations.
The new DNA sequence will result in a new sequence of
amino acids making up a protein.
Because of the degeneracy in the genetic code not all
changes in a DNA sequence will result in a new sequence
of amino acids.
Even with a change in amino acid sequence, protein
function may not be affected.
Original Unaltered Code
Original
DNA
Transcription
mRNA
Translation
Amino
Acids
Phe
Tyr
Glu
Glu
Val
Leu
Amino acid sequence forms a normal polypeptide chain
Set 2: Mutations
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OHT 9
Single Gene Mutations:
Missense Substitution
A single base is substituted by another.
This usually results in coding for a new amino acid in
the polypeptide chain.
If the third base in a triplet had been substituted, the
resulting amino acid may not be altered (due to
degeneracy in the code).
Mutation: Substitute T instead of C
Original
DNA
Mutant
DNA
mRNA
Amino
Acids
Phe
Tyr
Lys
Glu
Val
Leu
Polypeptide chain with wrong amino acid
Set 2: Mutations
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OHT 10
Single Gene Mutations:
Nonsense Substitution
A single base is substituted by another.
This results in a new triplet that does not code for an
amino acid.
The resulting triplet may be an instruction to terminate
the synthesis of the polypeptide chain.
Mutation: Substitute A instead of C
Original
DNA
Mutant
DNA
mRNA
Amino
Acids
Phe
Tyr
Mutated DNA creates a STOP codon which
prematurely ends synthesis of the polypeptide chain
Set 2: Mutations
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OHT 11
Single Gene Mutations:
Reading Frame Shift by Insertion
A single base is inserted, upsetting the reading
sequence for all those after it.
This results in new amino acids in the polypeptide chain
from the point of insertion onwards.
The resulting protein will be grossly different from the
one originally coded for (therefore non-functional).
Mutation: Insertion of C
Original
DNA
Mutant
DNA
mRNA
Amino
Acids
Phe
Tyr
Gly
Arg
Gly
Ser
Large scale frame shift resulting in a completely new sequence of amino
acids – the resulting protein is unlikely to have any biological activity
This type of reading frame shift can also be caused
by a base deletion.
Set 2: Mutations
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OHT 12
Single Gene Mutations:
Partial Reading Frame Shift
A single base is inserted and another is deleted at a
different location, resulting in a localised frame shift.
This results in a new amino acid sequence between
these points in the polypeptide chain.
Depending on how many amino acids are affected, the
resulting protein may have some useful function
(biological activity).
Mutation: Insertion of C
Mutation: Deletion of C
Original
DNA
Mutant
DNA
mRNA
Amino
Acids
Phe
Val
Arg
Lys
Val
Leu
Altered chain which may or may not produce a protein with biological activity
Set 2: Mutations
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OHT 13
Tautomerism
Some point mutations may result from bases with an
unusual number of hydrogen-bonding sites.
Results in mismatching of base pairs.
Irregular base configurations are called tautomers and
are indicated by abnormal base combinations below:
Usual base
combinations
Normal
partner
Normal
base
C
Guanine
A
T
Thymine
A
Abnormal
base
C
G
Cytosine
Abnormal base
combinations
A
Abnormal
Cytosine
Adenine
G
T
Adenine
Abnormal
Thymine
T
A
Adenine Thymine
Abnormal
partner
Abnormal
Adenine
Guanine
C
Cytosine
Hydrogen bonds
G
C
Guanine Cytosine
T
G
Abnormal
Guanine
Thymine
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OHT 14
Sickle Cell Disease
Synonym: Sickle cell anaemia
Incidence: Most common in people of African ancestry.
West Africans:
1% (10-45% are carriers)
West Indians:
0.5%
Gene Type: Autosomal recessive mutation which results
in the substitution of a single nucleotide in the HBB gene
that codes for the beta chain of haemoglobin.
Gene Location: Chromosome 11
HBB
p
q
Symptoms:
Include the following:
• Pain, ranging from mild to severe, in
the chest, joints, back, or abdomen
• Swollen hands and feet
• Jaundice
• Repeated infections, particularly
pneumonia or meningitis
• Kidney failure
• Gallstones (at an early age)
• Strokes (at an early age)
• Anaemia.
Set 2: Mutations
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OHT 15
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Sickle Cell Mutation
The mutation responsible for causing sickle cell disease is a
point substitution mutation.
Set 2: Mutations
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Haemoglobin molecules
are made up of 2 α-chains
and 2 β-chains linked together
Normal Red Blood Cells
containing normal
haemoglobin (soluble)
Beta (β) chain
Alpha (α) chain
Haemoglobin clustered together to
form fibres that deform the red blood
cells into a sickle shape
β-Chain
Haemoglobin
Chromosome 11
Sickle Cells
containing mutant
haemoglobin (less soluble)
HBB
gene
p
OHT 16
First base
q
DNA
Codes for the 1st amino acid
Normal base: T
Substituted base: A
The sickle cell mutation
involves the substitution of
one base for another in the
HBB gene, causing a single
amino acid to be altered.
Cystic Fibrosis
Synonyms: Mucoviscidosis, CF
Incidence: Varies with populations:
Asians: 1 in 10,000; Caucasians: 1 in 20-28 are carriers
Gene Type: Autosomal recessive. Over 500 different
recessive mutations (deletions, missense, nonsense,
terminator codon) of the CFTR gene have been identified.
Gene Location: Chromosome 7
CFTR
p
q
Symptoms:
• infertility occurs in males and
females.
Disruption of the following glands:
• the pancreas
• intestinal glands
• biliary tree (biliary cirrhosis)
• bronchial glands (chronic lung
infections)
• sweat glands (high salt content
of which becomes depleted in a
hot environment)
Set 2: Mutations
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OHT 17
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Cystic Fibrosis Mutation
The mutation responsible for causing most cases of
cystic fibrosis is a single gene deletion mutation.
Chromosome 7
Set 2: Mutations
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Base 1630
p
DNA
The 508th triplet is lost (not
present) in the mutant form
This triplet codes for
the 500th amino acid
q
CFTR
gene
CFTR Protein
Cl-
Cl-
Water
ClClOutside
the cell
Outside
the cell
OHT 18
Cell
Membrane
Cell
cytoplasm
Cell
cytoplasm
Cl-
Cl-
Cl- ClCl- Cl- Cl-
Normal CFTR Protein
Mutant CFTR Protein
correctly controls chloride ion
balance in the cell
allows chloride ions to remain in the cell
and leads to water entering the cell
ß-Thalassaemia
Synonyms: Cooley anaemia, Mediterranean anaemia
Incidence: Most common type of thalassaemia
affecting 1% of some populations. More common in
Asia, Middle East and Mediterranean.
Gene Type: Autosomal recessive mutation of the HBB
gene coding for the haemoglobin beta chain.
It may arise through a gene deletion or a nucleotide
deletion or insertion.
Gene Location: Chromosome 11
HBB
p
q
Symptoms:
• The result of haemoglobin with
few or no beta chains, causes a
severe anaemia during the first
few years of life.
• People with this condition are
tired and pale because not
enough oxygen reaches the cells.
Set 2: Mutations
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OHT 19
Huntington Disease
Synonym: Huntington’s chorea, HD (abbreviated)
Incidence: An uncommon genetic disease present in 1 in
20,000 people.
Gene: An autosomal dominant mutation of the HD gene
(IT15) caused by an increase in the length (36-125) of a
CAG repeat region (normal range is 11-30 repeats).
Gene Location: Chromosome 4
IT15
p
q
Symptoms:
• Mutant gene forms defective
protein: Huntingtin.
• Progressive, selective nerve cell
death associated with chorea
(jerky, involuntary movements)
• Psychiatric disorders
• Dementia (memory loss,
disorientation, impaired ability to
reason, and personality changes).
Set 2: Mutations
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OHT 20
Block Mutations
Causes of Block Mutations
Some can result from errors in the crossing over
process during meiosis.
Mutagens (e.g. X-rays) may cause some forms of
block mutation.
Types
Fate of Chromosome Fragments
Inversion
Pieces of chromosome are flipped upside
down so the genes appear in the reverse order.
Translocation
Pieces of chromosome are moved from one
chromosome into another.
Duplication
Pieces of chromosome are repeated so there
are duplicate segments.
Deletion
Pieces of chromosome are lost.
Block mutations cause genetic imbalances that usually
disrupt the development of an organism.
Set 2: Mutations
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OHT 21
Block Mutations: Deletion
Break occurs at two points on the chromosome
and the middle piece falls out.
The two ends then rejoin to form a chromosome
deficient in some genes.
Alternatively, the end of a chromosome may
break off and be lost.
Break
Break
Genes
A
B
C
D
E
F
G
H
G
H
M
N
O
P
Q
R
S
T
M
N
O
P
Q
R
S
T
A
B
Step 1
Chromosome
rejoins
A
B
G
H
C
D
E
F
M
N
O
P
Q
R
S
T
Segment
is lost
Step 2
Step 3
Set 2: Mutations
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OHT 22
Deletion on
Human Chromosome 1
Human chromosome 1 shows two forms of deletion.
These may involve deletion of either a chromosome tip
(left) or a middle segment with the tip rejoined (right).
This loss of genetic material may be harmful.
Tip Deletion
Before
After
Mid-Segment Deletion
Before
After
Tip
rejoins
1
1
Lost
1
Lost
1
Set 2: Mutations
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OHT 23
Block Mutations: Translocation
Translocation involves the movement of a group of
genes between different chromosomes.
A piece of one chromosome breaks off and joins
onto another chromosome.
Consequence: A chromosome deficient in genes.
Segment
Removed
Break
Genes
A
B
C
D
E
F
G
H
M
N
O
P
Q
R
S
T
1
2
3
4
5
6
7
8
9
0
Step 1
A
B
C
D
E
F
G
H
M
N
O
P
Q
R
S
T
1
2
3
4
5
6
7
8
9
0
Step 2
G
H
M
N
O
P
Q
R
S
T
A
B
C
D
E
F
1
2
3
4
Segments
join
5
6
7
8
9
0
Step 3
The large chromosome (green) and the small chromosome (dark blue) are not homologous
Set 2: Mutations
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O ve r h e a d Tr a n s p a r e n c i e s ( O H T s ) .
OHT 24
Translocation on
Human Chromosomes 9 & 22
Translocation can occur between human
chromosomes 9 and 22.
The tips of the two chromosomes are exchanged.
Before Translocation
After Translocation
22
22
The tips of the
chromosomes swap
9
9
This is the translocation observed in chronic
myeloid leukaemia.
Set 2: Mutations
Produced by:
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© 1993 – 2001
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OHT 25
Block Mutations: Inversion
The middle piece of the chromosome falls out
and rotates through 180° and then rejoins.
There is no loss of genetic material.
Break
Break
Genes
A
B
C
D
E
F
G
H
G
H
M
N
O
P
Q
R
S
T
M
N
O
P
Q
R
S
T
A
B
A
B
F
E
D
C
G
H
C
D
E
F
Step 1
Segment
Rotates 180°
Step 2
Segment
rejoins
M
N
O
P
Q
R
S
T
Step 3
Set 2: Mutations
Produced by:
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© 1993 – 2001
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OHT 26
Inversion on
Human Chromosome 2
A segment of chromosome 2 is inverted
(caused by looping of the chromosome).
Normal
Inversion
Flip
2
2
Set 2: Mutations
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OHT 27
Block Mutations: Duplication
A segment is lost from one chromosome and is
added to its homologue.
The chromosome on the left (below) was the
'donor' of the duplicated piece of chromosome.
The chromosome with the duplication will become
incorporated into a gamete, which may later
contribute to an embryo.
A
B
C
D
E
F
A
B
C
D
E
F
F
A
B
C
D
E
A
B
C
D
E
F
M
N
O
P
Q
M
N
O
P
Q
M
N
O
P
Q
M
N
O
P
Q
Segment
removed
Break
Genes
A
B
C
D
E
F
A
B
C
D
E
F
M
N
O
P
Q
M
N
O
P
Q
Step 1
Joins onto
Homologous
chromosome
Step 2
Step 3
Set 2: Mutations
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OHT 28
Duplication on
Human Chromosome 9
A segment of chromosome 9 is duplicated.
A segment is taken from its homologue and
inserted to produce double copies of some genes.
Some genes may be disrupted by this process.
Normal
Duplication
Duplicate
segment
A segment is tansferred
from one chromosome
into its homologue
Identical
segment
9
9
Set 2: Mutations
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OHT 29
Aneuploidy
The normal condition for a body cell is for
chromosomes to be present as homologous pairs
(a condition known as disomy).
Aneuploidy is a condition where one or more
chromosomes are missing from or added to the
normal body cell chromosome number.
Examples:
Nullisomy
0 homologues
Monosomy
1 homologue
Trisomy
3 homologues
Tetrasomy
4 homologues
May involve autosomes – examples are:
Down Syndrome:
Edward Syndrome:
Patau Syndrome:
Chromosome 21
Chromosome 18
Chromosome 13
May involve sex chromosomes – examples are:
Klinefelter Syndrome:
XXY
Turner Syndrome:
XO
Why are they called syndromes?
When a disease causes multiple effects it is called
a syndrome (virtually all chromosome
abnormalities are in this category).
Set 2: Mutations
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OHT 30
Down Syndrome
Chromosome affected: Trisomy 21
Incidence rate of 1 in 800 births in women giving birth
at 30 to 31 years of age (this is the most common
form of aneuploidy in humans).
ArtToday.com
The young boy below shows the typical appearance
of Down Syndrome:
Set 2: Mutations
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OHT 31
Causes of Down Syndrome
There are three causes of Down syndrome, each
producing a different severity of the syndrome:
92% of all cases result from non-disjunction of
chromosome 21 during meiosis (see the
karyotype shown below).
5%
result from translocation of chromosome 21
(usually onto chromosome 14).
3%
arise from a failure during mitosis (nondisjunction of chromosome 21) in a cell of a
very early embryo – the resulting individual is a
‘mosaic’ of normal and Down syndrome cells.
Set 2: Mutations
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OHT 32
Down Syndrome Phenotype
The expression of traits in the Down syndrome
phenotype varies greatly, depending on which of the
three chromosome abnormalities caused it.
The most common phenotypic traits are:
1. Skin fold over the eye.
2. Reduced mental capacity (varies greatly).
3. Short stature, stubby fingers, heart defects.
Small and arched palate
Big wrinkled tongue
Slanting eyes
Dental anomalies
Epicanthic eyefold
Short and broad hands
Back of head flat
Broad flat face
Short nose
Absence of one rib on
one or both sides
Abnormal ears
Congenital
heart disease
Many “loops”
on finger tips
Palm creases
Intestinal blockage
Special skin
ridge patterns
Big toes
widely
spaced
Enlarged colon
Umbilical hernia
Abnormal pelvis
Poor muscle tone
Set 2: Mutations
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OHT 33
Patau Syndrome
Chromosome affected: Trisomy 13
Incidence rate of 1 in 3,000 live
births (with a maternal age effect).
Set 2: Mutations
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OHT 34
Patau Syndrome Phenotype
Phenotype of an Patau syndrome child:
1. Multiple defects (see below).
2. Usually death by age 1 to 3 months.
Abnormal palm pattern
Polydactyly
(extra finger)
Structural eye defects
Faulty retina
Small eyes
Small head
Scalp defects
Cleft palate
and hare lip
Low set ears
Heart defects
Spinal defects
(meningomyelocele)
Polydactyly
(extra toe)
Set 2: Mutations
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OHT 35
Edward Syndrome
Chromosome affected: Trisomy 18
Incidence rate of 1 in 5,000 live births
(with a maternal age effect).
Set 2: Mutations
Produced by:
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OHT 36
Edward Syndrome Phenotype
Phenotype of an Edward syndrome child:
1. Ear deformities.
2. Heart defects.
3. Spasticity and other defects.
4. Usually death by age 1 year.
A small head
(microcephaly)
Small mouth and
unusually small jaw
Underdeveloped
or absent thumbs
Low set
malformed ears
Redundant skin folds,
especially over the
back of the neck
Big toe is
shortened and
frequently bent
backward
Small chest
Congenital anomalies
of the lung, kidneys
and ureters
Club feet
Nails are
underdeveloped
Clenched fists with
characteristic overlapping
index finger
Set 2: Mutations
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OHT 37
Maternal Age Effect
in Aneuploidy
Many aneuploidies show a ‘maternal age effect’,
with incidence increasing with age of the mother.
Example: Down syndrome is 100 times more
likely in children of mothers over 45 years, than in
those of mothers less than 19 years.
Estimated Rate of Down Syndrome (per 1000 births)
90
1 in 46
80
Maternal Age
(years)
Incidence per
1000 Live Births
70
< 30
30 - 34
35 - 39
40 - 44
> 44
<1
1-2
2-5
5 - 10
10 - 20
60
50
1 in 100
40
30
1 in 2,300
1 in 880
1 in 290
20
10
0
20
25
30
35
Maternal Age
40
45
50
Set 2: Mutations
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© 1993 – 2001
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O ve r h e a d Tr a n s p a r e n c i e s ( O H T s ) .
OHT 38
Causes of Maternal Age Effect
Maternal age effect probably arises because:
1.
All eggs are present at birth but are suspended in their
development in early prophase until puberty.
2.
A woman, on average, will produce about 400 eggs in
her lifetime (12 per year).
3.
Therefore, by the end of her reproductive life, the egg
cells that remain are old and there is a greater chance
that errors in meiosis will occur.
A similar, though less marked effect is exerted by
the age of the father.
Sperm from older men have
a slight tendency to be
deficient in chromosomes
Old egg cells
are prone to
faulty meiosis
Set 2: Mutations
Produced by:
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© 1993 – 2001
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OHT 39
Produced by:
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© 1993 – 2001
The Fate of Conceptions
For every million conceptions that occur, a significant number have
genetic abnormalities and fail to develop into a completely normal child.
Set 2: Mutations
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O ve r h e a d Tr a n s p a r e n c i e s ( O H T s ) .
Conceptions
1,000,000
Spontaneous Miscarriages
Live Births
150,000
850,000
Chromosome
Abnormalities
75,000
Other Causes
Children
Perinatal Deaths
75,000
833,000
17,000
With Chromosome Abnormalities
5,165
OHT 40
Trisomics ............. 39,000
XO ....................... 13,500
Triploids ............... 12,750
Tetraploids ............. 4,500
Sex Chromosome
Aneuploids
Autosomal
Trisomics
Other
Abnormalities
Others ................... 5,250
Male ............ 1,427
Trisomy 13 ........... 42
Trisomy 18 ......... 100
Trisomy 21 ...... 1,041
Total ............ 2,133
Female ........... 422
Aneuploidy in
Human Sex Chromosomes
The normal complement for human sex chromosomes is:
Male:
Female:
XY
XX
Unusual sex chromosome configurations can arise from
mistakes made during gamete formation (failure of sex
chromosomes to separate properly during meiosis).
Sex
Chromosomes
Apparent
Sex
Phenotype
XO
Female
Turner syndrome
XX
Female
Normal Female
XY
Female*
No pubertal development
XXX
Female
Apparently normal female, greater tendency to criminality
XXXX
Female
Rather like Down syndrome, low fertility/intelligence
XXXXX
Female
Rather like Down syndrome, low fertility/intelligence
YO
?
XY
Male
Normal male
XX
Male*
Short, broad chested, sterile, hypogonadism
XYY
Male
Jacob syndrome, apparently normal male, tall, aggressive
XXY
Male
Klinefelter syndrome
XXYY
Male
Resembles Klinefelter, sterile
XXXY
Male
Extreme Klinefelter, mentally retarded
XXXXY
Male
Down-like syndrome, very retarded
Not Known (non-viable)
* These apparent males and females appear to have the wrong
sex chromosome complement. this is due to hormonal
deficiencies or developmental errors.
Set 2: Mutations
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OHT 41
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Human Sex Aneuploidy Phenotypes
Four phenotypes of people with abnormal numbers of sex
chromosomes (together with the normal male and female ones):
Set 2: Mutations
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OHT 42
Normal
Male
Jacob
Syndrome
Klinefelter
Syndrome
Turner
Syndrome
Super
Female
Normal
Female
XY
XYY,
XYYY
XXY, XXXY,
XXXXY
XO
XXX, XXXX,
XXXXX
XX
Faulty Sperm Production
Aneuploidy in human sex chromosomes may result
from faulty sperm production.
Results from failure of the X and Y chromosomes to
separate during meiosis.
Male
Female
XY
XX
Primary
spermatocyte
Mistake
during
meiosis
X
XY
Faulty gametes
Offspring
XY
XY
X
X
X
X
X
XXY
XO
XXY
XO
Klinefelter
Syndrome
Turner
Syndrome
Klinefelter
Syndrome
Turner
Syndrome
Set 2: Mutations
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OHT 43
Faulty Egg Production
Aneuploidy in human sex chromosomes may
result from faulty egg production.
Results from failure of the two X chromosomes
to separate during meiosis.
Male
Female
XY
XX
X
X
Y
X
XXX
Super
Female
Y
Primary
oocyte
Mistake
during
meiosis
XX
Y
XXY
Klinefelter
Syndrome
XX
Faulty
gametes
XX
Offspring
XO
YO
Turner
Syndrome
Not viable
Set 2: Mutations
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OHT 44
Klinefelter Syndrome
Chromosome complement: 44 + XXY
Karyotype and phenotype:
Mildly impaired IQ
(intelligence)
Poor beard growth
Chest hair
is sparse
Frequently some breast
development (low levels
of testosterone)
Osteoporosis
Penis and testes
underdeveloped, low
levels of testosterone.
Always infertile.
Female type public
hair pattern
Limbs tend to be
longer than average
Set 2: Mutations
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OHT 45
Turner Syndrome
Chromosome complement: 44 + XO
Karyotype and phenotype:
Mental development normal,
difficulty with spatial memory
Characteristic
residual lateral
web neck
Low posterior hair line
Elbow
deformity
Poor breasts development
Constriction of aorta
Degenerate ovaries almost always infertile
Puffy fingers
with deep set,
hyperconvex
finger nails
Reduced stature - body is
typically short
brown spots (nevi)
Set 2: Mutations
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OHT 46
Polyploidy
An organism that has three or more complete sets of
chromosomes (3N or greater).
Types of Polyploidy:
Autopolyploid: A polyploid involving the duplication of
chromosomes from a single species.
Allopolyploid: A polyploid involving the duplication of
chromosomes in a hybrid between two species.
Amphiploid: Describes the result of the last stage of
allopolyploidy where a (usually) fertile hybrid is formed
by doubling of chromosomes in a hybrid.
Examples of Polyploid Plants
Name
Number
Name
Number
Common wheat ........6N = 42
Banana ...................3N = 27
Tobacco ....................4N = 48
Boysenberry ...........7N = 49
Potato .......................4N = 48
Strawberry ..............8N = 56
Many ferns are polyploid with chromosome number up to 400N
Polyploidy is common in plants because vegetative
growth can produce numerous individuals with the
same chromosome type/number.
Set 2: Mutations
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OHT 47
Polyploidy in Humans
Polyploidy is rarely observed in humans, but it is
thought to be one of the more common causes of
spontaneous abortion.
The example below shows a triploidy (3N = 69)
condition found in a baby that went to term (9
months gestation) and then died at birth.
The phenotype of this individual is unknown.
Set 2: Mutations
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OHT 48
Autopolyploidy
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Autopolyploidy is a type of polyploidy.
It involves a multiple of identical sets of chromosomes from the same species.
Hybrid may be fertile or sterile, depending on the number of chromosome sets.
Set 2: Mutations
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Hybrids with an even number of homologous chromosome sets (e.g. 4, 6, 8...28)
will be fertile because chromosome pairing can occur at meiosis.
Same
Species
Normal
haploid
gamete
Same
Species
BB
BB
B
BB
OHT 49
BBB
Sterile
hybrid
AA
Diploid
gamete
AA
AA
Diploid
gametes
AA
AAAA
Fertile
hybrid
Allopolyploidy
Allopolyploidy is a type of polyploidy.
It involves the combination of chromosomes from
two or more different species, to form a hybrid.
Fertile polyploids may arise from doubling of the
chromosome complement in the infertile hybrid
(a process called amphiploidy).
Many commercial plant varieties, being hybrids,
are polyploids of this type.
Species A
AA
Amphiploidy
A
AB
Species B
Infertile
hybrid
BB
AB
B
Species C
AB
Haploid
gametes
Diploid gametes
from identical
infertile hybrids.
(in flowering plants, this
could be self pollination)
AABB
Fertile
allotetraploid
Set 2: Mutations
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OHT 50
The Evolution of Wheat
The common wheat has developed as a result of
several polyploid events after the formation of
hybrids between different grass species:
Domesticated in
the Middle East
Interbreed
to form
sterile
hybrid
Wild Einkorn
Genome: AA
2N No.
X
14
Einkorn
Genome: AA
2N No.
14
Wild Grass
Sterile
Hybrid
Genome: BB
2N No.
AB
14
2N No. 14
Amphiploidy doubles
chromosome number
and creates fertile hybrid
Goat Grass
Interbreed
to form
sterile
hybrid
Emmer Wheat
Genome: DD
X
Genome: AABB
2N No.
2N No.
14
28
Sterile
Hybrid
ABD
Common Wheat
2N No. 21
Genome: AABBDD
2N No.
Amphiploidy doubles
chromosome number and
creates fertile hybrid
42
Set 2: Mutations
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OHT 51
Mutations: Overview
All new alleles originate by mutation.
New alleles introduce genetic variation upon
which natural selection can act.
Most mutations occur in somatic cells and
are not inherited.
Only mutations in gametes can be inherited.
Fitness of Mutations
Fitness describes the value of a mutation to the survival
and reproductive success of the organism. A mutation
may turn out to be:
1. Lethal Mutation
Many mutations are lethal and embryos are nonviable (causing spontaneous abortions).
2. Harmful Mutation
Non-lethal mutations may be expressed as effects
that lower survival or reproductive capacity.
e.g. Down syndrome; sickle cell disease.
3. Silent or Neutral Mutation
Most point mutations are probably harmless, with
no noticeable effect on the phenotype.
4. Useful Mutation
Occasionally mutations may occur which are useful,
particularly in a new environment.
e.g. DDT resistance in insects,
antibiotic resistance in bacteria.
Set 2: Mutations
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© 1993 – 2001
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O ve r h e a d Tr a n s p a r e n c i e s ( O H T s ) .
OHT 52
Evolutionary Significance
of Mutations
The role of chromosomal aberrations in speciation:
Polyploidy can result in the formation of “instant species”
by creating a barrier to chromosome pairing at meiosis
(common in plants).
Fusion of chromosomes (a form of translocation) may
result in a reduction in chromosome number – resulting
in reproductive isolation and therefore a new species.
Example: Fusion of chromosomes may have taken place
during the course of human evolution.
The chromosome number in the great apes is 2N = 48,
whereas in humans 2N = 46.
Possible fusion of
two chromosomes
to create the No. 2
chromosome in
humans.
Note the similar
banding patterns of
chromosomes from
related primate
species.
12
12
12
13
11
11
2
Human
Chimpanzee
Gorilla
Orangutan
Set 2: Mutations
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INTERNATIONAL
© 1993 – 2001
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OHT 53