Transcript Ch 10: Genetic Change and Variation

Ch 10: Genetic Change and Variation
Variation forms the basis of evolution.
 There are two basic forms:
1 Continuous variation where individuals in a population
shows a gradation from one extreme to the other.
2 Discontinuous variation where there is a limited
number of distinct forms within the population.
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10.1 Methods of Recording Variation
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10.1.1 Table of data
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10.1.2 Line graph
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10.1.3 Histogram
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10.1.4 Bar graph
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10.1.5 Kite graph
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10.1.6 Pie chart
10.2 Types of Variation
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10.2.1 Continuous variation
Characteristics within a population vary only very
marginally between one individual and the next
 a graduation from one extreme to the other
examples: weight, height, IQ, EQ, etc
Characteristics which show continuous variation are
controlled by the combined effect of a number of genes
(polygenes) - a polygenic character
10.2 Types of Variation
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The random assortment of genes during
metaphase I of meiosis ensures that
individuals possess a range of genes from any polygenic
complex:
all tall genes  very tall
all short genes  very short
about 1/2 tall and 1/2 short genes
 intermediate height
10.2.2 The normal distribution curve
10.2.2 The normal distribution curve
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The mean (arithmetic
mean) is the average of a
group of values.
The mode is the single
value of a group which
occurs most often.
The median is the central
or middle value of a set
of values.
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The standard deviation
is a value which gives an
indication of the range of
values on either side of the
mean.
10.2.3 Discontinuous (discrete) variation
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characters which do not show a gradation between
extremes but fall into a number of distinct forms
usually controlled by a single gene which may have 2
or more alleles
10.3 The chi-squared test (not required in syllabus)
10.4 The t-test (not required in syllabus)
10.5 Origins of Variation
1. Environment
2. Genetic change: reshuffling of genes and mutation
10.5.1 Environmental effects
 Phenotype is the result of its genotype and effect of the
environment.
 Because environmental influences are themselves very
various and often form gradations,
e.g. temperature, light intensity, etc.,
 they are largely responsible for continuous variation
within a population.
10.5 Origins of Variation
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10.5.2 Reshuffling of genes
- creating new combinations during sexual reproduction
by:
1 Mixing two different parental genotypes where cross
fertilization occurs
2 Random distribution of chromosomes during
metaphase I of meiosis
3 Crossing over between homologous chromosomes
during prophase I of meiosis
Mutation
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Mutation is any change in the structure or the amount
of DNA of an organism.
Most mutations occur in body cells and do not pass to
offspring. Only those that affect gametes can be
inherited and produce sudden and distinct differences
between individuals  discontinuous variation
10.5.3 Changes in gene structure
(point mutation)
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Gene mutation (point mutation) is a change in the
structure of DNA which occurs at a single locus on a
chromosome
gene mutation
 wrong sequence of amino acids
 no enzyme
 absence of a character, e.g. pigment
10.5.3 Changes in gene structure
(point mutation)
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There are many forms of gene mutation:
1. Duplication - a portion of the nucleotide chain
becomes repeated
2. Addition (insertion) - an extra nucleotide sequence
becomes inserted in the chain
3. Deletion – a portion of the nucleotide is removed
from the chain
4. Inversion – a nucleotide sequence separates and
rejoins at original position
5. Substitution – one of the nucleotides is replaced by
another with a different base
10.5.3 Changes in gene structure
(point mutation)
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example: sickle-cell anaemia
is the result of the replacement of just one base in the
DNA molecule causing the wrong amino acid being
joined into two of the polypeptide chains which make
up the haemoglobin molecule.
but the disease is resistant to malaria !
Sickle-cell
anaemia
Sickle-cell
anaemia
Sickle-cell
anaemia
10.5.4
Changes in whole sets of chromosomes
Polyploidy is the possession of more than 2 complete
sets of chromosomes.
e.g. triploid means 3 sets; tetraploid means 4 sets.
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Formation of tetraploid offspring:
fertilization of diploid gametes or
whole set of chromosomes doubles after fertilization
Formation of triploid offspring:
Fertilization of a diploid gamete with
a normal haploid gamete
Autopolyploidy – polyploidy within the same species
Autopolyploidy can be induced by colchicine
(a chemical) which inhibits spindle formation and so
prevents chromosomes separating during anaphase.
 Triploids are sterile because they cannot form complete
homologous pairings.
If, however, a hybrid has a chromosome number which is
a multiple of the original chromosome number,
a new fertile species is formed,
e.g. wheat (n=42) is the cross between
wild grass (n=14) and emmer wheat (n=28)
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Allopolyploidy –
A fertile species having a chromosome number which is
a multiple of the original haploid number
 Allopolyploidy is rare in animals, but relatively
common in plants, including many food plants,
e.g. wheat, coffee, banana, sugar cane, apple, tomatoes, etc
 The polyploid variety often have advantages,
e.g. large fruits, tomatoes have more vitamin C, etc.
10.5.5 Changes in chromosome number
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Non-disjunction occurs when one of the homologous
chromosomes (23 pairs) fails to segregate during
meiosis, gametes formed have 22 & 24 chromosomes.
This is often fatal.
Down's syndrome :
47 chromosomes (+ extra 21st
chromosome)
Down's syndrome often occurs in
ova formation rather than sperms,
especially in old age pregnancies.
Turner's syndrome:
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Have one missing X chromosome
XO with 45 chromosomes
Females with small stature & sexually immature
Klinefelter's syndrome:
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Genotypes are XXY, XXXY or XXXXY
Males with small testes but no sperms, with
breast development and female figures
It indicates that Y is the cause of maleness
10.5.6 Changes in chromosome structure
This occurs in meiosis when crossing over takes place.
1 Deletion
2 Inversion
3 Translocation
4 Duplication
84-II-4
10.6 Causes of Mutations
 There is natural mutation rate which varies from
one species to another.
 Animals with shorter life cycles show a greater rate of
mutation because of more frequent meiosis.
 This natural mutation rate can be increased artificially
by certain chemicals e.g. colchicine, formaldehyde,
nitrous acid & mustard gas
 or energy sources (mutagens), e.g. uv rays, X rays,
 rays,  &  particles and neutrons
10.7 Genetic Screening and Counselling –
To risk for a mother to have babies with certain genetic
diseases could be calculated, if enough information of
the disease in the family is known,
e.g. Down's syndrome, haemophilia.
 On the basis of this advice parents can choose whether or
not to have children.
 Doctors can diagnose certain genetic defects,
e.g. Down's syndrome, in a foetus, by studying samples
of cells taken from the amniotic fluid which surrounds
the foetus – a process called amniocentesis.
 Parents can then decide to have the pregnancy
terminated.
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10.7.1 Gene tracking
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To find out on which chromosome a defective
gene is located.
Blood groups are traced in families to act as gene
markers. Correlation between certain blood groups
alleles and the occurrence of a genetic disease can
determine whether or not the gene for the disease is on
the same chromosome as that for blood groups.
If one genetic marker is not linked to the disease in
question another must be tried and so on until the one
which shows linkage with the disease is found.
Linked markers are then used to work out if someone
carries a disease.
19. 88-II-4 (b)
What is the genetic basis of
(i)
Hybrid vigour,
(5 marks)
(ii)
and the determination of the ABO blood
groups?
(4 marks)