Download 10.1 Methods of Recording Variation

F 6 Biology - Ch 10: Genetic Change and Variation
Name:__________________________(
)
Variation forms the basis of evolution. There are two basic forms:
1.Continuous variation where
2.Discontinuous variation where
10.1 Methods of Recording Variation
10.1.1 Table of data
10.1.4 Bar graph
10.1.5 Kite graph
10.1.2 Line graph
10.1.6 Pie chart
10.1.3 Histogram
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F 6 Biology - Ch 10: Genetic Change and Variation
Name:__________________________(
)
10.2 Types of Variation
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:
- Characteristics which show continuous variation are controlled by the combined
effect of a number of genes (polygenes) - a polygenic character
- 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
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.
The standard deviation is a value which
gives an indication of the range of values
on either side of the mean.
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F 6 Biology - Ch 10: Genetic Change and Variation
Name:__________________________(
)
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 _____________ and effect of ____________________.
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.
10.5.2 Reshuffling of genes
- creating new combinations during sexual reproduction by:
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)
Gene mutation (point mutation) is
gene mutation → wrong sequence of amino acids → no enzyme
→ absence of a character, e.g. pigment
There are many forms of gene mutation:
1. Duplication 2.
Addition (insertion) -
3.
Deletion -
4.
Inversion -
5.
Substitution -
F 6 Biology - Ch 10: Genetic Change and Variation
Name:__________________________(
)
example:
sickle-cell anaemia
is the result of
but resistant to malaria !
10.5.4
Changes in whole sets of chromosomes
Polyploidy is the possession of more than
___________ sets of ___________________
e.g. triploid means
;
tetraploid means
Formation of tetraploid offspring:
Formation of triploid offspring:
Autopolyploidy -
Autopolyploidy can be induced by colchicine
(a chemical) which inhibits
Triploids are sterile because they cannot
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)
Allopolyploidy -
Allopolyploidy is rare in _________________, but relatively common in __________,
including many food plants, e.g. ________________________________________________
The polyploid variety often have advantages, e.g.
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F 6 Biology - Ch 10: Genetic Change and Variation
Name:__________________________(
)
10.5.5 Changes in chromosome number
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Non-disjunction occurs when
Down's syndrome:
Down's syndrome often occurs in ova formation
rather than sperms, especially in old age
pregnancies.
Turner's syndrome:
Klinefelter's syndrome:
10.5.6
Changes in chromosome structure
This occurs in meiosis when crossing over takes place.
There are 4 types:
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.
or energy sources (mutagens), e.g.
F 6 Biology - Ch 10: Genetic Change and Variation
Name:__________________________(
)
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10.7 Genetic Screening and Counselling - not required in syllabus
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.
10.7.1 Gene tracking
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.
E.g. Huntington's disease (neuropsychiatric disorder leading to the loss of control of movements)
fails to show itself until people are in the 40's or 50's. Any technique which allows the presence of
the gene to be detected early helps individuals to decide whether or not to have children, and also
shows the size of the risk of those children carrying the gene.