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Genetics and DNA
Contents
Genes
 Alleles
 Clones
 Tissue Culture
 Animal Cloning
 Evolution
 Mutation
 Evidence for Evolution
 Layout of Fossils
 Example of Evolution
 Extinction
 Variation
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Genes
Inside the nucleus  chromosomes
 Chromosome = 1000s of coils of genes
 Gene = comprise ‘DNA’ that decides our characteristics
 DNA = deoxyribose nucleic acid (chemical)
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Each gene acts as a code for a particular characteristic
Genes
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A human egg cell and sperm cell contains 23 chromosomes
Embryo therefore contains 23 pairs of chromosomes
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Each pair contains a gene from your mother, and a gene from
your father for a particular characteristic
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So each pair contains two “options” for a characteristic
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These options are called alleles
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e.g. gene = eye colour;
alleles = blue eye colour, brown eye colour
Alleles
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An Allele is an alternative form of a gene (one member of a
pair) that is located at a specific position on a specific
chromosome.
Alleles are dominant or recessive
Homozygous = two same alleles (purebred)
Heterozygous = two different alleles
Dominant
Dominant
Recessive
Recessive
+
+
+
+
recessive  dominant
dominant  dominant
dominant  dominant
recessive  recessive
e.g. BB, Bb = brown eyes; bb = blue eyes
Eye Colour
A heterozygous brown-eyed father and a
blue-eyed mother:
50:50 chance of
being either
brown eyed or
blue eyed
Clones
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Clone = organism that is genetically identical to its parent
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Cloning in nature:
– Potato tubers
– Strawberry runners
– Daffodils
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Tissue Cultures:
Plants multiply very quickly by human intervention
A number of cells are taken from the ‘parent’ plant and are
grown by mitosis in growth hormones
Tissue Culture
Advantages
Disadvantages
Many plants grow in a
short amount of time
Same genetic make-up:
vulnerable to disease
Little space is needed,
conditions are controlled
No new characteristics
can arise by chance
All new plants inherit the
desired characteristics
No variation: danger of
reducing gene pool
Animal Cloning
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Simple organisms reproduce by mitosis (e.g. amoeba) so
identical offspring are produced
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Artificial Clone Example: Dolly the Sheep (1996)
1)
Adult sheep egg removed from ovary – nucleus removed
Empty egg cell fused with DNA of udder cell of donor sheep
Fused cell developed, using donated DNA
Embryo implanted into uterus of foster-mother sheep
2)
3)
4)
Result: Dolly became genetically identical to donor sheep
Evolution
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Darwin made 4 key observations:
1) Living things tend to produce more offspring than survive
2) Population numbers in a species stay constant over time
3) Each species displays a wide variation in features
4) Some of these variations are passed on to offspring
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Living things are in continuous competition with each other
for food, space, mates…
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‘Survival of the Fittest’!
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Natural Selection: Survival of organisms best suited to
surviving and reproducing in their environment
Mutation
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During replication, an organism’s genetic make-up (DNA) can
change or mutate.
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If mutation is large…
- organism will probably not survive to reproduce
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If mutation is small…
- change might be beneficial. Offspring will flourish, doing
better than others in that species.
- Many more offspring will inherit this beneficial mutation and
will be better suited to that environment…
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Thus continues natural selection…
Evidence for Evolution
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Comes from rocks and fossils…
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The remains of organisms from millions of years ago are
preserved as fossils in sedimentary rocks
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Fossils are formed in one of two ways:
1) Organism decomposes, and minerals become implanted in
the tissue so that the organism turns to rock
2) Organism’s shape leaves an impression in the ground
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Fossils are formed in areas of insufficient oxygen to decay, in
low temperatures (glaciers) and high soil acidity (peat bog)
Example of Evolution
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The horse
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Fossils provide evidence for
the main stages of evolution
of the horse over 60 million
years
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Dog-sized  2m in height
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Multi-toed feet for walking
on forest floor  singletoed hooves for running
over open country
Extinction
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Species or whole families of organisms die out
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Any of 3 factors can contribute to extinction:
- environment changes too quickly
- new predator or disease kills them
- beaten by another species for competition for food
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The environment is slowly changing. Gradually, certain
characteristics will become favourable and those species
without these characteristics will die out
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The environment can change quickly. This affects great
numbers of species that cannot keep up with the changes
required for survival
Environmental Variation
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Causes:
climate, diet, lifestyle, culture, accidents
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Environment affects how our inherited characteristics develop
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Twins who grow up separately might become very different:
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e.g. fashion, taste, hair colour, build, personality, aptitudes
Genetic Variation
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Causes:
- Mixing of parent information during meiosis
- Gamete forms from a unique combination of genetic
information
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Siblings can have both similar and very different traits
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They are mixtures of their parents, each sibling can receive
different characteristics of their parents
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e.g. natural hair colour, eye colour, blood type
Continuous vs. Discontinuous
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Continuous Variation
- Small differences between individuals
- Greatly affected by environment
- e.g. height, shoe size, length of hair
- plotted on a line graph
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Discontinuous Variation
- Differences that are classed or categorised
- Not greatly affected by environment
- e.g. blood group, sex, hair colour, eye colour
- plotted on a bar chart or pie chart
Summary
Genes: instructions for our genetic make-up (e.g. eye colour)
 Alleles: different types of the same gene (e.g. blue eyes)
 Clones: genetically identical to the parent (mitosis)
 Tissue Culture: Many of a specialist organism type produced
 Animal Cloning: used for selective breeding
 Evolution: Survival of the fittest!
 Mutation: Cells mutate under certain conditions
 Evidence for Evolution: Rocks and fossils
 Layout of Fossils
 Example of Evolution: Horse
 Extinction: Occurs for 3 main reasons
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