Download 2.1 Selective breeding

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2.1 Selective breeding
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Sometimes animal breeders cross close relatives, like brothers and
sisters, or parents and offspring. This is called inbreeding. If this
continues for generations, the offspring start to decline, showing
various defects. This is sometimes noticeable with dogs or horses.
They may have weak hips, a bad heart or a nasty temperament.
Too much inbreeding reduces the gene pool and unwanted
recessive characteristics become common. Inbreeding leads to a
reduction in variation, often leading to a lowering of disease
resistance. The British Kennel Club introduced rules and
regulations to prevent inbreeding of pedigree dogs.
© Harcourt Education Ltd GCSE in Applied Science for OCR
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2.4 Genetics 2
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Each gene in every plant and animal has two alleles – one from
each parent. An allele is either recessive or dominant.
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Let’s call the allele for tongue-rolling T and the allele for nontongue-rolling t. Tongue-rolling is a dominant characteristic, so its
allele is dominant.
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If one allele is dominant and the other is recessive, the
instructions from the dominant allele show themselves. This
means that if the gene controlling tongue-rolling in a person has
the alleles Tt, that person can roll their tongue because T is
dominant over t.
This combination of alleles in the gene (here TT, Tt or tt) is
called the genotype.
The result from the combination of genes is called the phenotype.
Here, the phenotype is ‘you can roll your tongue’ or ‘you can’t
roll your tongue’.
When the alleles are the same (here tt or TT), the gene is called
homozygous. When the alleles are different, the gene is called
heterozygous.
a Write down the genotype for the heterozygous gene for
tongue-rolling. What is its phenotype?
Here is a Punnett square for this genetic cross. In this example the
father is heterozygous for tongue-rolling and the mother is
homozygous for non-tongue-rolling.
father’s
genes
mother’s
genes
T
t
Tt
t
t
tt
Tt
tt
possible
combinations
The first offspring from any genetic cross is called the F1
generation.
b List the phenotypes for the F1 generation. Write down their
genotypes.
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2.5 Genetic engineering
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Advantages
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There are many advantages to genetic engineering, such as the
following.
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• The gene that controls an important feature can be isolated,
extracted and inserted into a developing organism, so we can
sometimes control the characteristics of an organism.
• Crops can be developed that produce their own fertiliser.
• Eggs can be taken out of a woman’s body, fertilised and then
replaced. This is called fertility treatment. In the future, it might
be possible to change the genes in the egg while the egg is out
of the body. For example, harmful genes could be removed and
replaced with useful ones.
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Disadvantages
There are also disadvantages. Scientists still do not fully
understand what each gene is capable of controlling. There may
be harmful, unexpected effects. As yet, however, there is no clear
evidence of any harmful long-term effects.
The processes of genetic engineering
Genes are carried in the chromosomes of the cell. There are
thousands in each chromosome. Chromosomes are lengths of
DNA. Each gene is a small section of DNA.
When you take a gene out of an organism you need to cut it out
of the whole length of the chromosome. This is done by using
enzymes as ‘biological scissors’. Special enzymes are applied to
either side of the required section of DNA (or gene). The enzyme
breaks the bonds allowing the section of DNA to be removed.
Alternatively the gene can be extracted by replication.
When cells divide in mitosis every chromosome makes a copy of
itself. We call this process replication. Scientists can extract parts
of DNA and allow it to replicate itself in a test tube in a similar
way. This process is called gene cloning. Scientists use gene
cloning to separate the single gene of interest from the rest of the
genes extracted and make thousands of copies of it.
The modified gene is now ready to be inserted into the target
organism. A common method is to put the required gene into a
ring of DNA taken out of a bacterium. Enzymes are used to
connect the inserted gene onto the bacterium gene. The modified
ring of DNA is then placed inside a bacterium, which is allowed
to divide. The book shows how genetic engineering is used for
insulin production.
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Genetic engineering
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This stage of the process is referred to as transformation gene
insertion. We refer to the modified gene as the transgene. Since
plants have millions of cells, it would be impossible to insert a
copy of the transgene into every cell. Various techniques are used
to insert the transgene into cells. Some of the more common ones
include a gene gun, agrobacterium, microfibres and
electroporation. These methods allow the transgene to be
inserted into the nucleus of a cell without killing it. The diagram
shows how the gene gun method is used to transform plant cells.
gene gun
plant cells
transgenes
plant cells
plant cells
with transgenes
inserted
© Harcourt Education Ltd GCSE in Applied Science for OCR
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2.1 Selective breeding
1 A farmer is selectively breeding his cows to increase the milk
yield from his herd.
a Write down the meaning of the term ‘selective breeding’.
b Describe the different stages of the selective breeding
programme carried out by the farmer.
c Suggest one other feature a farmer may wish to selectively
breed into his animals.
2.2 Cross-breeding
1 Explain the difference between cross-breeding and selective
breeding.
2.4 Genetics 2
1 Explain the difference between chromosomes and genes.
2 Tim keeps pet rabbits. When Tim mated two black rabbits,
some of the babies were black and some were white. In the
questions, use B as the dominant allele for black fur, and b as
the recessive allele for white fur.
a Tim concluded that the parents must both carry a recessive
allele. Give reasons for Tim’s decision.
b Copy and complete the Punnett square to show the possible
genotype of the offspring.
B
b
c For Tim’s baby rabbits, write down the ratio of black fur
babies to white fur babies.
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© Harcourt Education Ltd GCSE in Applied Science for OCR
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2.5 Genetic engineering
1 James is diabetic. He needs to inject insulin into his body twice a
day. For many years this insulin has been extracted from the
pancreas of cattle, sheep and pigs. Scientists are now able to
produce human insulin using genetic engineering.
The diagram shows some of the stages involved in the production
of genetically engineered human insulin.
human cell
with insulin
gene on
chromosome
bacterium A
with ring of DNA
insulin
gene
the
chromosome
is removed
the insulin
gene is
removed from
chromosome
ring of DNA split open
the insulin gene put into
a ring of bacterial DNA
the DNA is inserted into
a bacterium A to get a
bacterium B
rapid multiplication to
produce a clone of bacterium B
a Describe how the insulin gene is removed from the human
chromosome.
b The clone of bacterium 2 produces large quantities of insulin.
i Describe what is meant by the term ‘clone’.
ii Explain why bacteria are suitable organisms to use for this
purpose.
c Explain the advantages of genetically engineered insulin
compared with insulin extracted from animals.
2.6 GM crops
1 Here is a newspaper cutting about GM crops.
Biotech Crops May Aid Farmers
A study on the impact of GM crops indicated that Kansas and Missouri
farmers could significantly improve their income with such crops.
a Describe what is meant by the term ‘GM crops’.
b Suggest why GM crops could improve the income of farmers.
c Suggest why some people do not like the idea of introducing
GM crops into our farmland.
© Harcourt Education Ltd GCSE in Applied Science for OCR
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Adapting animals and plants: questions
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1 A gardener has some roses that are a pale pink colour but too short. He also has
red roses that are the correct size.
a Describe the process the gardener could use to selectively breed pale pink
roses of the correct size.
[4]
b Explain why the process of selective breeding takes many years.
[2]
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The gardener crossed two red rose plants. He was surprised to find some of the
offspring produced pink roses.
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• R is the dominant allele for red colour.
• r is the recessive allele for pink colour.
c i Write down the genotype for the parent red rose plants.
ii Copy and complete the Punnett
square to show why these red rose
plants produced offspring with
pink roses.
[2]
iii Explain what the gardener would
need to do to produce all pink
rose plants.
[1]
[1]
2 Sam keeps pet rabbits. Some have brown fur and some have white fur. He
crosses a rabbit with brown fur and a rabbit with white fur.
• B is the dominant allele for brown fur.
• b is the recessive allele for white fur.
All of the offspring (F1 generation) had brown fur.
a i Write down the genotype of the brown fur and the white fur parents.
[2]
ii Explain why all of the offspring had brown fur.
[2]
b Sam later crosses two of the brown fur rabbits from the offspring (F1
generation). A quarter of their offspring (F2 generation) had white fur, and
three-quarters had brown fur. Copy and complete the grid to show the
genotypes of the parents and offspring.
[2]
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Adapting animals and plants: questions
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3 During mitosis the chromosomes replicate. Scientists carry out a process where they extract a
part of DNA and allow it to replicate itself in controlled conditions. They are able to separate the
single gene of interest from the rest of the genes extracted and make thousands of copies of it.
a Write down the name given to this process.
[1]
b The modified gene is then placed into a target organism.
This target organism is often a bacterium.
i Describe how the modified gene could be placed into the bacterium.
[2]
ii Explain why bacterium are useful target organisms.
[2]
c The modified gene at this stage is called the transgene. The transgenes are inserted into
plants at an early stage of their development.
i Explain why the transgene is introduced into plant cells at this stage of development. [2]
ii Write down the name of one technique used to insert the transgene into cells.
[1]
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