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4
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1 State that eukaryote chromosomes are made of DNA and proteins
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4
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2 Define gene, allele and genome
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Allele: one specific form of a gene, differing from other alleles by one or a few bases only and
occupying the same gene locus as other alleles of the gene
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4
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3 Define gene mutation
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4
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4 Explain the consequence of a base substitution mutation in relation to
the processes of transcription and translation, using the example of sickle-cell
anemia
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It is due to a mutation
on the Hb gene which codes for a polypeptide of 146 amino acids which is part of haemoglobin
(haemoglobin is an important protein component in red blood cells)
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This is called a base substitution mutation as
adenine (A) is replaced by thymine (T)
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Instead of the normal codon GAG, the messenger RNA
will contain the codon GUG
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In a healthy
individual the codon GAG on the messenger RNA matches with the anticodon CUC on the transfer RNA
carrying the amino acid glutamic acid
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So the base substitution mutation has caused glutamic acid to be replaced by valine on the
sixth position on the polypeptide
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This has an effect on the phenotype as instead of normal donut
shaped red blood cells being produced some of the red blood cells will be sickle shaped
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However, there is an advantage to sickle cell anemia
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Summary of important steps:
Normal Gene
Codon
Mutated gene
GAG
GTG
Transcription GAG on mRNA
GUG on mRNA
Translation
Anticodon CUC and amino acid glutamic Anticodon CAC and amino acid valine on
acid on tRNA
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Haemoglobin HbA
HbS
Phenotype
Normal donut shaped red blood cells
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Effects
Carry oxygen efficiently but are affected Do not carry oxygen efficiently but give
by malaria
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4
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1 State that meiosis is a reduction division of a diploid nucleus to form
haploid nuclei
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4
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2 Define homologous chromosomes
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4
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3 Outline the process of meiosis, including pairing of homologous
chromosomes and crossing over, followed by two divisions, which results in
four haploid cells
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Meiotic cells have an interphase stage before the start of meiosis I
which is similar to mitosis
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(See notes on mitosis) After meiosis I
there is another brief interphase stage which is followed by meiosis II
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In prophase I the chromosomes pair up so that the
chromosomes in each pair are homologous
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Crossing over is the exchange of genetic material between non-sister
chromatids
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The second stage is metaphase I
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The third stage is anaphase I
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This halves the chromosome number however each chromosome is still
composed of two sister chromatids
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The fourth stage is telophase I
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The membrane
then divides through citokinesis
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This stage does not include the S
phase
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Here the cell has divided into two daughter haploid cells
however the process does not end here as these two cells immediately start to divide again
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The second stage is metaphase II
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The third stage is anaphase II
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Each sister chromatid then becomes a
chromosome
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Here the nuclear membrane reforms around the four sets of daughter
chromosomes
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4
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4 Explain that non-disjunction can lead to changes in chromosome
number, illustrated by reference to Down syndrome (trisomy 21)
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A common problem is non-disjunction
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This leads the production of gametes that either have a chromosome too
many or too few
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When a zygote is formed from the fertilization of these gametes with
an extra chromosome, three chromosomes of one type are present instead of two
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Down syndrome is a disease in which the chromosomes failed to separate properly
during meiosis leading to three chromosomes of type 21 instead of two
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The non-disjunction can take place either in
the formation of the egg or the sperm
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Below is a diagram illustrating a non-disjunction:
4
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5 State that, in karyotyping, chromosomes are arranged in pairs according
to their size and structure
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4
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6 State that karyotyping is performed using cells collected by chorionic
villus sampling or amniocentesis, for pre-natal diagnosis of chromosome
abnormalities
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4
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7 Analyse a human karyotype to determine gender and whether nondisjunction has occurred
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The gender can be deduced by looking at the sex chromosomes
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We can distinguish this on with karyotyping as
the Y chromosome is smaller than the X
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If There should only be
two of each chromosome
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For example, if we notice that there are three chromosomes 21 then we can conclude that a
non-disjunction occurred
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(trisomy 21)
Below are two images of a karyotype
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The second image shows the karyotype a person with Down’s syndrome would get
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3
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Genotype: the alleles of an organism
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Dominant allele: an allele that has the same effect on the phenotype whether it is present in the
homozygous or heterozygous state
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Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote
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Homozygous: having two identical alleles of a gene
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Carrier: an individual that has one copy of a recessive allele that causes a genetic disease in
individuals that are homozygous for this allele
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4
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2 Determine the genotypes and phenotypes of the offspring of a
monohybrid cross using a Punnett grid
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3
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Some genes have more than two alleles (multiple alleles)
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3
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The ABO blood group is a good example of codominance and multiple alleles
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If there are more than two allele of a gene then they are called
multiple allele
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Both of these are dominant and so if IA and IB are present together
they form blood group AB (genotype IAIB)
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Codominant allele are pairs of allele that both affect the phenotype when present
together in a heterozygote
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However if
you have the genotype ii then you are homozygous for i and will be of blood group O
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PhenotypeGenotype
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii
Below are diagrams illustrating the inheritance of the ABO blood groups
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3
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There are two chromosomes which determine gender
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Females have two X chromosomes whereas males
have one X and one Y chromosome
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The Y chromosome on the other hand only contains a few
genes
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The male can pass on either the Y or the X chromosome from the sperm (male gamete)
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If the male
passes on the Y chromosome then the growing embryo will develop into a boy
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4
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6 State that some genes are present on the X chromosome and absent
from the shorter Y chromosome in humans
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4
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7 Define sex linkage
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4
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8 Describe the inheritance of colour blindness and hemophilia as examples
of sex linkage
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Since females have two X
chromosomes they have two copies of the sex-linked gene whereas males only have one since they
only have one X chromosome
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Hemophilia
XH is the allele for normal blood clotting and is dominate over Xh which is recessive and causes
hemophilia
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She can however pass the disease on to her offspring
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From our four possible outcomes we can see that a female child cannot get hemophilia but can be a
carrier
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Depending on whether the mother passes on the dominant or recessive allele will determine
if the female child is a carrier or is unaffected by the hemophilia
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So whether the child
has the disease or is unaffected depends on which allele the mother had passed on
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So there is a 50% chance of the child having hemophilia if it is male as half of the eggs produced by
the mother will carry the recessive allele
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Finally there is a 25%
chance overall that the offspring will be affected
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3
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A human female can be homozygous or heterozygous with respect to sex-linked genes
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3
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Female carriers for X-linked recessive alleles are always heterozygous since they require a dominant
allele and a recessive allele to be carriers
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For example hemophilia is a sex-linked disease
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The carrier mother can either pass on the dominate or recessive allele
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4
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11 Predict the genotypic and phenotypic ratios of offspring of monohybrid
crosses involving any of the above patterns of inheritance
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4
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12 Deduce the genotypes and phenotypes of individuals in pedigree
charts
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If it is a 50/50 ratio between men and women the disorder is autosomal
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Circles represent females
Shaded symbols represent affected individuals
Unshaded symbols represent unaffected individuals
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4
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1 Outline the use of polymerase chain reaction (PCR) to copy and amplify
minute quantities of DNA
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It can be useful when
only a small amount of DNA is available but a large amount is required to undergo testing
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The PCR requires high
temperature and a DNA polymerase enzyme from Thermus aquaticus (a bacterium that lives in hot
springs)
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4
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In gel electrophoresis, fragments of DNA move in an electrical field and are separated according to
their size
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4
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Gel electrophoresis of DNA is used in DNA profiling
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4
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Organisms have short sequences of bases which are repeated many times
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These repeated sequences vary in length from person to person
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Gel electrophoresis separates
fragmented pieces of DNA according to their size and charge
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This is called DNA profiling
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4
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5 Analyse DNA profiles to draw conclusions about paternity or forensic
investigations
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For a
paternity test, look for similarities between the child and the possible father
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4
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It is now easier to study how genes influence human development
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It allows the production of new drugs based on DNA base sequences of genes or the structure of proteins coded
for by these genes
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4
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7 State that, when genes are transferred between species, the amino acid
sequence of polypeptides translated from them is unchanged because the
genetic code is universal
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4
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8 Outline a basic technique used for gene transfer involving plasmids, a
host cell (bacterium, yeast or other cell), restriction enzymes (endonucleases)
and DNA ligase
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The bacterium can then synthesis insulin which can be
collected and used by diabetics
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The messenger RNA which codes for insulin is
extracted from a human pancreatic cell which produces insulin
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At the same time, a selected plasmid
is cut using restriction enzymes which cut the DNA at specific base sequences
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Once we have both the plasmid and the gene ready,
these are mixed together
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The plasmids with the
human insulin gene (called recombinant plasmids) can then be mixed with host cells such as
bacterium
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4
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9 State two examples of the current uses of genetically modified crops or
animals
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The transfer of a gene that gives resistance to the herbicide glyphosate from bacterium to crops so that the crop
plants can be sprayed with the herbicide and not be affected by it
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4
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It is quite common to see genetic modifications in crop plants
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This is done because maize crops are often destroyed by insects that eat the corn and so by adding
the Bt toxin gene this is prevented as the toxin kills the insects
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The table
below summarizes the benefits and possible harmful effects of genetically modifying the maize crops
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Harmful Effects
We are not sure of the consequences of humans and animals
eating the modified crops
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Since there is a higher crop yield, Other insects which are not harmful to the crops could be killed
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crops
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There is a reduction in the use of Cross pollination can occur which results in some wild plants being
pesticides which are expensive
and may be harmful to the
genetically modified as they will contain the Bt gene
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certain insects and so some plants may become endangered
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4
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11 Define clone
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4
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12 Outline a technique for cloning using differentiated animal cells
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These cells were than cultured
in a low nutrient medium to make the genes switch off and become dormant
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The nucleus of this egg cell was removed by using a micropipette and
then the egg cells were fused with the udder cells using a pulse of electricity
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These embryos were then implanted into
another sheep who’s role was to be the surrogate mother
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Dolly was genetically identical to the sheep from which the udder cells were taken
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4
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There are many ethical issues involving therapeutic cloning in humans
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Arguments for
Arguments against
Embryonic stem cells can be used for therapies that save lives and Every human embryo is a
reduce pain for patients
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developing
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many are killed
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There is a risk of embryonic stem
cells developing into tumour cells