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Learning Outcomes

Candidates should be able to:

(a) [PA] describe, with the aid of diagrams, the behaviour of

chromosomes during meiosis, and the associated behaviour of

the nuclear envelope, cell membrane and centrioles (names of

the main stages are expected, but not the sub-divisions of

prophase);(b) explain how meiosis and fertilisation can lead to variation;

(c) explain the terms locus, allele, dominant, recessive,

codominant, homozygous, heterozygous, phenotype and

genotype (see pages 35

37 and 39

41);(d) use genetic diagrams to solve problems involving monohybrid

and dihybrid crosses, including those involving sex linkage,

codominance and multiple alleles (but not involving autosomal

linkage or epistasis);


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Learning Outcomes

Candidates should be able to:

(e) use genetic diagrams to solve problems involving test crosses;

(f) [PA] use the chi-squared test to test the significance of

differences between observed and expected results (the

formula for the chi-squared test will be provided);

(g) explain, with examples, how mutation may affect the

phenotype;

(h) explain, with examples, how the environment may affect the

phenotype;

(i) explain how a change in the nucleotide sequence in DNA mayaffect the amino acid sequence in a protein and hence the

phenotype of the organism;

(j) use the knowledge gained in this section in new situations or to

solve related problems.


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Sexual reproduction


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Meiosis (Middle prophase I)


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Meiosis (late prophase I)


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Meiosis (Metaphase I)


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Meiosis (Anaphase I)


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Meiosis (Telophase I)


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Meiosis (Meiosis II-Prophase II and

Metaphase II)


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Meiosis (Anaphase II)


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Meiosis (Telophase II)


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Meiosis


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Meiosis


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Meiosis


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Meiosis I

During Interphase IChromosomes _________

ie. 2n _____

During Prophase IChromosomes _________

ie. see _______________

____________ chromosomes _________

ie. form _________ / _________

ie. may have __________ at _________* crossovers only occur between NON-SISTER chromatids

Nuclear membrane _______________

double

4n

thicken

sister chromatids

homologous pair up

bivalent synapsis

crossover chiasma

breakdown


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Meiosis I

During Metaphase ISpindle fibers attach

to ___________

(at ____________)

____________chromosomes

______ at _________

During Anaphase I

Spindle fibers _________

____________ chromosomes _________

move to opposite poles

homologous

homologous

centromerekinetochore

align equator

shorten

separate


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Meiosis I

During Telophase IEach separated homologous

chromosome reaches pole

Spindle fibers _________

Nuclear membrane _________

At this time,

~ cells may rest

~ cells may continue immediately into_________

disappear

reform

Meiosis II


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meiosis and fertilisation can lead to

variation

In sexually reproducing organisms, three

processes lead to most genetic variation:

1. independent orientation (independent

assortment )of chromosomes in meiosis

(metaphase I)

2. crossing over of chromosomes in meiosis

(late prophase I)

3. random fertilization


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Independent orientation of chromosomes in meiosis

and random fertilization lead to varied offspring


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Homologous chromosomes can carry

different versions of genes

Separation of homologous chromosomes during

meiosis can lead to genetic differences between

gametes

Homologous chromosomes may have differentversions of a gene at the same locus

One version was inherited from the maternal parent,

and the other came from the paternal parent

Since homologues move to opposite poles during

anaphase I, gametes will receive either the maternal

or paternal version of the gene


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Differing genetic information on

homologous chromosomes

Simplified example in mice:

Coat color and eye color

C (brown) c (white)

E (black) e (pink)

Two possible outcomes for

The genes

C i (d i l h I)


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Crossing over (during late prophase I)

further increases genetic variability

Genetic rearrangement between homologues Sister chromatid exchange

Chiasma Site of crossing over Happens between chromatids within tetrads

Produces new gene combinations Genetic recombination

Can occur several times in various location within onetetrad

Zygote combination possibilities are much greater than 64trillion

Two individuals cannot produce identical offspring fromseparate fertilizations

Everyone is a unique genetic entity never seen before

and never to be seen again


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Figure: How

crossing over

leads to

genetic

recombination


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Random fertilization

The combination of each unique sperm with

each unique

egg increases genetic variability:

Combination 1 + Combination 2 is different from

Combination 3 + Combination 4, etc.


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Paper 4 JO5Figure is a diagram of pair of homologous

chromosomes during meiosis.

(a) State the stage of meiosis shown. [1]

metaphase 1 / (late) prophase 1

(b) Describe what has occurred between the

two homologous chromosomes. [3]- homologous chromosomes) pairing /

synapsis

- to chiasma / crossing over

- exchange of genetic material

- between non-sister chromatids

(c) Explain how this can lead to variation. [2]

- breakage of linkage groups / ref. new

linkage groups ;

- may have different alleles ;

- creates new combinations of alleles ;

- when sister chromatids separate ;

(d) Describe two other sources of variation that

are possible as a result of meiosis. [4]

- idea of random orientation at metaphase

I and II / random alignment of homologous

chromosomes on spindle equator ;

- subsequently leads to independent

assortment ;

- 2n possible combinations when n is

number of chromosome pairs ;

- ref. to chromosome mutation qualified ;

- ref. gametes haploid (so can fuse) ;

- random fusion of gametes ;


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Important terms

Locus- a locus (plural loci) is the specific location of a gene or

DNA sequence on a chromosome.

Allele- a different form of gene (e.g. represented with letters for

the different types of alleles PP, Pp, pp)

Dominant- allele that always expresses itself in the phenotype

when present/ allele which influences the phenotype even in

the presence of a alternative allele.

Recessive- allele which does not have its effect in heterozygote/

allele which only effect in homozygotes/ affects phenotype if

dominant allele is absent.


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Important terms

Codominant- both alleles, influence phenotype/ are

expressed; more than 2 phenotypes possible; phenotype

of heterozygote different from either homozygote

Homozygous- having two identical alleles of gene (PP, pp)

Heterozygous- having two different alleles of gene (Pp)

Phenotype- the observable characteristics of an organism,

Genotype- the alleles possessed by an organism


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G l i i


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Gene loci

Homozygousfor the

dominant allele

Dominantallele

Homozygousfor the

recessive allele

Heterozygous

Recessiveallele

Genotype:

P Ba

P

PP

a

aa

b

Bb

Figure: Matching gene loci on homologous chromosomes.


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Test cross

Test cross always involves crossing an organism

showing the dominant phenotype with one which is

homozygous recessive.


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The inheritance of coat color in horses is complex but all horses have one of

two base colors, red (chestnut) or black. The base color is controlled in a

simple monohybrid way.

Draw a genetic diagram to show how two parents with black coat colorcan produce a chestnut foal and the probability of such an eventoccurring. Choose a letter symbol to represent coat color. [4]

Paper 4 (J09)


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Paper 4 (N04)

Figure shows four generations of a family in which some members of the

family suffer from sickle cell anaemia.

Using the symbols HN for the allele

for normal haemoglobin and HS for the

allele for sickle cell haemoglobin, statethe genotypes of the following

individuals. [1]A:

______________________________C:

______________________________Draw a genetic diagram to show theprobability of the parents A and B

producing another child with sickle cell

anaemia. [4]


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Multiple alleles

Most genes have more than

two alleles, e.g. human

blood groups.

The four groups A, B, AB, O

are all determined by a

single gene.

Three alleles of this exist IA,

IB, and IO.

IA, IB are codominant, while

IO is recessive to both IA, IB .

A diploid cell can carry only

two alleles, the possiblegenotypes and phenotype

are shown in table 17.2.


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S i h it


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Sex inheritance The X chromosome contains many different genes.

One of them is a gene that codes for the production of aprotein needed for blood clotting, called factor VIII.

There are two alleles of this gene, the dominant one Hproducing normal factor VIII, and the recessive one h resultingin lack of it.

People who are homozygous for the recessive allele sufferfrom the disease haemophilia, in which the blood fail to clotproperly.

This gene is on the X chromosome, and not on an autosome,

affects the way that it is inherited. Females, who have two X chromosomes, have two copies of

the gene. Males, have one copy of the gene. Therefore, thepossible genotypes for men and women are different.

Sex linked- is found on a part of the X chromosome not

matched by the Y, and therefore not found on Y chromosome.


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Paper 4 (J08)


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Paper 4 (J08)

Color blindness is a condition

characterized by the inability ofthe brain to perceive certain

colors accurately.

(a) Explain the meaning of the

terms allele and recessive. [2]Allele:

Recessive:

(b) Explain why females are lesslikely than males to have RGC.

[2]

(c) With reference to the figure,

and using the symbols R for

dominant allele and r for the

recessive allele, state the

genotypes of the individuals 1,

4, 6 and 7. [4]


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Dih d id ( ti 1 1 1 1)


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Dihydrid crosses (ratio 1:1:1:1) Dihybrid crosses look at the inheritance of two genes

at once. In tomato plants, there is gene which codes for stem

colour. This gene has two alleles:

A different gene, at a different locus on a differentchromosomw, codes for leaf shape. There are twoalleles:

Dih d id ( ti 1 1 1 1)


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Dihydrid crosses (ratio 1:1:1:1)

What will happen if a plant which is heterozygous for

both of these genes is crossed with a plant with

green stem and potato leaves?

At fetilisation, any of four type of gamete from theheterozygous parent may fuse with the gametes

from the homozygous parent. The genotypes of the

offspring will be:

Dih d id ( ti 1 1 1 1)


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From this cross, we would expect approximately equalnumbers of the four possible phenotypes.

This 1: 1: 1: 1 ratio is typical of a dihybrid cross between aheterozygous organism and a homozygous recessiveorganism, where the alleles show complete dominance.

Dih d id ( i 9 3 3 1)


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If both parents are

heterozygous. There are sixteen

possibilities.

9 purple, cut:

3 purple, potato:3 green, cut

1 green, potato

9:3:3:1 ratio

This is typical dihydridcross between twoheterozygousorganism.

Dih d id


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Dihydrid crosses

Paper 4 (J06)


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Paper 4 (J06)

The summer squash plant produces fruits that are either

white of yellow in color and are either shaped like a discor a sphere. The dominant phenotypes are white and disc-shaped fruit. Using the symbols A for white and a foryellow and B for disc and b for sphere, draw a geneticdiagram to show what proportion of offspring will have

yellow and sphere-shaped fruit if a white and disc-shapedfruit plant, heterozygous for both genes, is self fertilized.[6]

Paper 4 (J07)


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Paper 4 (J07)

A gene for feather color inchickens is carried on anautosome. This gene has twoalleles, black (CB) and splashed-white (CW). When a malechicken with black feathers ismated with a female chickenwith splashed-white feathers,all the offspring have bluefeathers. This also occurs whena male chicken with splashed-white feathers is crossed with afemale with black feathers.

Another gene may cause stripson feathers (barred feathers).

This gene is carried on the Xchromosome. The allele forbarred feather (XA) is dominantto the allele for non-barredfeathers (Xa). In chickens themale is homogametic and has

two chromosomes while thefemale is heterogametic andhas one X chromosome and oneY chromosome.

Paper 4 (J07)


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Paper 4 (J07) A male chicken with black,

non-barred feathers was

crossed with a female chicken

with splashed-white, barred

feathers. All the offspring had

blue feathers, but the males

were barred and the femaleswere non-barred. Using the

symbols given above, draw a

genetic diagram to show this

cross. [5]

Explain how a farmer coulduse a breeding program to find

out the genotype of a male

chicken with blue, barred

feathers. [3]

P 4 (J07)


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Paper 4 (J07)

The X2 (Chi squared) test


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The X2 (Chi-squared) test

Two plant produced a total of 144 offspring.

If the parents really were both heterozygous, and if the purplestem and cut lead alleles really are dominant, and if the

alleles really do assort independently, then we would expect

the following numbers of each genotype to be present in the

offspring:

But image that, amongst these 144 offspring, the results weactually observed were as follows:

Purple, cut 86 green, cut 24

purple, potato 26 green, potato 8



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We might ask, are these results sufficiently close to ones we

expected that the differences between them have probably justarised by change, or are they so different that something

unexpected must be going on?

We can use a statistical test called the X2 (Chi-squared) test.

This results allows us to compare our observed results with theexpected results, and decide whether or not there is a significant

difference between them.

The first stage in carrying out this test is to work out the expected

results, as were have already done.

These, and the observed results, are then recorded in table like the

one overleaf.

We then calculate the difference between each set of results, and

square each different. (squaring it get rid of any minus signs- it is

irrelevant whether the differences are negative or postive)

Th X2 (Chi d) t t


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The X2 (Chi-squared) test Then we divide each squared

difference by the expected value,and add up all of these answer:



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Now we have X2 value.

We look in a table that relates X2 values to probabilities. The probability given in the table are

E.g. a probability of 0.05 means that we would expect thesedifferences to occur in 5 out of every hundred experiments,or 1 in 20 just by chance.

In biological experiments, we usually take a probability of 0.05as being the critical one.

IfX2 value represents a probabilities of0.05 or larger, thenwe can be fairly certain that the differences between ourobserved and expected results are due to chance- the

differences between them are not significance.



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There is one more aspect of our result to consider, before we

look up our value of X2 in the table- degree of freedom. This takes into account the number of comparisons made.

(Remember that to get our value of X2 , we added up all our

calculated values, so obviously the larger the number of

observed and expected values we have, the larger X2

is likelyto be, we need to compensate for this)

To work out the number of degrees of freedom, simply

calculate the (number of classes of data-1). (4-1)= 3

We can look a the table to determine whether our resultsshow a significant deviation from what we expected.



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Our calculated value is 0.79, the value is a much smaller valuethan the one we have read from the table.

It would be way off the left hand side, representing a

probability of much more than 0.1 (1 in 10) that the differencein our results is just due to chance.

So we can say that the difference between our observed andexpected results is almost certainly due to chance, and thereis no significant difference between what we expect, and

what actually got.


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Paper 4 N09


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Paper 4 N09Complete the missing values in

the table. [3]

Calculate the value for chi-

square. [1]

Table below relates chi-square

values to probability values. As

four classes of data were counted,

the number of degrees of freedom

was 4-1=3. Table gives values of

chi-square where there are three

degrees of freedom.

Explain whether or not the

observed results were significantly

different from the expected results.

[2]


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Mutation


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Mutation

Mutation- an unpredictable change in the genetic material

of an organism. 2 type of mutation

1. Gene mutation- a change in the structure of a DNAmolecule, producing a different allele of a gene.

2. Chromosome mutations- mutation changes in thestructure of or number of whole chromosomes in a cell.

Mutations may occur completely randomly with no obviouscause, but there are several environment factors thatsignificantly increase the chances of a mutation occurring

(mutagen)

1. ionising radiation- (, , )

2. Ultraviolet radiation

Note: Mutagen- a substance that can cause mutation.

Mutation (gene mutation)


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Addition



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Base additions or deletions usually have a very significant

effect on the structure, because they alter every set of threebases that follow them in the DNA molecule. Therefore thefunction, of the polypeptide that the allele codes for.

They are said to cause frame shifts in the code. It mayintroduce a stop triplet part way through a gene, so that a

complete protein is never made at all. Base substitution may have no effect at all.

A mutation that has no apparent effect on an organism is saidto be a silent mutation. E.g. CCT to CCA or CCG makes still

make Gly. However, base substitution can be very large effect. E.g. base

sequence ATG (coding for Tyr) mutated to ATT, this hasproduced a stop triplet, so the synthesis of protein would

sop at this point.


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Sickle cell anaemia

Mutation due to base substitution at the genethat codes for polypeptide chains ofhaemoglobin

CTT which codes for glutamic acid mutates toCAT which codes for valine

Mutant polypeptide chains are less solubleand forms fibres in RBCs Sickle-shapeInefficient in O2 transportation and get stuckin blood capillaries


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Sickle cell anaemia

Recessive allele HS codes for the production of

mutant polypeptide chains that causes sickle

RBCs

HSHS Sickle cell anaemia

HNHS Carrier

HNHN Normal

Common in Africa


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Sickle cell anaemia and malaria

In malarial region, carriers of sickle cell allele(HNHS) can survive better, so HS allele isfrequent

HS

HS

Die from sickle cell anaemia HNHN Die from malaria

HNHS Not suffer from sickle cell anaemia and

less likely to suffer from malaria

Able toreproduce and pass on the HS alleleSelective advantage


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Phenylketonuria (PKU)

Phenylketonuria (PKU) a genetic disease resulting from amutation from in gene that codes for an enzyme involved in

the metabolism of Phenylketonuria.

Mutation happens on gene that codes for production of

enzyme phenylalanine hydroxylase

Complications of PKU:

Less melanin Lighter skin and hair colour

Phenylalanine accumulates in blood and tissue fluid which

cause mentally retarded and brain damage in children.Phenylalanine hydroxylase

Phenylalanine tyrosinemelanin


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Converted

into Malanin

Phenotype affected by both


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1. Diet and nutrients- Height and weight

2. Low temperature

- Dark fur color on Himalayan rabbits

3. High temperature

- Gender of reptiles

4. Ultraviolet rays

- Production of melanin5. Wavelength of light

- Flowers and fruit color

Phenotype affected by bothenvironmental and Gene

d f l


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Production of Insulin

Injection of insulin is needed for diabetes

mellitus patients

Insulin extracted from animal sources is

expensive, has side-effects and ethical /religious issues

Therefore, production of insulin through

genetic engineering is encouraged



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1. Isolating the insulin gene- mRNA for insulinextracted from cells(human pancreas)

- Incubate mRNA withreverse transcriptaseenzyme

- Complementary DNA(cDNA) is formed

- DNA polymerase is addedto form complete double-helix structure



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2. Inserting the gene into avector

- Vector Small carrier inwhich fragments of DNAcan be inserted, eg. Plasmid

- Plasmids are centrifugedand isolated from bacteria

- Enzyme restrictionendonuclease is used to cutthe plasmids with stickyends

- DNA ligase to join insulingene into plasmidsRecombinant DNA



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3. Inserting the gene into the bacteria

- Plasmids with recombinant DNA mix with bacteria

- Bacteria that take up plasmids are isolated and cultured in

large scale

- Extraction and purification of recombinant insulin




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U f h l


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Uses of gene technology Medical: Synthesis of growth hormones, recombinant Factor

VIII to treat haemophiliac and production of pharmaceuticaldrugs.

The human gene for making factor VIII has been inserted intohamster kidney and ovary cells that are then cultured infermenters. The cells constantly produce factor VIII, which is

extracted and purified before being used to treat people withhemophilia.

Before availability of the recombinant factor VIII, came fromhuman and this carried risks of infection, such as HIV.

Agriculture: Genetically modified crops against pests andinsecticides, genetically modified organisms with desirabletraits and better yields

Industry: Biological washing powder and production ofgenetically modified food

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