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Mark schemes for Topic 3
3.1
Define the
terms gene and allele and
explain how they differ. 4
marks
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3.1
Describe the
consequences of a base
substitution mutation with
regards to sickle cell
anemia. 7 marks
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3.1
Explain the
consequences of a
base substitution
mutation in relation
to the processes of
transcription and
translation.
gene is a heritable factor / unit of inheritance
gene is composed of DNA
gene controls a specific characteristic / codes for a
polypeptide / protein
allele is a form of a gene
alleles of a gene occupy the same gene locus / same
position on chromosome
alleles differ (from each other) by one / a small number of
bases(s)/ base pair(s)
the sequence of nucleotide bases in DNA codes for the
sequence of amino acids in proteins
DNA is transcribed into mRNA, which is translated into
amino acids of protein
normal (ß chain) hemoglobin gene / DNA produces
normal (ß chain) hemoglobin protein / amino acids
substitution= the replacement of one (or more) nucleotide
base with another
caused by a copying mistake during DNA replication
as a result of a mutagen / X-rays / chemical / UV radiation
/ other mutagen
mutation in normal (ß chain) hemoglobin gene alters the
sequence of nucleotide bases
normal nucleotide sequence = CTC altered to CAC
resulting in altered mRNA (GAG to GUG) during
transcription
resulting in altered sequence of amino acids in (ß chain)
hemoglobin protein (glutamic acid to valine) during
translation
causing red blood cells to change shape / sickle under
low oxygen conditions
causing sickle cells anemia when two copies of the
mutated gene are inherited
producing a sickle cell carrier when one copy of the
mutated gene is inherited
sickle cells anemia reduces oxygen flow to organs,
leading to their deterioration
mutation is a change in the genetic make-up;
base substitution mutation occurs when one (nitrogenous)
base in DNA chain is
replaced by another;
this is a gene mutation / change in the base sequence of
a gene;
effect of mutation ranges from no effect / no change in
amino acid sequence to
drastic changes;
sickle-cell anaemia involves change in gene for one of
polypeptides in
hemoglobin / Hb / HBA;
GAG has mutated to GTG (on DNA);
adenine replaced by thymine in DNA;
transcription of DNA produces the triplet GUG instead of
GAG on mRNA;
one codon is different in mRNA;
new codon is for valine rather than glutamic acid;
tRNA brings amino acid to ribosome during translation;
different amino acid placed in polypeptide chain being
formed by translation;
the two amino acids differ in solubility / have different
properties / valine causes
S
HB to be less soluble;
causes red blood cells to become sickle shaped / carry
oxygen less efficiently;
S
HB allele causes sickle-cell anaemia but gives resistance
to malaria;
3.2
. Karyotyping involves
arranging the
chromosomes of an
individual into pairs.
Describe one application
of this process, including
the way in which the
chromosomes are
obtained. 5 marks
3.2
State the two classes of
compounds that
compose
chromosomes in
animal cells. (1)
3.2
Describe how sexual
reproduction
promotes genetic
variation within a
species. (4 marks)
3.3
Outline the formation of
chiasmata (bivalent)
8 max
application of karyotyping {2 max}
 find gender / test for Down's syndrome / other
chromosome abnormality
 identify sex chromosomes / numbers of chromosome 21 /
other chromosomes counted
 XX = female and XY = male / third chromosome 21
indicates Down's syndrome / other chromosome
abnormality (e.g. Klinefelter's syndrome)
obtaining chromosomes {3 max}
 fetal cells obtained from amniotic fluid / amniocentesis /
other named source
 white blood cells obtained
 cells encouraged to divide
 cells accumulated / blocked in metaphase
 prepare slide / chromosomes examined
DNA and protein
(segregation of alleles involves) meiosis;
crossing over / chiasma formation in prophase I / meiosis (do not
allow if
wrong phase of meiosis given);
random orientation / assortment of homologues at metaphase I;
fertilization by chance / one of many male gametes;
number of different gametes is 2n (ignoring crossing over);
genes / alleles combined from two parents;
Accept the points below in an appropriately annotated
diagram.
during crossing over. 5
marks
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crossing over/chiasmata formed during prophase I of
meiosis;
pairing of homologous chromosomes/synapsis;
chromatids break (at same point); (do not accept
chromatids overlap)
non-sister chromatids join up/swap/exchange
alleles/parts;
X-shaped structure formed / chiasmata are X-shaped
structures;
chiasma formed at position where crossing over occurred;
chiasmata become visible when homologous
chromosomes unpair;
chiasma holds homologous chromosomes together (until
anaphase); 5 max
3.3
Explain how an error in
meiosis can lead to Down
syndrome. 8 marks
Accept the points below in an appropriately annotated
diagram.
 non-disjunction;
 chromosomes/chromatids do not separate / go to same
pole;
 non-separation of (homologous) chromosomes during
anaphase I;
 due to incorrect spindle attachment;
 non-separation of chromatids during anaphase II;
 due to centromeres not dividing;
 occurs during gamete/sperm/egg formation;
 less common in sperm than egg formation / function of
parents' age;
 Down syndrome due to extra chromosome 21;
 sperm/egg/gamete receives two chromosomes of same
type;
 zygote/offspring with three chromosomes of same type /
trisomy / total 47 chromosomes;
3.3
Compare the processes of
mitosis and meiosis. 6
marks
answers must be pair-wise comparisons to receive any marks.
 Mitosis: one cell division & Meiosis: two divisions / reduction
division
 Mitosis: chromosome number does not change & Meiosis:
converts diploid to haploid cells
 Mitosis: products genetically identical & Meiosis: products
genetically diverse
 Mitosis: separation of sister chromatids in
anaphase & Meiosis: separation of homologous
chromosomes in anaphase I and sister chromatids in
anaphase II
 Mitosis: no crossing over & Meiosis: crossing over in
prophase I
 Mitosis: no formation of tetrads / no synapsis & Meiosis:
formation of tetrads / synapsis
 Mitosis: produce cells for growth/repair/asexual
reproduction & Meiosis: produce sexual cells / gametes for
sexual reproduction
 Mitosis: two cells produced & Meiosis: four cells produced
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3.3
Outline how meiotic
division results in
almost an infinite
genetic variation in
the gametes
produced. 2marks
Mitosis: daughter cells with both copies of
chromosomes/random assortment does not
occur & Meiosis: random assortment of maternal/ paternal
chromosomes
Mitosis: replication of DNA in interphase & Meiosis:
replication of DNA in interphase I
Mitosis: four phases: prophase, metaphase, anaphase,
telophase & Meiosis: same four phases twice
crossing over / chiasmata;
shuffles alleles;
random orientation of chromosomes;
at metaphase I:
at metaphase II;
2 max
3.3
Describe, with the aid of
a diagram, the
behaviour of
chromosomes in the
different phases of
meiosis.
(Total 5 marks)
chromosomes condense / coil /
become shorter and fatter during prophase I;
(homologous) chromosomes pair up in prophase I;
crossing over / chiasmata formation in prophase I;
movement of pairs of chromosomes /
bivalents to the equator in metaphase I;
movement of half of the chromosomes to each pole in
anaphase I;
movement of chromatids to opposite poles in
anaphase II;
decondensation / uncoiling in telophase II;
[4 max] if no diagram is shown.
Do not award a mark for a statement if a diagram has
been drawn that does not fit in with the statement.
For example, if the candidate states that pairs of
chromosomes move to the equator in metaphase I
but shows single chromosomes, do not award that
mark.
3.3
Explain how meiosis
random orientation of bivalents / pairs of chromosomes;
and fertilization
maternal and paternal chromosome could go to either
can give rise to
pole;
genetic variety.
2n combinations;
(Total 6 marks)
eg over 8 million in humans;
crossing over;
exchange of material between homologous
chromosomes /
non-sister chromatids;
segregation of alleles in meiosis;
combinations of alleles are broken up;
fertilization brings together genes /
alleles from two different parents;
fertilization generates new combinations of genes /
alleles;
random fertilization /
many possible combinations of male and female
gamete;
eg over 64 million million in humans (ignoring crossing
over); [6]
3.4
Outline one example of
inheritance involving
multiple alleles. 5 marks
multiple alleles means a gene has three or more alleles /
more than two alleles
 ABO blood groups / other named example of multiple
alleles
 ABO gene has three alleles / equivalent for other example
 IA IB and i shown (at some point in the answer) /
equivalent for other example
accept other notation for alleles if clear
 any two of these alleles are present in an individual
 homozygous and heterozygous genotye with phenotypes
(shown somewhere)
 all six genotypes with phenotypes given (shown
somewhere)
 example / diagram of a cross involving all three alleles
3.4
Describe the inheritance
of ABO blood groups
including an example of
the possible outcomes of
a homozygous blood
group A mother having a
child with a blood group
O father. 5 marks
example of co-dominance
multiple alleles / 3 alleles
(phenotype) O has (genotype) ii
B can be IB IB or IB i
A can be IA IA or IA i
AB is IA IB
(P are) i i x IA IA
(gametes) i and IA
(F1 genotype) IA i
(F1 phenotype) blood group A
accept other notations if used consistently and if phenotype and
genotype are clearly distinguished
3.4
Outline sex linkage. 5
marks
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3.4
Explain, using a named
example, why many sexlinked diseases occur
more frequently in men
than women. 9 marks
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gene carried on sex chromosome / X chromosome / Y
chromosome
inheritance different in males than in females
males have only one X chromosome therefore, only one
copy of the gene
mutation on Y chromosome can only be inherited by
males
women can be carriers if only one X chromosome
affected
example of sex linked characteristics (e.g. hemophilia /
color blindness)
example of cross involving linkage
named example of sex-linked disease
caused by recessive allele
on the X chromosome
example of pair of alleles (e.g. X H and X h) (reject if alleles
do not correspond)
females are XX and males are XY
females have two alleles of the gene and males have only
one
allele causing the disease is rare / uncommon
probability of females inheriting rare allele twice as low
calculation of squaring the gene frequency
female would have to inherit the allele from her father
who would have suffered from the disease
so females can carry the gene but still be normal
but males (with the gene) will have the disease
non-disjunction
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3.4
State how
chromosome number
can increase in human
beings.
1 mark
3.4 Define the term coalleles of gene / pairs of alleles which both affect the
dominant alleles.
phenotype / both expressed
(1)
(when present together in an individual / in the
heterozygote);
1
Reject: both dominant / both recessive
3.5 Describe the
technique for
the transfer of
the insulin gene
using E. coli.
(6)
3.5
3.5
Describe the
application of DNA
profiling to determine
paternity and also in
forensic investigations.
mRNA is extracted;
DNA copy of RNA is made using reverse transcriptase;
plasmids are cut open with endonucleases (at specific
sequences);
insulin gene and plasmid are mixed together;
addition of “sticky ends” to the DNA copy (so that it
will combine with
the cut plasmid);
DNA ligase will seal the plasmid;
recombinant plasmid is inserted into E. coli;
E. coli is cultured;
E. coli begins to make insulin;
6 max
Mark Scheme
A. sample of DNA / blood / saliva / semen is obtained;
B. satellite DNA / repetitive sequences used for profiling;
C. reference samples of DNA are obtained;
D. PCR used to amplify / produce more copies of the DNA;
E. DNA broken into fragments by restriction enzymes;
F. DNA fragments are separated by gel electrophoresis;
G. separation according to the length of the fragments;
H. pattern of bands obtained / different pattern of bands
with DNA from different individuals;
I. bands compared between different DNA samples;
J. if pattern of bands is the same then DNA is (almost
certainly) from same source;
K. if some bands are similar then individuals are (almost
certainly) related;
L. used for criminal investigations / example of use in criminal
investigation;
M. used to check paternity / who is the father / mother /
parent;
N. used to check whether two organisms are clones;
Outline three outcomes
of the sequencing of the
complete human
genome.
3.5
Discuss the ethical
issues for and
against the use
of transgenic
plants. 6 marks
Mark Scheme
A. may lead to an understanding of
genetic / inherited diseases /
conditions;
B. may lead to the production of
gene probes to detect carriers of
genetic diseases;
C. may lead to the production of
pharmaceuticals based on DNA
sequences;
D. study of similarities / differences
between human race / population;
E. find location of genes / produce a
complete gene map;
F. study of human origins / migration /
relationships with other species;
Responses must have at least two arguments for and two
arguments against
to receive full marks.
arguments for:
transgenic plants do not survive long in wild;
reduced cost of food production / reduce amount of
land used for food production;
transfer of genes occurs naturally;
DNA and proteins are digested (unlikely to cause
problems);
longer shelf life for certain products;
increased yield / bred for faster growth;
better adaptation to certain conditions / eliminate
possible plagues;
healthier food produced because use of pesticides
can be decreased eg Bt corn /
increase food quality eg albumin gene has increased
sulfur amino acid content of
alfalfa used to feed sheep;
arguments against:
uncertain long-term health effects from eating
transgenic crops / may contain
animal genes;
new proteins in food crops could cause allergies;
genes could escape from transgenics to wild plants;
create superweeds / damage ecosystems;
monopoly (created by large companies) adversely
affects small farmers;
[6]
3.5
Outline a basic
technique used for gene
transfer involving
plasmids, a host cell
(bacterium, yeast or
Mark Scheme
A. mRNA is extracted;
B. DNA copy of RNA is made using reverse transcriptase;
C. plasmid is a small piece of circular DNA;
D. plasmid removed from (host) cell;
other cell), restriction
enzymes
(endonucleases) and
DNA ligase.
E. plasmids are cut open with endonucleases (at specific
sequences);
F. genes / DNA fragments from another organism cleaved by
same restriction enzyme;
G. gene of interest and plasmid are mixed together;
H. addition of “sticky ends” to the DNA copy (so that it will
combine with the cut plasmid);
I. DNA ligase will seal the plasmid;
J. recombinant plasmid is inserted into E. coli/host cell;
K. E coli is cultured;
L. E coli begins to make protein coded by the gene of interest;
Discuss the potential
benefits and possible
harmful effects of one
example of genetic
modification.
Mark Scheme
Possible benefits:
A. benefits include more specific (less random) breeding than
with traditional methods;
B. faster than traditional methods;
C. some characteristics from other species are unlikely in the
gene pool / selective breeding cannot produce desired
phenotype;
D. increased productivity of food production / less land
required for production;
E. less use of chemicals (eg pesticides);
F. food production possible in extreme conditions;
3.5
H. eg pharmaceuticals in milk;
I. human insulin engineered so no allergic reactions;
J. may cure genetic diseases;
K. allows use of herbicide on growing crop;
L. higher yield due to less weed competition;
M. weeds that are very similar to the crop plants can be
controlled;
Possible harmful effects:
N. some gene transfers are regarded as potentially harmful to
organism (especially animals);
O. release of genetically engineered organisms in the
environment;
P. can spread and compete with the naturally occurring
varieties;
Q. some of the engineered genes could also cross species
barriers;
R. technological solution when less invasive methods may bring
similar benefits;
S. reduces genetic variation / biodiversity;
T. uncertainty about long-term effect;
U. uncontrollable superweeds might be produced;
V. foreign DNA in the crop plant might cause allergies in humans;
W. fewer weeds for wildlife that feed on them;
Examples:
X. name of organism that was genetically modified;
Y. source of the DNA / gene used to modify organism;
Z. effect of the gene / characteristic coded for by the gene;
AA. reference to gene transfer between species being a natural
process (with viral vectors);
BB. wheat / maize / other crop plant;
CC. Salmonella typhimurium;
DD. resistance to glyphosate / roundup herbicide;
3.5
Discuss the ethical issues
of therapeutic cloning in
humans.
genetic screening is testing for the presence or absence of
gene /
chromosome;
screening for chromosomes can involve karyotyping;
genetic screening is controversial;
advantages: [4 max]
parents can choose to avoid having children with disorder;
parents can prepare for a child with a disorder;
parents can use IVF to select embryos that are normal;
can use gene therapy to correct the problem;
treatment can start to prevent symptoms;
fewer children with the disorder are born;
disadvantages: [4 max]
frequency of abortion can increase;
parents can select embryos for sex of the child;
can have harmful side effects such as depression if you
know you will
develop a disorder later;
can create a genetic underclass;
health insurance / treatment can be denied if there is
genetic predisposition;
8 max
3.5
Outline how the
process of
meiosis
can lead
to Down’s
Syndrome.
(4)
Accept any of the points below if clearly drawn and
correctly
labelled in a diagram.
in metaphase homologues in centre of cell / spindles
attached;
homologues are separating;
one pair doesn’t separate / non-disjunction;
in telophase cells divide into two;
cells have either one more / one less chromosome;
can occur in second division of meiosis;
sister chromatids fail to separate;
fertilization with one gamete / sperm / egg carrying extra
chromosome;
Down’s syndrome is trisomy of chromosome 21;
4 max
3.5
State two examples of
transgenic techniques in
agriculture involving
animals
sheep (milk) produce factor IX for blood clotting;
winter flounder fish gene to make tomatoes frost resistant;
α-1 antitrypsin in sheep;
interferon in cow’s milk;
spider silk protein into sheep;
2 max
Award any other examples which involve animals as
sources of genes or as the genetically modified organism.
(5 marks)
3.5
State two
procedures used for
the preparation of a
DNA profile.
polymerase chain reaction and electrophoresis