Meiosis, Variation & Genetics · Final Mark Scheme 2804/01 January 2004 Question Expected Answers...

OCR 215

Meiosis, Variation & Genetics

Mark Scheme

2804 Mark Scheme June 2003

Question Expected Answers Marks

2 (a) parental genotypes CrCr x CwCw;gametes Cr , Cw;

F1 genotypes (all) CrCw

F1 phenotypes (all) pink;

(F1 genotypes and phenotypes 1 mark )

gametes Cr , Cw Cr , Cw;

F2 genotypes CrCr CrCw CrCw CwCw

F2 phenotypes red pink (pink) white;

(F2 genotypes and phenotypes 1 mark)

F2 ratio 1:2:1;

accept other symbols if key given. accept r and w as symbols without key. 6

(b) (i) 65; 130; 65; 3

(ii) 0.138 + 0.007 + 0.061; (or other suitable working) 0.206 – 0.208; 2 marks for correct value if no working shown ecf for both marks but calculated value must be to three decimal places 2

(iii) support, figure lower than 5.991 / figure lower than critical value; R ‘support’ on its own. ecf applies if value in (ii) is incorrect 1

(c) named characteristic; named environmental factor;

mark first example only 2

[Total: 14]

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2804 Mark Scheme January 2007

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4 (a) (i) crossing over; treat chiasma(ta) as neutral 1 (ii) prophase; 1 (iii) have different, alleles/base sequence of DNA;

A sister chromatids have same alleles/non sister have different alleles

1 (b) two different genes represented in each gamete ie Q or q and R or r;

four correct combinations ie Q and R, Q and r, q and R, q and r; 2 (c) (i) (parental genotypes:) AaBb x aabb;

(gametes:) AB, Ab, aB, ab (all) ab; (offspring genotypes:) AaBb, Aabb, aaBb, aabb; (offspring phenotypes:) grey body/normal wing, grey body/bent wing, black body/normal wing, black body/bent wing; [sequence of phenotypes must match genotypes for mark] (phenotypic ratio:) 1 : 1 : 1 : 1; apply ecf. accept alternative symbols if a key is given, but if no key given max 4 5

(ii) 80,80,80,80; 1 (iii) (working) 0.1125 + 0.3125 + 0.05 + 0.45;

= 0.925; A 0.9/0.92/0.93 2 marks for correct answer with no working. ecf if correctly use wrong figures from (ii) 2

(iv) yes (but no mark for yes on own)

as calculated figure is smaller than 7.82; ecf applies to value calculated in part (iii) 1

[Total: 14]

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Final Mark Scheme 2804/01 January 2004


4 (a) species

similar morphological, physiological, biochemical and behavioural features ;(require 3 features for mark) (inter)breed / reproduce ;produce fertile offspring ;reproductively isolated ;occupy same (ecological) niche ; 3 max

(b) geographical / mountains / distance ;allopatric ; 2

(c) 1234

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associated with human settlements ;mouse population scattered ;mountain ranges / geographical barriers / distance ;no breeding / gene flow / mixing, between populations / separate breeding populations ;random mutations occur ;different selection pressures / example of, in different areas ;different alleles selected for or change in, gene pool / allele frequency ;develop different chromosome numbers ;AVP ; e.g. founder effect genetic drift 5 max

(d) 1

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6

unable to reproduce / unsuccessful breeding / infertile offspring / no offspring, produced ;due to different (parental) chromosome numbers / AW ;no pairing of chromosomes (in offspring) ;meiosis unable to take place ;no gametes produced (by offspring) ;

hybrid vigour / heterosis ; 4 max

[Total: 14]

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Final Mark Scheme 2804/01 June 2004


5 (a) A A no trim A as no trim A ac no trim; three correct for 1 mark

as as sable as ac sable; two correct for 1 mark

ac ac copper; 1 mark

alternative symbols accepted if key given - ecf section (b) 3

(b) parental genotypes A ac A ac ; gametes A ac A ac ;

(gamete and genotype marks can be credited in Punnet square)

offspring genotypes A A A ac A ac ac ac ; offspring phenotypes no trim no trim no trim copper ;

If only one heterozygote shown in offspring genotypes lose genotype mark but ecf for offspring phenotypes 4

(c) (i) test cross / backcross; A cross with, copper / ac ac 1

(ii) homozygous, all offspring sable; if any offspring are copper; must be heterozygous / must carry ac allele; 3

(d) multiple alleles more than two; forms / varieties, of a gene;

locusposition of a gene / allele; on a chromosome / length of DNA 4

[Total : 15]

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3 (a) parental genotypes RrBb x Rrbb ; gametes RB Rb rB rb Rb rb ; offspring genotypes RRBb RrBb (RrBb) Rrbb RRbb (Rrbb) rrBb rrbb ; offspring phenotypes rough black rough white smooth black smooth white ; expected ratio 3 : 3 : 1 : 1 ; accept correct gametes, offspring genotypes and offspring phenotypes in Punnett square use ecf except for ratio Reject the ratio 6 : 6 : 2 : 2 ratio not a stand alone mark – there must be some correct working to support it 5

length of DNA ; codes for a (specific), polypeptide / protein / RNA ; found at a, locus / particular position on, a chromosome ; 2

(b) (i)

variety / form of a gene ; R type of gene A type of a gene 1 (ii) assume the allele = coat colour allele

(coat colour) gene / alleles, only on X chromosome ; A no (coat colour), gene / allele, on Y chromosome male cats, XY / only have one X chromosome ; (males have) only one (coat colour) allele / cannot have two (coat colour) alleles ; need black and orange alleles for tortoiseshell colour ; max 2

(c) 1

2 3 4 5 6 7 8 9

ref to operon ; normally repressor substance bound to operator ; prevents RNA polymerase binding (at promoter) / prevents transcription ; lactose binds to repressor ; changes shape of protein molecule ; unable to bind (to operator) ; RNA polymerase binds (at promoter) / transcription occurs / genes switched on ; production of lactose permease ; production of beta – galactosidase ; max 5

[Total: 15 ]

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5 (a) RR RR - low, do not have enough vitamin K in diet / ref to figures ; RRRS - high, (warfarin resistant) and have enough vitamin K / ref to figures ; RSRS - low , will be killed by warfarin / ref to effects of warfarin ; If quote probabilities for survival less than 50% is low and over 50% is high 3

(b) (i) mutation / named mutation ;

change in DNA base sequence ; max 1 (ii) variation within population ;

some individuals produce enzyme not susceptible to warfarin ; these individuals survive / selective advantage ; reproduce / breed ; pass, resistance / advantageous allele , to offspring ; R gene those without resistance die ; ref to selective pressure of warfarin ; max 5

(c) does not directly involve humans ;

environment selects individuals that will reproduce ; max 1 (d) resistant allele / RR , will decrease and , susceptible allele / RS , will increase ;

RRRR at a disadvantage due to vitamin K requirements / RSRS at an advantage due to warfarin being removed ; A frequencies of both alleles will stay the same ; must be linked to second statement no longer any selective pressure / no directional selection ; max 2

[Total: 12 ]

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Abbreviations,

annotations and

conventions used in

the Mark Scheme

/ ; NOTR ( ) ___ ecf AW A ora

= = = = = = = = = =

alternative and acceptable answers for the same marking point separates marking points answers which are not worthy of credit reject words which are not essential to gain credit (underlining) key words which must be used to gain credit error carried forward alternative wording accept or reverse argument


1 (a) (i) Q, S, P, N, M, R ; 1 (ii) accept correct names of stages

Q ; A prophase 1 M ; A anaphase 2 Q / S ; A prophase 1 / metaphase 1 S ; A metaphase 1 R ; A telophase 2 5

(iii) DNA replication ;

synthesis of proteins / named protein ; A transcription / translation synthesis of membrane ; synthesis of, organelle(s) / named organelle ; respiration ; AVP ; e.g. centrioles replicate ; 2 max

(b) (i) Individual 2 - XHY ;

Individual 5 - XhY ; Individual 6 - XHY ; Individual 9 - XHXh ; max 2 if sex chromosomes not shown 4

(ii) half / 0.5 / 50% / 1 in 2 ; A 1:1, 50:50 R 1:2 1 (iii) carriers have, both / H and h / dominant and recessive, alleles ; A are heterozygous

R two alleles females have two X chromosomes / ora ; 2

[Total: 15]

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6 (a) form of a gene ; position of, gene / allele on, chromosome / DNA ; 2

(b) 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19

Woodland more, dark / unbanded, snails or fewer, light / banded, snails ; better camouflaged / ora ; against, leaf litter / uniform background ; relevant woodland data quote on colour and banding ; Grassland more, yellow / banded, snails or fewer, dark / unbanded, snails ; better camouflaged / ora ; (only award if missed point 2) against, pale / yellow / green / variable, background ; relevant grassland data quote on colour and banding ; survivors posses advantageous alleles / ora ; reproduce ; pass alleles on (to, offspring / next generation) ; ref to stabilising selection (in both habitats) ; ref to other named selection pressure(s) ; not a very mobile population or little, immigration / emigration ; separate gene pools described ; little mutation taking place ; A no new camouflage method over time habitat stable ; ref to why unfavourable alleles have not disappeared ; AVP ; e.g. calculated average figures for both habitats max 8

QWC – clear, well organised using specialist terms ;

clear and well organised and must include marking points 4 and 8 1

[Total: 11]

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2 (a) (i) protein / polypeptide; R enzyme, peptide 1

(ii)123

binds / forms complex, (with lactose); alters structure / changes shape; repressor molecule detaches / cannot bind; Ignore where it binds

max 2

(iii) operator; 1

(b) (i) observable characteristics of an organism / AW; A expression of an allele

1

(ii)1234567

altered DNA; altered mRNA (of repressor); altered amino acid sequence (of repressor); A no repressor produced change to, structure / shape (of repressor); (repressor protein) unable to, bind to / block, operator; RNA polymerase can bind to promoter; transcription / translation (continues);

max 4

(c) (i) Xr Xr ; Xr Y; A Xr - XR Xr / XrXR ; XR Y; A XR –

4

(ii) altered shape/ non functional / no, carrier protein; less / no, (re)absorption of phosphate or more / all, phosphate excreted; from glomerular filtrate; low phosphate levels in blood / bones;

max 2 [Total: 15]

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Question Expected Answers Marks2 (a) (i) class ;

order ; Canis ; lupus ; A Canis lupus / C. lupus 4

(ii) capable of interbreeding ; A reproducing, mating producing fertile offspring ; belong to same gene pool / reproductively isolated ; have a common ancestry ; occupy the same (ecological) niche ; have similar morphological, biochemical, behavioural, physiological and anatomical features (must name two of these for mark) ; 2 max

(b) 1 2/3

45

6

7

selected by humans ; two examples of characteristic ; ; e.g. long back, muscular back, long legs, small feet, fast allowed to, mate / AW ; offspring with ideal characteristics chosen to mate ; A best offspring chosen (repeated) over many generations ; allele frequency for desired characteristics increases / ref to directional selection ; precautions against, inbreeding / selection of undesirable traits ;

5 max

(c) (i) Jackal - sympatric / behavioural / ecological / temporal ; ignorereproductive isolation unqualified Dingo – geographical / allopatric ; 2

(ii) 1

2

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all forms (fertile) hybrids with domestic dog ; A one named species forming (fertile) offspring with domestic dog (evidence suggests) domestic dog not separate species ; must be linked to mp1 red wolf and coyote form hybrids ; R fertile hybrids may / may not, be fertile ; may / may not, be separate species ; (three types of) jackals do not interbreed ; (evidence suggests) they are separate species ; must be linked to mp 6

3 max

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Question Expected Answers Marks4 (a) more than two ;

forms / varieties, of a gene ; interaction of genotype and environment ; observable / measurable, characteristics (of organism) ; A physical features 4

(b) parental genotypes: IAIo IBIo ;

gametes: IA Io IB Io ; children genotypes: IA IB IAIo IBIo IoIo ; children phenotypes: AB A B O ;

max 3 if fail to use symbols given 4

(c) ¼ X ¼ ; 1/16 / 1 in 16 / 0.0625 / 6.25% ; apply ecf if incorrect phenotypes in section b

2 marks awarded for correct answer without any working

2

1

2

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idea of identifying those most at risk / ref screening ; A early diagnosis identify faulty, genes / alleles / DNA , or identify carriers ; (prenatal) diagnosis can allow early treatment / termination / selection of embryo ; genetic counselling / life style advice ; drugs targeted for individual / AW ; gene therapy ; problem with, false positives / negatives ; idea of identifying disorders for which there is no treatment ; leading to e.g. discrimination / insurance problems / employment problems ; 3 max

[Total: 13]

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Question Expected Answers Marks3 (a) crossing over

(equivalent portions) of homologous chromosomes / (non sister) chromatids ; are exchanged / AW ; new combinations of alleles ; during prophase 1 ; at chiasma(ta) ; independent assortment of homologous chromosomes bivalents / pairs of (homologous) chromosomes ; line up on, equator ; during metaphase 1 ; each pair lines up independently of others / randomly ; results in gametes that are genetically unique / AW ; max 6

(b) artificial selection natural selection

selection pressure by humans environmental selection pressures ; genetic diversity lowered genetic diversity remains high ; inbreeding common ; outbreeding common ; loss of vigour ; increased vigour ; proportion of heterozygotes proportion of heterozygotes in in population is reduced population remains high ; genetic isolation mechanisms genetic isolation mechanisms operate ; do not operate (usually) faster than natural (usually) slower than artificial selection selection ; selected feature not always selected feature always to organism’s to organism’s advantage ; advantage ;

ignore ethical and economic differences max 3

(c) (i) hhCRCW ; 1

(ii) Parental genotypes: hhCRCW HhCWCW ;

apply ecf if hornless genotype is HH Gametes: hCR hCW HCW hCW ; Offspring genotypes: Hh CRCW , hh CRCW, HhCwCW , hhCWCW ; Offspring phenotypes: hornless roan, horned roan, hornless white, horned white ; Phenotypic ratio: 1 1 1 1 ; 5

[Total: 15]

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(a) (i) T ; R ;V ; W ; S ; only mark if one letter in each box 5

2

(ii) E –centromere ; F-chromatid / locus ; G-chromosome ; 3

(iii) centrioles absent ; aster absent ; involved in spore formation in plants and gamete formation in animals ; no, telophase 1 / nuclear membrane formation in telophase 1 ;

1 max

(iv) 3 ; 1

(b) 123456789

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121314

15

crossing over ; (between), chromatids / homologous chromosomes ; A within bivalent prophase 1 ; (linked to crossing over / description of crossing over) ref to chiasma(ta) ; (results in) new combinations of alleles; A recombinants chromosome mutation ; independent / random, assortment ; bivalents / chromosomes ; (linked to 7) metaphase 1 ; (linked to 7) (independent assortment) of chromatids ; metaphase 2 ; (linked to 10) 6 max from points 1 to 11 and 15

gametes genetically different / AW ; random fusion of gametes ; random mating (within population) ; AVP ; e.g. linked genes separated during crossing over, named chromosome mutation annotated diagrams are acceptable 7 max

QWC – clear well organised using specialist terms

Need to use four of the following: crossing over, chiasma(ta), allele, mutation, bivalent, independent / random assortment, gametes. 1

[Total: 18]

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5 (a) (i) 0.6 X 100 or 0.8 x100 ; 11.2 9.4 or 100 – (10.6/11.2 X100) or 100 – (8.6./9.4 X100) ; beak length: 5.4% (A 5.36%) ; beak depth: 8.5% (A 8.51%) ; If no working shown 2 marks for one correct answer 3 marks for two correct answers. 3

(ii) food / seed, shortage / AW ; A competition for food competition from large ground finch / interspecific competition ; R competition unqualified 2

(iii) no, competition from large ground finch / interspecific competition ; A no large ground finches present. seed size larger ; A size / hardness, of seeds has changed 1 max

(iv) 12

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10

variation (in beak size) ; those with large beaks in competition with large ground finch ; A struggle for survival idea medium ground finch out competed ; smaller size beaks for opening small seeds ; less (interspecific) competition for small seeds ; birds with small beaks more likely to survive ; reproduce / AW ; pass on, small beak / favourable, alleles ; A ora over a number of generations ; increase in frequency, small beak / favourable alleles ; A ora 5 max

(b) disagree (no mark on own)

change in phenotypic feature is taking place ; A beaks have changed directional selection taking place ; 1 max

[Total: 12]

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4 (a) (i) P – prophase 1 ; R prophase 2 Q – anaphase 1 ; R anaphase 2 P prophase and Q anaphase 1 mark

2

(ii) R- centromere ; S – chromosome / (pair of) chromatids ; R chromatid T – bivalent / pair of homologous chromosomes ; A chromosome, pair of chromatids R chromatid V – chiasma ; A chiasmata, chromatid R chromatids

4

(b) crossing over (in prophase 1) ; A description of crossing over independent / random, assortment of (homologous) chromosomes ; ignorestage independent / random, assortment of chromatids at metaphase 2 ; chromosome mutation / named example of chromosome mutation ; max 3

(c) (i) RRYY RrYY RrYy RRYy

1

(ii) test / back, (cross) ; 1

(iii) RrYy rryy ; RY, Ry, rY, ry ry (ry ry ry) ; RrYy Rryy rrYy rryy ; round/yellow, round/green, wrinkled/yellow, wrinkled/green ; 1:1:1:1 ; A 4:4:4:4 etc apply ecf once if parental genotypes correct.

ie incorrect parental genotypes no marks. 5

[Total: 16]

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5 (a) (i) bind to membrane proteins ; inhibit, active transport / facilitated diffusion ; prevent uptake of, mineral ion / named mineral ion ; R nutrients unqualified ref to function of ion ; inhibit enzymes ; detail of inhibition ; e.g. change in shape of active site / AW. Not simply competitive or non - competitive. inhibit respiration or any stage of respiration ; reduced ATP production ; inhibit photosynthesis ; reduced respiratory substrate ; prevent PGR production / inhibit PGR ; A named PGR inhibit protein synthesis ; decreased water potential of soil ; reduced water uptake ; inhibit mitosis ; new cells not produced ;

max 2

(ii) temperature ; pH (of culture solutions) ; light intensity ; carbon dioxide (concentration) ; volume of solutions ; R amount of solution conc. of other ions in solution ; sample size ; time before measuring roots ;

max 2

(b) 1 2 3 4 5 6 7 8 9

10 11 12

13 14

15

mutation ;alleles for (copper) tolerance ; variation in population ; copper is selection pressure ; directional selection occurs ; those with, (copper) tolerant alleles / selective advantage, survive ; reproduce / breed ; A asexual reproduction in correct context offspring are copper tolerant / pass on copper tolerant alleles ; over many generations ; tolerant allele frequency increases ; plants at D less (copper) tolerant ; (tolerance) no, advantage / disadvantage in areas where low copper levels ; mean root growth reduced by 80% ; the more tolerant plants are found where copper levels are high(est) / in A or B ; mean root growth reduced by 45% - 52% ; max 7

QWC – clear well organised using specialist terms; Need to use three of the emboldened terms in correct context. 1

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(c ) (i) reproductive ; geographic ; A allopatric behavioural / ecological / temporal / polyploidy / mechanical ; A sympatric A prezygotic and postzygotic for two marks max 2

(ii) 1. flies, breed / mate on species of plant they emerge from ; 2. different times of emergence ; 3. therefore population separate from hawthorn population / reproductively isolated ; 4. different selection pressures / named difference ; 5. sympatric speciation ; 6. two separate gene pools ; 7. differences (in gene pool) accumulate ; 8. until two cannot breed successfully ; max 5 [Total: 19]

2804/01 Mark Scheme June 2010

5


3 (a) chinchilla – CCh C Ch C Ch C H C Ch Ca ;

agouti - CACA CA Cch CA CH CACa ;

2

(b) (i) test cross ; A back cross 1

(ii) Himalayan rabbit either CHCH or CHCa ;A correctly derived gametes in genetic diagram

albino rabbit must be CaCa ;

albino offspring produced if Himalayan rabbit is heterozygous / ora ;

if genetic diagrams given with no annotations, max 2

3

(c)

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stability

limited, food supply / space ;competition ;predation ;disease ;reached carrying capacity / death rate = birth rate ; internal max 3

gene pool

individuals show variation; variation due to, combination of alleles / mutations ;best adapted survive / ora ; A survival of fittest ideareproduce ;pass alleles to offspring ;frequency of favourable alleles will, increase / be maintained ; A ora internalmax 3

max 5

[Total: 11]

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2 (a) (i) sympatric; 1 (ii) ranges of two species, overlap/close together/AW;

no geographical barrier; ref to behavioural/genetic/physiological/prezygotic barrier; correct ref to named area of map; max 2

(b) ref to mate selection by size; ie large with large or small with small

ref to monogamy; ref to intermediate sizes, at disadvantage/selected against/ora; intermediate do not pass on alleles/ora; suggested reason why intermediate at disadvantage/ora max 3

(c) female produces a lot of eggs;

selects male, that can store lots of eggs/has a large pouch/ora; large males fertilise many eggs/ora; chance of more offspring surviving; or large female and small male produce intermediates/ora; intermediates at disadvantage/ora; max 2

[Total: 8]

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Final Mark Scheme 2806/01 January 2004


5 (a) (i) note - one of these marking points gets two marks

is passed down without skipping generation ;no evidence of carriers passing it on ;example ; 7 / 8 inherit from 2

9 / 10 from 4 14 / 15 / 18 from 8 and 9

16 /17, unaffected but their parents (8 and 9) both have achondroplasia ; ; oraaccept ‘When both parents are affected, unaffected offspring can result’ for two marks (so the allele for normal height must be recessive / ora) 2 max

(ii) sex linked genes cannot pass from father to son ; (are on the X chromosome)4 passes achondroplasia to his, son(s) / offspring / 9 and/or 10 ; 2 max

(b) genotypes of the parents - both Aa, gamete genotypes - A and a, possible genotypes of children - AA Aa aa ;possible phenotypes of children - with achondroplasia or unaffected ;

one mark deducted for each error or omission 2

(c) ref. to mutation ;shows in the phenotype (immediately) as it is dominant ;as mutation is a rare chance event the siblings or other family are very unlikely to be affected ;(the mutant individual is Aa) half the gametes will be A and half a ; 2 max

(d) achondroplasia is caused by, genes / genotype / mutation ;A it is a genetic disease ;

not a deficiency disease ; A correct ref. to diet only limbs are affected so, calcium, metabolism / nutrition, must be normal ;AVP ; 2 max

[Total: 10]

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Final Mark Scheme 2806/01 June 2004


2 (a) (i) both types of haemoglobin can be present in one (individual) sheep;if both sheep are Hb-I and Hb-II their offspring have all three phenotypes / AW;

see row three of Table 2.1if not codominant only two phenotypes would be produced;shows that individuals with both haemoglobin types, are heterozygous / carry one of each type of allele / both alleles are expressed (in the heterozygotes); max 2

(ii) Hb-II (only); A description of phenotype 1

(b) eitherless oxygen in the blood / AW;ormore, oxyhaemoglobin would have dissociated / haemoglobin gives up oxygen;

because(muscles) respire more / use up more oxygen;orstimulated by, high CO2 / heat / low pH / lactate; A lactic acid orAVP; e.g. ref to Bohr effect max 2

(c) (i) Hb-I = 63 - 63.5 (%), Hb-II = 76 - 76.5 (%); 1

(ii) accept ora for this question

oxygen concentration / pO2, is lower in, hills / high altitudes;

Hb-II has higher affinity for oxygen / AW;adaptation (to high altitude);comparative figures from graph;

ref to selection / selection described; e.g. ‘survive and breed’ max 3

(iii) carbon dioxide / pCO2, influences the saturation of Hb with oxygen; A ideaincrease in CO2 concentration causes, dissociation / unloading / Hb to give up O2;

if awarding this point give marking point 2 as well

ref to Bohr effect;AVP; e.g. ref to validity / a variable that must be controlled max 2

[Total: 11]

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2806/01 Mark Scheme January 2010

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Question Expected Answers Marks4 (a) (i) no, nucleus / nuclear membrane ;

no membrane-bound organelles ; circular DNA ; naked DNA / no histones ; cell less than 1�m ; small (18nm/70S) ribosomes ; bacterial / peptidoglycan / murein, cell wall ; AVP ; other valid prokaryote feature such as mesosome, plasmids 3 max

(ii) Latin, universal / same, name all over world ; first name / genus, shows relationship to other close species ; second name / species, unique to this species ; 2 max

(b) number of erythromycin doses was increasing up to 1988 ; falls, in 1989 / after 1988 ; largely a downward trend since ;ref. plateau ; campaign has been successful ;figs. to support ; 3 max

(c) (i) mutation ; resistant bacteria brought in from elsewhere ; conjugation / horizontal transfer / plasmid, from a different species ;transformation ; 1 max

(ii) 1. natural selection / ref. selection pressure ; 2. resistant bacteria, selected for / have advantage / outcompete rest ; 3. as erythromycin kills other strains / only resistants, survive / are left ; 4. (resistants) reproduce ;5. increasing numbers with, plasmid / resistance gene, (in population) ; 6. ref., asexual reproduction / binary fission / cloning, (in bacteria) ; 7. time lag between erythromycin used falling and % resistant infections

falling ; 4 max

(iii) erythromycin now less of a selection pressure ; non-resistant, survive / have selective advantage / out-compete resistant

strains ; orabacterial resources not used for resistance ; may be diverted to, growth / reproduction ; 2 max

[Total: 15]

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4 (a) chinchilla – CChCCh CChCH CChCa ; agouti – CACA CACCh CACH CACa ; 2

(b) (i) test cross ; A back cross 1 (ii) Himalayan rabbit either CHCH or CHCa ;

A correctly derived gametes in genetic diagram

albino rabbit must be CaCa ; albino offspring produced if Himalayan rabbit is heterozygous / ora ; if genetic diagrams given with no annotations max 2 3

(c)

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max 3 from points 1 to 5 limited, food supply / space ; competition ; predation ; disease ; reached carrying capacity / death rate = birth rate ; marking points 1 – 5 linked to keeping population stable

individuals show variation ; variation due to, combination of alleles / mutations ; best adapted survive / ora ; A survival of fittest idea reproduce ; pass alleles to offspring ; frequency of favourable alleles will, increase / be maintained ; A ora 5 max

[Total: 11]

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Meiosis, Variation & Genetics · Final Mark Scheme 2804/01 January 2004 Question Expected Answers...

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Transcript of Meiosis, Variation & Genetics · Final Mark Scheme 2804/01 January 2004 Question Expected Answers...