Genetics sample answers

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  • Created on: 02-04-16 11:15
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Edgbaston High School For Girls
Genetics mark scheme
M1. (a) males are XY and females XX / males have one X chromosome and
females two X chromosomes
males only have one allele (of the gene) present / recessive allele
always expressed
colour blindness is masked in heterozygote / female needs 2 recessive
alleles to be colour blind
2 max
(b) (i) 5 hh X Y
b
6 Hh XB
X b
2
(ii) h X
b
, h Y, and H X, h X, H X, hX
B
B b b
1
(iii) 1/8 or 12.5% or 0.125
either
genetic diagram to show genotypes Hh X X , Hh X Y, hh Xb
X,
b B B b
hh X Y, HHX
B
X , Hh X Y, hh X
b b
X hh X
b
Y b b b
1/8
or
P (boy) = 0.5, P (colour blind) = 0.5, P (white streak) = 0.5
(0.5 × 0.5 × 0.5 =) 0.125
2
[7]
M2. (a) mutations
which are different/at different positions in the gene
2
(b) (i) either dominant or recessive allele
1
(ii) aa
h h
BB, a aBB, a
h
a Bb, a aBb h h h
(allow 1 mark for 2 or 3 correct answers)
2
(iii) temperature lower at extremities
enzyme active/ not denatured
2
(c) if allele A is present (normal) tyrosinase/enzyme is produced, so it does
not matter what other allele is present / explanation of why heterozygote is
same phenotype as double dominant in terms of enzyme produced
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Edgbaston High School For Girls
phenotype/rabbit is black as both have alleles A and B
2
[9]
M3. (a) (i) 1. Parents are heterozygous
2. Kittens receive white allele from parents /black cat
1. Accept carriers/carries white allele
1 max
(ii) 1:1
Answer must be expressed as a ratio that could be reduced
to 1 : 1
1
(b) (i) Black,
Chocolate,
Black
All three correct for the mark
1
(ii) Parental phenotypes Chocolate male Black female
1.…read more

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Edgbaston High School For Girls
2. gametes may not be produced in equal numbers/
3. fertilisation/fusion of gametes is random/
4. small sample
1
(iv) 1. Possible if parents homozygous/ bb
2. Don't know genotype of chocolate cat / chocolate cat could be
homo or heterozygous / chocolate cat could be bb or bb i
3. Two chocolate cats could give cinnamon kittens
2 max
[9]
M4.…read more

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Edgbaston High School For Girls
1
(ii) Give credit for any method which would ensure collection of a
random sample from trees e.g. beating tray
Q Note that specification does not require specific knowledge
therefore the use of specific terminology such as "beating
tray" is not required here.
1
(b) Two marks for correct answer of 49% red and 51% black
One mark for incorrect answer in which p/frequency of black allele/B is
Identified as 0.3 and q/frequency of black allele/B as 0.…read more

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Edgbaston High School For Girls
B allele more eumelanin deposited in hairs
2
[9]
M8. (a) (i) Two, as white blood cells are diploid cells/alleles are present
on each chromosome of an homologous pair/one maternal
and one paternal
1
(ii) A and B
(reject I
A
and I)
B
1
(b) 1 in 8/ 1/8 / 12.5%/ 1:7/ 0.…read more

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Edgbaston High School For Girls
Do not negate mark if candidate refers to gene rather than
allele.
Answers including correct and incorrect evidence = zero
marks evidence and explanation.…read more

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Edgbaston High School For Girls
[9]
M10. (a) (i) Only seen in males / not in females
1
(ii) Unaffected parents/mother child with M.D./
(1 ×)2 5 / (3 ×) 4 11 / 8 (× 9) 13
1
(b) 5 = XY d
6 = XY D
7 = XX AND
D d
XX D D
8 = XX D d
All 4 correct = 2 marks
2 or 3 correct = 1 mark
max 2
(c) ¼ / 0.…read more

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Edgbaston High School For Girls
(ii) 1. Parental genotypes correct: both W W R S
(ACCEPT `RS')
AND
W (ACCEPT `S'
S
) /gamete from each parent
2. W WS
(ACCEPT `SS')
S
/ offspring formed and identified
as susceptible
If different symbols:
­ defined : max 2 marks
­ not defined max 1 mark (= pt.2)
2
(iii) 1. Description: decrease + rate of decrease slows with time
Explanation: Any three from:
2.…read more

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Edgbaston High School For Girls
separate gene pools / no interbreeding
4 max
(c) selection of mate dependent on colour pattern
prevents interbreeding / keeps gene pools separate
2
[8]
M13 is a repeat of question 1
M14.…read more

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Edgbaston High School For Girls
Accept: genotype/explanation of genotype
Accept: alternative wording, e.g. there is no gene
flow/genetic drift for genetically isolated.
3
(b) White/deaf cats unlikely to survive/selected against
Will not pass on allele (for deafness/white fur) (to next
generation)/will reduce frequency of allele
Accept: alternative wording, e.g. have a disadvantageous
phenotype
Neutral: will not breed
2
(c) In Paris/London frequencies (of these alleles) add up to more than 1
Can be shown by correct figures to be more than 1
e.g. 0.…read more

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