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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…

Page 2

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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…

Page 3

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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…

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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)…

Page 5

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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…

Page 6

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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.
2




(ii) Evidence (not a mark)

3 would not be/is Rhesus positive/would be Rhesus negative

Explanation (not a mark)

3 would…

Page 7

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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…

Page 8

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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
­…

Page 9

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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. (a) is always expressed(in the phenotype) / produces (functional) proteins
1

(b)…

Page 10

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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…

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