Download Q1. In humans, the sex chromosomes X and Y determine whether

Q1.
In humans, the sex chromosomes X and Y determine whether the baby will be male or
female (its gender).
(a)
(i)
Draw a genetic diagram to show how gender is inherited. The male has XY
chromosomes and the female has XX.
(2)
(ii)
What is the likelihood of obtaining a male child?
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(1)
(b)
In the 16th century Henry VIII was the King of England. He blamed some of his wives for
giving birth to daughters instead of sons. With our present day knowledge of genetics this
mistake could not be made today. Explain why Henry VIII was wrong.
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(2)
(Total 5 marks)
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Q2.
The diagrams show four ways in which human twins may be formed.
Which diagram, A, B, C or D, shows the process which will produce genetically identical twin
boys?
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Explain the reason for your choice.
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(Total 3 marks)
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Q3.
(a)
(i)
The diagram shows a normal body cell which has six chromosomes.
Complete the diagram below to show one cell produced from this cell by mitosis.
(3)
(ii)
Complete the diagram below to show one cell produced from the original cell by
meiosis.
(2)
(b)
Thalassaemia is a blood disease. It is determined by a single recessive allele. A person
with one recessive allele does not get the disease but does act as a carrier. People with
this pair of recessive alleles can become ill.
(i)
Draw a genetic diagram to show the inheritance of' this disease if both parents are
heterozygous.
[Use the symbols T = dominant allele and t = recessive allele]
(3)
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(ii)
What are the chances of a baby inheriting the disease?
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(1)
(iii)
What are the chances of a baby being a carrier if both parents are heterozygous?
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(1)
(Total 10 marks)
Q4.
For many years scientists studied the organisms in an area of grassland.
One of the animals was a species of black fly. In this population only one allele B existed for
colour. All the flies were homozygous BB.
A mutation occurred which produced a new recessive allele b which could produce a green
colour.
(a)
Draw two genetic diagrams to show how the single b allele in just one fly was able to
produce homozygous bb green flies in two generations.
First generation
Second generation
(4)
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(b)
Although this new allele was recessive and the mutation only occurred once, a large
proportion of the fly population was soon green.
Suggest in terms of natural selection why the recessive b allele was able to spread
through the population.
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(3)
(Total 7 marks)
Page 5 of 9
M1.
(a)
(i)
gametes correct
allow by implication from line diagram
only need on X from female
1
offspring genotype correctly derived
on suitable diagram
or
1
(ii)
1:1 or 50% or ½ or 0.5 or 1 in 2
or 1 out of 2 or 50 : 50
do not accept 50/50
accept ‘equal’ (probability)
1
(b)
Y chromosome needed for male child
1
only male has the Y or wives had only X (chromosomes)
or sex determined by the sperm
1
[5]
M2.
D
idea that twins have come from one (fertilised) egg
idea that Y sperm / Y chromosome produces boys
each for 1 mark
allow 1 mark if candidate selects A and states that Y sperm / Y
chromosome produce boys (reject Y gene unqualified) OR allow 1
mark if candidate selects C and states that twins must have come
from one (fertilised) egg
[3]
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##
(a)
(i)
if two nuclei drawn then maximum two marks
1
6 chromosomes
1
same 3 homologous pairs
1
nuclear membrane drawn
1
(ii)
3 chromosomes
1
1 from each homologous pair
1
(b)
(i)
parent line must be separate
heterozygous parents Tt × Tt
maximum of 2 marks if parental genotype is wrong
gametes correct T t T t
1
genotypes TT Tt Tt tt
1
(ii)
correct analysis of chance i.e. 1 in 4
or 25%
1
(iii)
50% or 1 in 2
1
[10]
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M4.
(a)
First Generation
or as matrix
allow one mark for being able to produce a correct genetic cross
(even if from an incorrect starting point)
Second generation
or as a matrix
4
(b)
•
green colour gives an advantage/camouflage
•
more green flies dm black flies survive to breed*
•
pass on their genes to the next generation
•
(* but implied by 3rd bullet point)
for 1 mark each
3
[7]
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