Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Answers to end-of-chapter questions
the recessive allele may code for defective
product or no product;
production of some correct product may cure
the disorder;
adding a recessive allele cannot block a
faulty dominant allele; [max. 5]
[Total: 7]
no vector needed for drug, whereas virus
vector for therapy may cause side-effects;
[3]
[Total: 15]
8 a
Gametes A a
A
a
AA
normal
Aa
normal, but
with cystic
fibrosis trait
Aa
normal, but
with cystic
fibrosis trait
aa
cystic fibrosis
correct gametes;
correct genotypes;
correct phenotypes;[3]
b 1 in 4 / 0.25 / 25%;[1]
[Total: 4]
9 a i chloride channel;
chloride moves out of cell by active
transport;[2]
ii upper face because of presence of
carbohydrate chains;[1]
b i allele: variant form of a gene;
recessive: only affects phenotype when
dominant allele is not present;[2]
ii thick, sticky mucus produced;
mucus accumulates;
reduced gas exchange;
more infections; [max. 3]
c normal dominant CFTR allele added to
viral DNA;
virus inserts DNA into cell;[2]
d i translation stopped at ‘stop’ codon;
protein chain not completed;[2]
ii comparisons include: drug easily taken up
by cells, whereas therapy is poorly taken
up;
drug taken orally, whereas therapy must be
inhaled into lungs;
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