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GCSE Biology Paper 2
Inheritance
Cystic Fibrosis
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Sophia Robinson
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Cards (23)
What inherited disorder is discussed in this video?
Cystic fibrosis
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What should you be able to do by the end of this video?
Describe cystic fibrosis and complete Punnett squares
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What are alleles?
Different versions of a
gene
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What is the difference between dominant and recessive alleles?
Dominant
alleles are
expressed
with
one copy
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What is the symbol for the dominant allele for normal cell membrane function?
Capital C
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What is the symbol for the recessive allele for defective cell membranes?
Lowercase c
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How many copies of the defective allele must a person inherit to have cystic fibrosis?
Two copies
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What do we call a person with one defective allele and one normal allele?
A
carrier
of the
cystic fibrosis
allele
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What is the genotype of a man who is a carrier of the cystic fibrosis allele?
Heterozygous
(one normal, one defective)
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What is the genotype of a woman who does not have cystic fibrosis and is not a carrier?
Homozygous
for the normal
allele
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What is the first step in constructing a Punnett square for cystic fibrosis?
Write down the
genotypes
of the gametes
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What percentage of offspring will be carriers when a heterozygous man and a homozygous normal woman have children?
50%
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What is the genotype of the offspring that are carriers of cystic fibrosis?
Capital C
lowercase c
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What is the ratio of carriers to non-carriers from the cross of a heterozygous man and a homozygous normal woman?
1:1
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What is the probability that all offspring could be carriers or all could be unaffected?
These are just
probabilities
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What happens when two carriers of the cystic fibrosis allele have children?
25%
will have cystic fibrosis
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What is the ratio of offspring from two carriers of cystic fibrosis?
1:2:1
ratio
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What is the percentage of offspring that are not affected at all from two carriers?
25%
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What is the percentage of offspring that are carriers from two carriers?
50%
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What is the percentage of offspring that have cystic fibrosis from two carriers?
25%
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What are the key points about cystic fibrosis inheritance?
Cystic fibrosis is a disorder of
cell membranes
.
Controlled by a
single gene
with two
alleles
.
Dominant allele (normal function):
Capital C
.
Recessive allele (defective function):
Lowercase c
.
Two copies of the
defective allele
are needed for cystic fibrosis.
Carriers
have one normal and one defective allele.
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What are the outcomes of genetic crosses involving cystic fibrosis?
Heterozygous
man and
homozygous
normal woman:
50%
carriers
, 50%
not affected
(
1:1 ratio
).
Two carriers:
25% not affected, 50% carriers, 25% with cystic fibrosis (
1:2:1 ratio
).
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What should students practice regarding genetic crosses?
Complete
Punnett square
diagrams.
Express outcomes as percentages and ratios.
Understand
probabilities
in genetic outcomes.
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