Download Genetics: biology homework revision questions

8. Genetics
1.
Explain what is meant by each of the following terms.
Gene .....................................................................................................................................
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Allele ....................................................................................................................................
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(Total 4 marks)
2.
Huntington’s disease is a genetic disease that is caused by a defect in the huntingtin protein. The
disease causes the degeneration and death of neurones. It is an unusual disease as it is caused by
a dominant allele. However, the symptoms of the disease do not appear until a person is
between 30 and 50 years of age.
(a)
The diagram below shows a family tree in which the Huntington’s disease phenotypes
are shown for the family members.
A
C
B
D
Key
Female, unaffected
Female, Huntington’s disease
Male, unaffected
Male, Huntington’s disease
AS Unit 1: Lifestyle, Transport, Genes and Health: Topic 2: Genes and Health
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8. Genetics
(i)
Using the symbols H to represent the dominant allele and h for the recessive allele,
indicate the genotypes of the following family members.
Member A ...........................
Member B ...........................
Member C ...........................
Member D ...........................
(2)
(ii)
The disease does not show itself until the sufferer is 30 years of age or older.
Suggest why this is a problem in preventing the inheritance of the disease.
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(1)
(b)
The disease is associated with a repetitive sequence of nucleotides, CAGCAGCAG, near
the start of the huntingtin gene. The triplet CAG codes for the amino acid glutamine.
People with 40 or more glutamines at the start of the huntingtin protein will suffer from
the disease but people with 30 or fewer do not.
Suggest ways in which a normal huntingtin gene could be changed into the defective
form.
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(2)
(c)
Research shows that the defective protein binds to an enzyme which is involved in the
process of transcription. This results in a reduced level of gene transcription.
Suggest how the defective protein could inhibit the functioning of the enzyme.
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(3)
(Total 8 marks)
AS Unit 1: Lifestyle, Transport, Genes and Health: Topic 2: Genes and Health
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8. Genetics
3.
In guinea pigs, the colour of the hair in the coat is determined by a gene, where the allele for
black hair (b) is dominant to the allele for white hair (b). Another gene determines whether the
hair is short or long, with the allele for short hair (H) being dominant to the allele for long hair
(h). A guinea pig with the genotype BBhh was crossed with another guinea pig with the
genotype bbHH.
(a)
(i)
Give the phenotypes of these two guinea pigs.
BBhh ..............................
bbHH ..............................
(1)
(ii)
Give the genotype of the offspring of this cross.
Genotype ..............................
(1)
(b)
One of these offspring was crossed with a guinea pig with white long hair.
(i)
Construct a genetic diagram, using the same symbols, to show the expected
genotypes and phenotypes of the offspring of this cross.
(4)
(ii)
Suggest how the results of the cross in (b)(i) demonstrate that independent
assortment has been involved in the production of the gametes.
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(2)
(c)
Compare how mutation and independent assortment can increase genetic diversity.
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(2)
(Total 10 marks)
AS Unit 1: Lifestyle, Transport, Genes and Health: Topic 2: Genes and Health
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8. Genetics
4.
(a)
Explain what is meant by the term genetic carrier.
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(2)
(b)
Sickle cell anaemia is caused by the allele
of a single gene. People who are
homozygous for this allele have sickle
cell anaemia and are severely anaemic.
The photograph opposite shows red blood
cells taken from a person with sickle cell
anaemia.
(i)
Suggest two reasons why oxygen transport by cell A may be less efficient than a
normal red blood cell.
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(2)
AS Unit 1: Lifestyle, Transport, Genes and Health: Topic 2: Genes and Health
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8. Genetics
(ii)
A couple have a number of children, only one of whom has sickle cell anaemia.
Using a genetic diagram, show how this child inherited sickle cell anaemia.
Use the symbols HbA to represent the normal allele and HbS to represent the
sickle cell allele.
(3)
(iii)
Give the probability that this couple’s next child is a genetic carrier of sickle cell
anaemia.
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(1)
(c)
One of the polypeptide chains in the haemoglobin of a person with sickle cell anaemia
has a change in one of its amino acids. Explain how a change in DNA can lead to a
change in a single amino acid in a polypeptide chain.
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(4)
(Total 12 marks)
AS Unit 1: Lifestyle, Transport, Genes and Health: Topic 2: Genes and Health
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