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3B Inheritance
Time: 1 hour 17 minutes
Total marks available: 77
Total marks achieved: ______
Andy Todd
Q1.
The following passage describes inheritance.
Complete the passage by writing a suitable word or words on each dotted line.
(6)
A gene is a section of a double-stranded molecule known as
............................................................ .
This molecule is found within the ............................................................ of the cell,
contained in threadlike structures called ............................................................ .
The two strands form a double helix linked by a series of paired bases.
The base adenine is always linked to ............................................................ and the base
cytosine is always linked to ............................................................ .
Sometimes the genetic material of a cell changes. This is known as a
............................................................ .
These changes occur very rarely but their incidence can be increased by chemicals or
ionising radiation.
(Total for question = 6 marks)
Andy Todd
Q2.
FH (familial hypercholesterolaemia) is an inherited condition. People with FH have
high levels of blood cholesterol and an increased risk of heart disease.
A dominant allele (D) results in high levels of blood cholesterol. A recessive allele
(d) results in low levels of blood cholesterol. This means that people who inherit the
dominant allele are most at risk of FH.
(a) (i) What is meant by the term recessive?
(1)
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(ii) What are the two different genotypes of people who are at risk of FH?
(2)
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(b) (i) In the boxes below give the genotypes of the parents, and the genotypes of
all the possible children, for a cross between a heterozygous father and
a heterozygous mother. You should use the symbols D for the dominant
allele and d for the recessive allele in your answer.
(2)
(ii) What is the probability of these parents producing a child with FH?
(1)
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(iii) What is the phenotype ratio of the children produced?
(1)
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(c) High levels of blood cholesterol can lead to narrowing of arteries. Suggest how
this might affect the ability of the heart to function.
(5)
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(Total for question = 12 marks)
Andy Todd
Q3.
Achondroplasia is an inherited condition in humans. Adults with achondroplasia are
much shorter than average height.
This condition is controlled by a gene with two alleles. The dominant allele (A) codes
for shorter than average height and the recessive allele (a) codes for average height.
(a) Two parents both had achondroplasia. They had a child who grew up to be of
average height.
Use a genetic diagram to show:
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the genotype of each parent
the gametes they produced
the genotypes of all the possible offspring
the phenotypes of all the possible offspring
(4)
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(b) The parents had a second child.
State the probability that this child grew up to be of average height.
(1)
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(c) Achondroplasia is caused by a dominant allele.
(i) Explain what is meant by the term dominant allele.
(2)
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(ii) Suggest why the number of people with achondroplasia is low, even though it
is a dominant condition.
(2)
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(Total for question = 9 marks)
Andy Todd
Q4.
Human blood group is an example of a phenotype determined by alleles that show
codominance.
The table shows the different blood groups and their genotypes.
(a) Explain what is meant by the term codominance.
(1)
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(b) A man with blood group A married a woman with blood group B. They had four
children, each with a different blood group.
Use a genetic diagram to show this cross.
(3)
Genotypes of the parents
Gametes
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Genotypes of the offspring
(Total for question = 4 marks)
Andy Todd
Q5.
The photographs show an adult insect called an ash borer and an adult insect called
a wasp.
Ash borers reproduce by laying eggs which develop into maggots. The maggots eat their
way into ash trees and feed on carbohydrates in the trees. This can kill the trees because
the root cells lack the carbohydrate needed to release energy for the absorption of
mineral ions.
(a) (i) Suggest why the maggots need to feed on carbohydrate.
(1)
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(ii) Name and describe the process used by root cells to absorb mineral ions.
(2)
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(iii) Describe how magnesium ions are used to help trees to grow.
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(b) Wasps defend themselves from predators by using a sting. This means that predators
avoid attacking wasps.
Ash borers look very similar to wasps.
Use your knowledge of natural selection to explain why ash borers have evolved
to look like wasps.
(4)
Andy Todd
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(Total for question = 9 marks)
Andy Todd
Q6.
Most simple genetic crosses, such as those studied by Mendel, investigate phenotypes
determined by a pair of alleles, where one allele is dominant over the other allele.
Mendel crossed homozygous tall pea plants with homozygous dwarf pea plants.
All the offspring were tall.
(a) Use a genetic diagram to show the parent genotypes, the gametes formed and
the genotypes of the offspring.
(3)
(b) Mendel then made sure that the tall offspring plants could only self-pollinate,
rather than being pollinated by other plants. The seeds produced grew to give
new plants with a 3 : 1 phenotypic ratio.
(i) Suggest how Mendel made sure that the pea plants self-pollinated.
(2)
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(ii) Give the genotypes of the offspring he obtained from the self-pollinated pea plants.
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(c) What is meant by the term dominant allele?
(1)
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Andy Todd
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(d) Some phenotypes are controlled by codominant alleles.
What is meant by the term codominant alleles?
(2)
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(Total for question = 9 marks)
Andy Todd
Q7.
Mendel crossed together pure breeding pea plants with purple flowers and pure
breeding pea plants with white flowers. The offspring plants all had purple flowers.
(a) Explain which phenotype is dominant.
(2)
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(b) In a second cross, the purple offspring plants were self-pollinated (pollen from a
flower put on the stigma of the same flower).
Suggest how Mendel made sure that all the purple offspring plants were
self-pollinated.
(2)
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(c) (i) Use a genetic diagram to show the ratio of plants expected by crossing the
purple offspring plants in part (b).
Show the parental genotypes and gametes, and the offspring genotypes and
phenotypes. Use F and f to represent the alleles.
(4)
Parent genotypes
Gametes
Offspring genotypes
Offspring phenotypes
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(ii) The plants actually produced in this cross were 36 purple and 8 white
flowered plants.
Calculate the ratio of purple to white flowered plants.
(1)
Answer ...........................................................
(iii) Suggest why the actual ratio of phenotypes differs from the one predicted by
your genetic diagram.
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(d) A student suggested that purple flowers are more likely to be visited by bees than
white flowers.
Use your knowledge of natural selection to suggest how this might affect the
number of purple and white flowers in the wild.
(5)
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(Total for question = 16 marks)
Andy Todd
Q8.
In mammals, males have two different sex chromosomes (X and Y) and females have
two similar sex chromosomes (X and X).
However, in birds males have two similar sex chromosomes (Z and Z), while females
have two different sex chromosomes (Z and W).
(a) Complete the genetic diagram to show how sex is inherited in birds.
(4)
(b) In birds, the egg is extremely large and contains food molecules that the growing
embryo needs in order to develop.
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(i) Suggest one food molecule that is present in a bird's egg and describe the
role of this molecule in the growth of the bird embryo.
(2)
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(ii) Suggest an advantage of the shell of the egg being made of a hard substance.
(1)
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(iii) Chicken eggs are a rich source of vitamin A.
Give another source of vitamin A and describe the function of vitamin A in
humans.
(2)
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(c) The number of chromosomes in eggs and in sperm is less than the number of
chromosomes in the body cells of animals.
Explain how the number of chromosomes in animals is maintained in their offspring.
(3)
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(Total for question = 12 marks)
Andy Todd
Q1.
Candidates had to fill in the gaps with the appropriate word in a passage
describing inheritance. Many scored full marks but some confused which bases
paired with adenine and cytosine. Some spelling errors were tolerated with
thymine and guanine, yet others could not be credited as they were not
recognisable.
Q2.
This question was on inheritance of a dominant allele causing familial
hypercholesterolaemia. In part a) the best candidates were able to write
what is meant by the term recessive. The weaker candidates just stated
that it was not dominant or was dominated by the dominant allele. Most
candidates could however give the two genotypes that were at risk of FH. In
part b) almost all candidates could fill in a simple genetic diagram to show
the genotypes of the parents and the children. Most were also able to give
the probability of producing a child with FH and the phenotype ratio of the
children. Some candidates did not understand what probability is and others
confused genotype with phenotype. Part c) required candidates to suggest
how high levels of cholesterol might affect the ability of the heart to
function. Many candidates earned full marks for their answers.
Q3.
(a) required candidates to show the parents, gametes and possible
offspring genotypes and phenotypes of a cross between parents with
achondroplasia. Most were able to do this with some losing credit by failing to
identify the phenotype of the offspring. Some seemed to think a sex linked allele
was involved. In part (b) most could see that the probability of a second child
being of average height is unaffected by the phenotype of the first child. Some
candidates tried to express a probability as a ratio rather than a decimal or a
fraction or percentage. In part (c) (i) the best candidates could explain a
dominant allele as one that if present in the genotype is always expressed in the
phenotype. Poorer responses used terms like stronger or more powerful and
some candidates described a dominant allele as being more likely to be passed
on. Part (c) (ii) candidates were asked to suggest why the number of people with
achondroplasia is low, even though it is a dominant condition. We were expecting
responses to refer to the allele having selective disadvantage and less
achondroplasics reproducing leading to a low allele frequency.
Q4.
Finally this question described the inheritance of A B O blood groups in humans. In part (a) about
half of the responses correctly explained the meaning of codominance. Students did better in
part (b) with most being able to identify the genotypes of the parents and gametes that would
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lead to 4 different blood groups in one family.
Q5.
This question presented some novel information to candidates on ash borers. In
(a) most were able to suggest that feeding on carbohydrates would give the
maggots energy from respiration. Some were able to name active transport and
describe how it enables absorption of ions against a concentration gradient. Most
could also describe how magnesium ions are used for chlorophyll molecules
required in photosynthesis. In part (b) candidates needed to explain how ash
borers had evolved to look like wasps. Quite a number of candidates thought that
the ash borer could mate with the wasp to acquire similar characteristics. The
best answers described a mutation that caused the ash borer to inherit an allele
that conferred a similarity to the wasp. This allele would mean a wasp like
phenotype would be less likely to be attacked by predators. This advantage
meant that these ash borers would be more likely to survive and mate. The allele
for wasp features would then be passed on to their offspring. Over many
generations the ash borer would evolve to look like a wasp.
Q6.
Part (a) of this question tested candidates' understanding of a simple genetic
cross. Most chose a sensible upper and lower case letter to represent the
parental genotype but a surprising number chose different letters. In these
cases, no mark was given to the parental genotype but the concept of
transferred error was used to credit the gamete and offspring genotypes. In
(b)(i), a pleasing number appreciated the need to keep the plants isolated,
particularly from agents of cross-pollination such as insects. Many suggested
that Mendel himself transferred pollen from the anther to the stigma of the
same flower using a brush and due credit was given. Credit was also given to
those candidates who described self-pollination. In (b)(ii), no credit was given
for combinations of different letters such as T and D or T and d or T and S.
Defining genetic terms is a challenge for students but many appreciated that a
dominant allele is the one expressed in the heterozygote and that codominant
alleles are both expressed in the heterozygote.
Andy Todd
Q7.
This question provided a genetics question on flower colour in peas. In part (a)
almost all could identify the dominant phenotype and most could explain
how they knew. In (b) candidates had to suggest how Mendel could ensure
that the purple F1 plants self-pollinated. The best responses suggested how
he could isolate the plants and transfer pollen using a paintbrush. In (c) (i)
candidates had to use a genetic diagram to show the offspring of selfing the
F1 plants. Most could do this but some failed to gain full credit as they did
not clearly show the gametes or the phenotypes of the offspring. In (i) most
could calculate the ratio but only some could explain why the actual and
observed ratio are different. In (d) candidates had to suggest how natural
selection could affect the numbers of flowers since purple are more
attractive to bees. Most candidates scored some marks with the best
responses including the increased likelihood of purple pollen being carried
and transferred to purple flowered plants. How this would lead to increased
reproduction of purple flowered plants and how the purple allele would be
passed to future generations and how this would increase purple flowers
and reduce white flowers.
Q8.
Part (a) of this question tested understanding of sex inheritance and was well answered by most
students. A minority lost a mark for not stating the sex of the offspring and a very small number
lost credit for using X and Y symbols rather that the ones given in the question. The remaining
parts of this question were extremely well answered by the vast majority of students. Protein
providing amino acids for growth was the most popular response in part (b)(i), most appreciated
the role of the shell in protecting the developing embryo in (b)(ii), and a pleasing number knew a
source and a function of vitamin A in (b)(iii). In part (c), most were able to gain full marks though
there was occasionally evidence of confusion about the role of meiosis and mitosis.
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Q1.
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Q2.
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Q3.
Q4.
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Q5.
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Q6.
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Q7.
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Q8.
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