Download Genetic Diagrams and Disorders

StudyPacks
STUDY.
KS4
SCIENCE
Genetics
Inherited Diseases
This Study Pack aims to cover:
1. Genetic disorders
2. How to draw and analyse Genetic diagrams to show genetic disorders.
SB29
Study Packs are prepared by Qualified Teachers and Specialists and are a complete range of comprehensive
compiled resources based on the UK National Curriculum covering the Primary and Secondary Frameworks
including SATs and GCSE examinations.
Student Name
Contents
Inherited diseases notes
2-6
Practice Questions
7
Exam questions
8-17
Mark scheme
18-23
1
2
3
Sex determination
Most body cells contain chromosomes in matched pairs. The number of pairs of chromosomes varies
between species. Human body cells have 23 pairs of chromosomes in the nucleus. One of these pairs
controls the inheritance of gender - whether offspring are male or female,

in males, the two sex chromosomes are different. They are XY

in females, the two sex chromosomes are the same. They are XX.
A set of human chromosomes can be separated from its cell, spread out on a microscope slide and
magnified many thousands of times. When stained and photographed, they look like this:
Chromosomes from a male
Chromosomes from a female
Gametes
Gametes are sex cells. The male gametes are the sperm, and the female gametes are the eggs.
Gametes contain half the number of chromosomes as body cells do. They contain half the genetic
information that body cells do.
Alleles
Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene.
These genes may have different forms.
Different forms of the same gene are called alleles (pronounced al-eels). The gene for eye colour has
an allele for blue eye colour and an allele for brown eye colour.
Alleles are dominant or recessive, meaning:

the characteristic controlled by a dominant allele develops if the allele is present on one or
both chromosomes in a pair
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
the characteristic controlled by a recessive allele develops only if the allele is present on
both chromosomes in a pair.
For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An
individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only
have blue eyes if they inherit two copies of the allele for blue eyes.
Individuals A and B have brown eyes - only individual C has blue eyes
Higher tier
An individual is homozygous for a certain gene if they have two identical alleles. They are heterozygous
for a certain gene if they two different alleles.
The genotype is the genetic makeup of an individual. For example, it is the particular combination of
alleles in each of the three examples above.
The phenotype is the characteristics expressed by an individual. For example, it is the actual eye colour
for the three examples above
Inherited disorders
Some disorders are inherited, such as:

red-green colour blindness

sickle cell anaemia

cystic fibrosis.
Inherited disorders are caused by faulty genes, which are mostly (but not always) recessive alleles. For
example, cystic fibrosis is an inherited disorder that affects the cell membranes, causing the production
of thick and sticky mucus. It is caused by a recessive allele, which means that it must be inherited
from both parents.
Genetic testing
Scientists are now able to test adults and unborn babies for alleles that can cause genetic disorders.
However, the scientific information produced raises many issues that science cannot address. For
example, should a couple with a one in four risk of having a child with cystic fibrosis take the gamble,
or decide not to have any children at all? If a woman becomes pregnant with a child that is going to
have cystic fibrosis, should she have the child, or choose to have an abortion? These are questions
about values that science cannot answer. Different people will have different views
5
Genetic diagrams- Higher tier
Genetic diagrams are used to show the possible outcomes of a particular cross. A dominant allele is
shown by a capital letter, and a recessive allele by a lower case letter.
Cystic fibrosis
Cystic fibrosis is an inherited disorder caused by a recessive allele. This genetic diagram shows the
possible outcomes when both parents are heterozygous for the faulty allele. There is a one in four
chance of the offspring being homozygous for the faulty allele, and so having cystic fibrosis.
Mother and father both have one F and one f allele
This genetic diagram shows the possible outcomes when only one parent carries the faulty allele. There
is no chance of the offspring being homozygous for the faulty allele and therefore having cystic fibrosis.
Mother has two F alleles, father has one F and one f allele.
Gender determination
Female gametes (eggs) contain X chromosomes, while male gametes (sperm) contain either X
chromosomes or Y chromosomes. This genetic diagram shows that equal numbers of male and female
offspring should be produced.
Females are XX and males are XY
6
Worksheet 1 inherited conditions in humans
7
Exam questions
Q1.
Complete each sentence by choosing the correct terms from the box.
23
dominant
46
female
ADH
male
DNA
XX
recessive
XY
strong
YY
weak
A gene is made up of a substance called ............................. . Genes are found on chromosomes
and most human cells contain ................................. pairs of chromosomes. In females the two
sex chromosomes are ........................, but in males the two sex chromosomes are .................. .
Alleles are alternative forms of a gene. Two healthy parents can sometimes have a child with a
genetic disorder such as cystic fibrosis. This is because cystic fibrosis is caused by a
..................................... allele. The two parents are healthy because they also have the
..................................... allele.
(Total 6 marks)
Q2.
In humans, sex chromosomes control whether a person is male or female.
(a)
Use letters X and Y to complete the Punnett square for sex inheritance.
Eggs
or
XX..........
............................
............................
............................
...........
Sperm
or
(3)
zb)
A couple already have three boys.
What is the probability that their next child will be a girl?
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(1) (Total 4 marks)
Q3.
Sometimes babies are born with extra fingers or toes as shown in the photograph.
This condition is called polydactyly.
©CNRI / SCIENCE PHOTO LIBRARY
The diagram shows the inheritance of polydactyly in a family.
(a)
Polydactyly is caused by a dominant allele, D.
The recessive allele of the gene is represented by d.
Use one genetic diagram to show the inheritance of the polydactyly gene by R and S.
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(4)
(b)
In this question you will be assessed on using good English, organising information
clearly and using specialist terms where appropriate.
Embryos can be screened for genetic disorders.
Many people would favour the use of embryo screening for cystic fibrosis but not for
polydactyly.
Compare the issues involved in the use of embryo screening for cystic fibrosis and for
polydactyly.
You should use your knowledge and understanding of the process and the two conditions.
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(6)
(Total 10 marks)
10
The passage below is an extract from an article in The Independent newspaper.
This Mouthwash could mean
a lot to your Children
A SIMPLE TEST SCREENS WOULD-BE PARENTS FOR
THE DEFECTIVE GENE THAT CAUSES
CYSTIC FIBROSIS
It is not every week that a university professor tells you to spit in front of him. But
Bob Williamson, professor of biochemistry and molecular genetics asked me to do
just that.
In future, some people may choose their marriage partners on the basis of the
contents of their spit.
The professor handed me a sample bottle containing sterile salt solution.
I rinsed my mouth out and spat back into the tube. Incredible though it may sound,
the mouthwash now contained enough cells from the inner lining of my cheek for an
assistant to dissect my DNA and check whether I carry any of the mutations
responsible for cystic fibrosis.
Professor Williamson’s team can locate genetic markers, distinctive segments of
DNA, that are inherited along with the mutant genes in people affected with cistic
fibrosis.
About 16 000 people who bought The Independent this morning unwittingly carry a
cystic fibrosis gene. The statistics indicate that 23 of the staff of this newspaper are
unknowing carriers. Carriers are normal healthy individuals who do not have the
disease.
(a)
Describe, as fully as you can, where genes are located inside cheek cells.
.....................................................................................................................................
.....................................................................................................................................
(2)
(b)
The gene for cystic fibrosis has two forms called alleles. Only the recessive allele causes
cystic fibrosis.
Explain how two healthy carriers of the cystic fibrosis allele could produce a child with the
disease. Use the symbol A for the normal allele of the gene and a for the allele which produces
the disease. You may use a diagram if you wish.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
..................................................................................................................................... (4)
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(d)
In the test used to identify the ‘genetic markers’, DNA is extracted from the cheek cells.
The DNA molecules are then made to produce hundreds of millions of copies of
themselves.
(i)
Explain, as fully as you can, how the structure of DNA molecules allows them to
replicate themselves.
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(6)
ii)
Explain how DNA controls the structure of proteins.
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(3)
(Total 15 marks)
Q5.
The diagram shows a family tree in which some individuals have an inherited disorder,
which may cause serious long-term health problems.
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(a)
What proportion of the children of A and B have the disorder?
.....................................................................................................................................
(1)
(b)
Explain the evidence from the diagram which shows that the allele for the disorder is dominant.
Use the appropriate letters to identify individuals in your answer.
You may use genetic diagrams in your explanation. There is space for you to draw a
genetic diagram at the top of the facing page.
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(3)
(c)
(i)
What is meant by ‘embryo screening’?
...........................................................................................................................
...........................................................................................................................
(1)
(ii)
A doctor suggests that couple C and D should have their embryos screened but that
couple G and H do not need this procedure.
Explain the reasons for the doctor’s suggestions.
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(3)
(Total 8 marks)
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Q6.
Cystic fibrosis is an inherited disease which causes the tubes in the lungs to be blocked
with sticky mucus. Two parents who do not have the disease can still produce children who do
have the disease.
(a)
Explain how children can inherit this disease from parents who do not have it (use a
genetic diagram in your answer if you want to).
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(4)
(b)
Mucus contains protein. The information for the production of this protein is stored
in a gene.
Explain how a change in a gene causes a different protein to be produced.
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(3)
(Total 7 marks)
Q7.
Cystic fibrosis is an inherited disorder that can seriously affect health.
(a)
Which one of these is affected by cystic fibrosis?
Draw a ring around your answer.
blood
cell membranes
kidneys
nervous system
(1)
(b)
The diagram shows the inheritance of cystic fibrosis in a family. The allele that produces
cystic fibrosis is recessive.
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(i)
Explain why Alice inherited cystic fibrosis.
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(2)
(ii)
Explain why Ted did not inherit cystic fibrosis.
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(2)
(c)
Bob and Carol know that there is a risk that their next baby will have cystic fibrosis.
Embryos can be screened for the allele that produces cystic fibrosis.
Many people support the screening of embryos, but others do not.
(i)
Suggest one reason why many people support the screening of embryos for the
cystic fibrosis allele.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(ii)
Suggest one reason why many people are against the screening of embryos for the
cystic fibrosis allele.
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..........................................................................................................................
(1)
(Total 7 marks)
Huntington’s chorea is a disease found in 5 out of every 100 000 people.
Q8.
(a)
Describe, as fully as you can, how the disease is likely to affect these people.
.....................................................................................................................................
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(3)
(b)
Some members of the Smith family suffer from Huntington’s chorea. The family tree
gives details.
(i)
Are Mary and John Smith’s children likely to have Huntington’s chorea?
Explain your answer as fully as you can. Use a diagram if it will help.
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(4)
(ii)
Suppose Mary and John both carry one gene for Huntington’s chorea.
How likely are their children to have Huntington’s chorea?
Explain your answer as fully as you can. Use a diagram if it will help.
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(2)
(Total 9 marks)
16
Q9.
The black pigment in human skin and eyes is called melanin. Production of melanin is
controlled by a single pair of genes. A person who is homozygous for a recessive allele of the
gene has no melanin and is said to be albino.
(a)
A man is albino. His wife is heterozygous for the melanin-producing allele.
(i)
The fertilised egg cell produced by the couple divides to form two cells.
Name the process of cell division involved.
..........................................................................................................................
(1)
(ii)
How many albino genes would there be in each of these two cells?
..........................................................................................................................
Explain you answer.
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..........................................................................................................................
(3)
(b)
(i)
Albino people are more likely than people with melanin to suffer mutations that cause cancer in
their skin. Suggest why albino people have an increased chance of mutation in their skin cells.
..........................................................................................................................
..........................................................................................................................
(1)
(ii)
Sometimes, mutation in skin cells leads to cancers in other organs, such as
the liver.
Explain how.
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(2)
(Total 7 marks)
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Answers to Exam questions
M1.
in the correct order
DNA
1
23
1
XX
1
XY
1
recessive
1
dominant
1
[6]
M2.
(a)
(X)
(XX)
XX
Y
XY
XY
1 mark for Y in sperm box
1 mark if XX box correct
1 mark if both XY boxes correct
3
(b)
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
[4]
M3.
(a)
(genotype / gametes from P / father) D and d (*)
1
(genotype / gametes from Q / mother) d and d / accept d(*)
1
offspring genotypes correctly derived from correct gametes(*)
1
offspring phenotypes R and S identified
1
(*) eg may be in punnett square
allow own upper and lower case symbols or
allow any symbol correctly used with key
(b)
Marks awarded for this answer will be determined by the
Quality of Written Communication (QWC) as well as the standard
of the scientific response.
No relevant content.
0 marks
There is a brief description of the issues involved in
screening for at least one condition.
Level 1 (1–2 marks)
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There is some description of issues involved in screening
for both conditions but there is a lack of both pros and
cons for the two conditions.
Level 2 (3–4 marks)
There is a clear, balanced and detailed description of the
issues involved in screening for both conditions, giving
pros and cons for each condition.
Level 3 (5–6 marks)
examples of biology points made in the response
For cystic fibrosis
pros:
•
reduce number of people with cystic fibrosis (in population)
•
reduce health-care costs
•
allows decision / emotional argument,
eg allows people to make choices about termination
cons:
•
possible damage / risk to embryo / fetus / baby
•
possible harm / risk to mother
•
(may) have to make ethical / moral / religious decisions
for polydactyly:
•
cures ‘disfigurement’
•
but condition not life threatening
•
so risks to foetus / mother unjustified
[10]
M4.
(a)
on chromosomes/DNA within the nucleus
each for 1 mark2
(b) parental genotypes correct i.e. Aa Aa;
gamete genotypes correct i.e. A or a A or a/correct lines;
F1 genotypes correct i.e.
AA Aa Aa aa;
aa recognised as child with cystic fibrosis
each for 1 mark
4
(d) (i)
molecule has two long strands/double helix;
idea of held together by (weak) bonds;
each strand has 4 different types of base;
)
which pair with specific bases in opposite strand; )
when strands separate;
) OWTTE
each strand acts as a ‘complementary’ template; )
makes 2 identical strands
)
each for 1 mark
6
(ii) order of bases acts as a code;
which controls the order;
in which amino acids are assembled into protein;
read in triplet
each for 1 mark
3
[15]
19
M5.
(a)
1 in 4 / 1/4 / 1: 3 / 25% / 0.25
do not accept 3:1 / 1:4 / 2:6
1
(b)
either from C and D
accept synonyms for dominant / recessive eg
Normal / faulty
accept genetic diagram if clearly referring to correct individuals or genotypes on family tree
allow ‘gene’ for ‘allele’
any three from:
•
C and D have disorder
ignore ‘C & D are carriers’
•
I/J don’t have disorder
•
C and D have dominant and
recessive alleles
•
recessive alleles from C and D passed to I/J
or I/J have two recessive alleles
NB if allele was recessive then all offspring of C and D would have the disorder = 3 marks
or from A and B
assume response refers to A + B unless contradicted
•
A is homozygous recessive / rr, and B is heterozygous / Rr can be shown
in words or symbols
allow any symbol
•
offspring can be rr or Rr described
allow without key
(c)
(i)
3
(embryos) checked for inherited / genetic disorders / conditions
accept diseases for disorders
1
(ii)
any three from:
•
C/D have disorder / have dominant allele
accept disease / condition
accept ‘gene’ for ‘allele’
ignore reference to ‘carriers’
•
chance of embryo / foetus / child having disorder
or may pass on alleles for disorder to their offspring
•
C/D might want to decide on termination or prepare
for child with disorder
•
G and H don.t have disorder / both homozygous
recessive / have no dominant alleles (for this disorder)
•
so offspring (of G and H) cannot / don.t have disorder
3
[8]
M6.
(a)
•
caused by a recessive* gene / allele
(allow non / not dominant)
•
both parents heterozygous / carry the gene / allele
for 1 mark each
offspring needs two recessive genes to have / inherit disease
20
for 2 marks
or
•
Nn × Nn
•
NN
Nn
Nn
nn
for 1 mark each
nn identified as having the disease*
for 2 marks
4
(b)
any reference to DNA
gains 1 mark
but
different genes means difference in DNA
gains 2 marks
idea of
different codes / instructions for making proteins
or
different (order of) amino acids (in proteins)
for 1 mark
3
[7]
M7.
(a)
cell membranes
1
(b)
(i)
two recessive / cystic fibrosis / faulty / diseased / the allele(s) / genes
two can be implied by second marking point
ignore chromosomes
1
from Bob and Carol / both parents / the parents
if no other marks awarded ‘Carol is a carrier’ gains 1 mark
1
(ii)
(inherited) dominant / normal allele / gene
1
from Carol / mother
ignore references to recessive allele / gene from father / Bob
if no other marks awarded he has just / only one recessive allele gains 1 mark
1
(c)
(i)
reduce number of people with cystic fibrosis (in population)
or
reduce health-care costs
or
expensive to have baby with cystic fibrosis
accept to allow decision / emotional argument qualified
eg allows abortion
or
allows people to make choices about termination
or
help to prepare financially / emotionally etc
1
21
(ii)
any one from:
•
possible damage / risk to embryo / fetus / baby
allow possible harm / risk to mother
•
screening / it is expensive
•
(may) have to make ethical / moral / religious decisions
ignore not natural / playing God / unethical / immoral / religious unqualified
•
right to life
1
[7]
M8.
(a)
idea:
mental/brain deterioration
involuntary muscular movement/fidgety
starts in 40/50’s/middleage
for 1 mark each
3
(b)
ideas that:
mother homozygous *
father heterozygous * (*these terms not essential) ) these marks
) can be gained
allele dominant (H. chorea)
) on diagram*
children normal or heterozygous
chance 50/50
Diagram gains max. 3 marks
for 1 mark each
4
(c)
3 in 4 chance (or 3:1) (of Huntington’s chorea)
gains 1 mark
or equivalent in words
gains 1 mark
2
[9]
M9.
(a)
(i)
mitosis
for 1 mark
1
(ii)
1
fertilised egg cell has
identical cells /
1 albino gene from father splits to produce
produced by mitosis
each for 1 mark
3
(b)
(i)
less protection from UV light / UV radiation
for 1 mark
1
(ii)
ideas of uncontrolled multiplication of mutated cells reject fast /
rapid cell division cells invade of other parts / cells transported in blood
each for 1 mark
2 [7]
22
Worksheet 1 Answers
1 a A genetic disorder which causes extra fingers or toes.
b The faulty allele is dominant, so only one parent needs to have the allele and pass it on for the
offspring to be affected.
c A Pp only – as produced a child that was unaffected.
B Pp – because mother must pass on a recessive allele to produce two unaffected children.
C-E – could be PP or Pp as each parent has the genotype Pp.
2 a Carriers have a normal dominant allele, so their bodyworks normally.
b CF (cystic fibrosis) recessive – must inherit one from each parent to get the disease – but if parents
had the disease themselves, they would almost certainly be infertile so parents must be carriers.
3 Genetic diagram based on Figure 3 in Student Book, showing how the cc (cystic fibrosis) arises.
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