Download Cell division and inheritance (Student Support)

Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
-1-
Biology workbook
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
WORKBOOK 7 – CELL DIVISION & INHERITANCE
Characteristics are passed on from one generation to the next in both plants and
animals. Simple genetic diagrams can be used to show this. There are ethical
considerations in treating genetic disorders.
Candidates should use their skills, knowledge and understanding to:
■ explain why Mendel proposed the idea of separately inherited factors and why the
importance of this discovery was not recognised until after his death
■ interpret genetic diagrams, including family trees
■ construct genetic diagrams of monohybrid crosses and predict the outcomes of
monohybrid crosses and be able to use the terms homozygous, heterozygous,
phenotype and genotype
■ predict and /or explain the outcome of crosses between individuals for each possible
combination of dominant and recessive alleles of the same gene
■ make informed judgements about the social and ethical issues concerning the use of
stem cells from embryos in medical research and treatments
■ make informed judgements about the economic, social and ethical issues concerning
embryo screening.
Additional guidance:
Candidates should be familiar with the principles Mendel used in investigating
monohybrid inheritance in peas. They should understand that Mendel’s work preceded
the work by other scientists which linked Mendel’s ‘inherited factors’ with
chromosomes.
Additional guidance: HT only
Foundation Tier candidates should be able to interpret genetic diagrams of monohybrid
inheritance and sex inheritance but will not be expected to construct genetic diagrams
or use the terms homozygous, heterozygous, phenotype or genotype.
Subject Content: Cell division
a) In body cells the chromosomes are normally found in pairs. Body cells divide by
mitosis.
b) The chromosomes contain the genetic information.
c) When a body cell divides by mitosis:
■ copies of the genetic material are made
■ then the cell divides once to form two genetically identical body cells.
d) Mitosis occurs during growth or to produce replacement cells.
e) Body cells have two sets of chromosomes; sex cells (gametes) have only one set.
f) Cells in reproductive organs – testes and ovaries in humans – divide to form
gametes.
g) The type of cell division in which a cell divides to form gametes is called meiosis.
h) When a cell divides to form gametes:
■ copies of the genetic information are made
■ then the cell divides twice to form four gametes, each with a single set of
chromosomes.
i) When gametes join at fertilisation, a single body cell with new pairs of chromosomes
is formed. A new individual then develops by this cell repeatedly dividing by mitosis.
Additional guidance:
Knowledge and understanding of the stages in mitosis and meiosis is not required.
Candidates should develop an understanding of the relationship from the molecular
level upwards between genes, chromosomes, nuclei and cells and relate these to
tissues, organs and systems.
For Foundation Tier, knowledge of meiosis is restricted to where the process occurs
-2-
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
and that gametes are produced by meiosis.
Subject content – cell differentiation & stem cells
j) Most types of animal cells differentiate at an early stage whereas many plant cells
retain the ability to differentiate throughout life. In mature animals, cell division is mainly
restricted to repair and replacement.
k) Cells from human embryos and adult bone marrow, called stem cells, can be made
to differentiate into many different types of cells, eg nerve cells.
l) Human stem cells have the ability to develop into any kind of human cell.
m) Treatment with stem cells may be able to help conditions such as paralysis.
n) The cells of the offspring produced by asexual reproduction are produced by mitosis
from the parental cells. They contain the same alleles as the parents.
Subject Content - Genetic variation
a) Sexual reproduction gives rise to variation because, when gametes fuse, one of
each pair of alleles comes from each parent.
b) In human body cells, one of the 23 pairs of chromosomes carries the genes that
determine sex. In females the sex chromosomes are the same (XX); in males the sex
chromosomes are different (XY).
c) Some characteristics are controlled by a single gene. Each gene may have different
forms called alleles.
d) An allele that controls the development of a characteristic when it is present on only
one of the chromosomes is a dominant allele.
e) An allele that controls the development of characteristics only if the dominant allele
is not present is a recessive allele.
f) Chromosomes are made up of large molecules of DNA (deoxyribo nucleic acid)
which has a double helix structure.
g) A gene is a small section of DNA.
h) Each gene codes for a particular combination of amino acids which makes a specific
protein.
i) Each person (apart from identical twins) has unique DNA. This can be used to
identify individuals in a process known as DNA fingerprinting.
Additional guidance:
Knowledge and understanding of stem cell techniques is not required.
Candidates are not expected to know the names of the four bases or how
complementary pairs of bases enable DNA replication to take place.
HT only - Candidates should understand that genetic diagrams are biological models
which can be used to predict the outcomes of crosses.
Subject content - Genetic disorders
a) Some disorders are inherited.
b) Polydactyly – having extra fingers or toes – is caused by a dominant allele of a gene
and can therefore be passed on by only one parent who has the disorder.
c) Cystic fibrosis (a disorder of cell membranes) must be inherited from both parents.
The parents may be carriers of the disorder without actually having the disorder
themselves. It is caused by a recessive allele of a gene and can therefore be passed
on by parents, neither of whom has the disorder.
d) Embryos can be screened for the alleles that cause these and other genetic
disorders.
Additional guidance:
Knowledge and understanding of genetic fingerprinting techniques is not required.
Attention is drawn to the potential sensitivity needed in teaching about inherited
disorders.
Knowledge and understanding of embryo screening techniques is not required.
-3-
Wellsway school science dept.
1
Curriculum 2011 – AQA Additional Science
Biology workbook
Structures in the nucleus.
Discuss with your teacher the role of the nucleus and how the nucleus,
chromosomes, genes and DNA are all linked.
Using labelled diagrams show this relationship in the space below:
If you have finished….. Find out how DNA’s coded information affects an organism’s
inherited characteristics:
DNA codes for the production of specific PROTEINS which in
turn produce all our inherited characteristics.
-4-
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
DNA
1. (Optional practical.)
Using the method provided, prepare some DNA from Kiwi Fruit or Onion.
2. Write down 2 reasons why we might we need to extract DNA from cells.
a. To
match it with DNA taken from a crime scene
b. Paternity
/ maternity testing – to prove that a child is/is not
related to its alleged parents.
3. An interesting calculation!
Calculate how many times to the moon and back a human’s DNA would reach if it was
removed from each cell and each strand was laid end to end. Each cell nucleus in a
human holds about 2 metres of DNA. A typical adult is comprised of approx. 50 trillion
cells (A trillion is 1 000 000 000 000). The distance from the Earth to the moon is
380000 kilometres.
50 x 2 trillion m of DNA in all our cells = 100 000 000 000 000 m of DNA
= 100 000 000 000 km of DNA in all our cells.
100 000 000 000 / 380 000 = 263157 times!!
2
Genetic Fingerprinting
Discuss Genetic fingerprinting with your teacher.
The DNA in your cells is as unique as your fingerprints. Unless you have an identical
twin your ‘genetic fingerprint’ is different from anyone else’s
1. Complete the following paragraph:
DNA can be analysed by genetic FINGERPRINTING. This involves
CUTTING the DNA into short PIECES and then separating them into
bands. The PATTERN of the bands can then be COMPARED
to show SIMILARITIES and differences in the DNA.
WORD LIST:
-5-
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Cutting
compared
fingerprinting
2. Look at the genetic fingerprints below.
patterns
pieces
Biology workbook
similarities
The fingerprints labelled 1-7 are suspects in a murder investigation. The fingerprint in
the middle of the picture was DNA found at the scene of the crime.
Answer the questions below:
a. Who do you think committed the murder?
b. Explain your answer clearly:
3. Go to the following website:
http://library.thinkquest.org/C0125833/english/whodunit.php
In the interactive menu option complete the “Whodunit”? scenario (DNA fingerprint
game)
The 3 suspects are Jason Chua, Melvin Tan and Danny Lee.
a. Write down who was most likely to have committed the crime & explain your answer:
Melvin Tan – his genetic fingerprint shows he was at the crime
scene
b. Why can you not be certain that your suspect committed the crime?
It only shows he was at the crime scene – not that he actually
committed the murder.
4. Read P292 ‘Biology For You’ and write down who you think is the guilty suspect.
-6-
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
Suspect 2 – genetic fingerprint matches that from crime scene.
3
Cell Division – Mitosis
Watch the video or PowerPoint presentation on Mitosis & answer the questions below:
1. Write down the three reasons why it is so important that cells must divide.
a. Growth – addition of new cells
b. Replacement of dead / damaged cells & repair of tissues
c. Asexual reproduction – in some animals and many plants
2. How many sets of chromosomes are present in normal body cells? Where does
each set originate from?
TWO – one from each parent’s gamete (egg or sperm)
3. How many “daughter” cells are formed from each cell which divides by mitosis?
TWO
4. How does the number of chromosomes and genetic information in each daughter
cell compare with that in the parent cell?
Same number of chromosomes and genetic information is
identical
5. What is the modern term for the production of identical offspring by mitosis?
Cloning
6. Draw a diagram showing the production of 2 daughter cells from one parent cell:
-7-
Wellsway school science dept.
4
Curriculum 2011 – AQA Additional Science
Biology workbook
Cell Division – Meiosis
Watch the video or PowerPoint presentation on Meiosis & answer the questions below:
1. What is the purpose of meiosis?
To produce HAPLOID GAMETES – i.e. sex cells with HALF the number of
chromosomes as normal body cells; e.g. egg, sperm, pollen.
2. What is the purpose of gametes (sex cells)?
To fuse together at fertilisation. The fertilised egg now has the normal
number of chromosomes and can divide and grow into a new individual.
3. Where does meiosis take place?
In GONADS – special sex glands; e.g. ovaries, testes, anthers.
4. TRUE or FALSE: meiosis takes place in egg and sperm cells.
(If the answer is false, write out a correct statement about egg and sperm):
FALSE – meiosis takes place in GONADS and produces egg and sperm
cells.
5. How many sets of chromosomes are present in the cells which produce gametes?
TWO – they are DIPLOID.
6. How many sets of chromosomes are present in gametes? Why is this important?
ONE – they are HAPLOID. this is important so that the normal DIPLOID
number of chromosomes is restored when the sex cells fuse at fertilisation.
7. How many times does a cell divide during meiosis to produce a gamete?
TWO
8. How many “daughter” cells are formed from each cell which divides by meiosis?
FOUR (Usually – in human females 3 of these break down to leave one
egg, per month).
9. How does the number of chromosomes and genetic information in each daughter
cell compare with that in the parent cell?
Number of chromosomes is HALVED and the genetic information is
shuffled during meiosis so that the daughter cells (the gametes) are
always genetically different.
6. Draw a diagram showing the production of 4 daughter cells from one parent cell:
-8-
Wellsway school science dept.
5
Curriculum 2011 – AQA Additional Science
Biology workbook
Cell differentiation & stem cells
Read your textbook or support material on growth in animals and plants.
1. Describe the main differences between growth in animals and plants in terms of:
a. where growth takes place.
Animals – all over the body, producing a compact body + limbs
Plants - only at growing tips and cambium (=meristems) producing a
branching shoot (photosynthesis) & root system (anchorage; absorption of
water & mineral ions)
b. how long animals and plants can keep growing.
Animals – until they reach a finite, maximum (adult) size e.g. humans, 18
years
Plants - continue growing in height & girth until they die
c. ability of tissues and organs to regenerate from undifferentiated cells.
Animals – generally poor, but new research with stem cells shows that
some can
Plants - generally much better e.g. many plants will grow new roots from
cuttings.
d. which regions of plants and animals retain the ability to divide by mitosis.
Plants divide ONLY at MERISTEMS - Specialised regions of plants which
retain the ability to divide by mitosis throughout the life of the plant, giving
rise to new tissues .e.g. root meristem – in root tip, shoot meristem – in
shoot tip
Animals – most organs are capable of dividing throughout the life of the
animal, although in some very specialised organs this is very limited e.g.
brain & nervous system.
2. a. Find out what stem cells are. Use your textbook, support materials, or an internet
search.
Undifferentiated cells which are capable of dividing and differentiating into
different cell types.e.g Bone marrow cells, umbilical cord cells, embryo
-9-
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
cells.
b. Explain how stem cells can be used to treat certain disorders, e.g. paralysis.
Stem cells can be introduced into an area where cells are damaged e.g.
motor nerve cells, and can differentiate into new cells to replace the
damaged ones. If the damaged motor nerve cells are replaced, the
paralysis might be cured.
c. Discuss the advantages and possible ethical problems of using stem cells, especially
those from human embryos, to replace damaged tissues.
Advantages:
1. Stem cells are immortal – keep dividing indefinitely
2. They can be used to repair many damaged tissues and organs e.g. spinal cord,
pancreas, brain.
3. If they are taken from embryos cloned from a person’s own cells, they will be
genetically identical, and less likely to be rejected by that person’s immune
system.
4. This could reduce the need for transplant operations, with patients waiting for
organs from donors (often dead ones).
5. They can be used to test drugs, so reducing the need to test them on animals.
Disadvantages; moral/ethical problems (mainly of using embryo stem cells):
1. Stem cells can be obtained from human embryos. Is it morally right to kill these
tiny balls of cells which could develop into human beings?
2. The S. Korean fraud shows that scientists are desperate to make
breakthroughs in this field, because it could earn huge amounts of money for
those who succeed, and this might become more important than the medical
advantages.
3. Many egg cells are needed from young women. This is expensive & risky – a
needle is injected into their ovaries, and fertility drug treatment is needed. Is it
worth the risk?
4. Research is still at a very early stage and not many trails have been successful
yet. There could be unforeseen problems.
5. Other techniques, especially using adult stem cells, are available and highly
promising – so using embryo stem cells might not even be necessary.
6. Some research suggests that stem cells are more likely to mutate to form
cancer cells which will produce tumours.
6
Boy or Girl
Chromosomes are usually found in pairs in the nucleus of each body cell. These are
called homologous pairs.
Humans have 23 pairs (46) of chromosomes. Different species of plants and animals
have different numbers of chromosomes e.g. cats have 19 pairs. In all mammals,
males have one X and one Y chromosome, but females have two X chromosomes.
The pictures (A and B) below show matched sets of chromosomes in the male and
female of an organism.
A
B
- 10 -
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
a. What species of mammal have these 2 pictures come from? Explain your answer:
Homo sapiens – humans! 46 chromosomes = 23 pairs
b. Which one is male and which is female? Explain your answer:
A = male – X and Y chromosome; B=female – 2 X
chromosomes
Discuss with your teacher how the sex chromosomes can determine the sex of an
individual.
Think about which chromosomes, X or Y can be carried by the egg and sperm
(remember – a gamete can only carry ONE of a pair of chromosomes).
Your teacher will explain to you about how to draw and use a PUNNETT SQUARE.
c. Complete the diagram to show the probability of a couple having a boy or girl baby:
PARENT:
Male
Female
SEX CHROMOSOMES: (2)
XY
XX
SEX CHROMOSOME
IN GAMETES:
(1)
X
PUNNETT SQUARE:
- 11 -
or
Y
X
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
CHROMOSOME
IN
GAMETES
Biology workbook
X
Y
X
XX
XY
X
XX
XY
Probability
of a boy baby:
Probability of a girl baby:
50%
50%
d. The sex (gender) of a baby is ultimately determined by the type of sex chromosome
the sperm carries. Explain why.
The egg can only carry an X chromosome.
The sperm can carry either an X or a Y chromosome and it is this
which determines the gender of the baby.
e. Henry VIII either divorced or beheaded wives who were unable to give him a boy
child. Was Henry correct in his assumption that his wives had failed him? Explain your
answer clearly:
No – he was not! Only he could produce a boy baby by supplying
a Y chromosome to the egg!
7
Genetics crosses
a. Gregor Mendel
Read the passage below and answer the questions:
Gregor Mendel was an Austrian monk born in 1822. His work on pea plants in 1865
marked the start of modern genetics. He investigated the height of pea plants which
were all either tall or dwarf.
He started off by taking plants which was pure-breeding for tallness( this means when
bred with itself or other tall plants they only produced tall plants)and plants which were
pure-breeding for dwarfness (this means when bred with itself or others dwarf plants
they only produced dwarf plants). He then cross-fertilized these two plants by taking
pollen from each one.
To Mendel’s surprise he found that all the plants produced from the cross were tall. He
concluded that “When pure breeding plants with contrasting traits (e.g. tall and dwarf)
are crossed, all the offspring will resemble one parent”.
He then crossed several of these tall plants which he had produced. Again to his
surprise he found there was a 3:1 ratio of tall to dwarf plants. He concluded that ‘for
every characteristic each individual must have two determiners or factors (now known
as genes).
- 12 -
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
a. What did Mendel investigate? The
inheritance of height in pea plants
He investigated the height of pea plants which were all either tall
or dwarf.
b. What are pure breeding plants?
Plants which always produce the same characteristic as
themselves when bred together
c. When Mendel cross-pollinated pure breeding tall and dwarf plants what did he find?
That all the plants were TALL
d. When he cross-pollinated these tall plants which he had produced, what did he find?
That TALL and DWARF pea plants were produced, in a 3:1 ratio.
e. What did he conclude (find out) from this?
He concluded that for every characteristic each individual must have two
determiners or factors.
Also that TALL genes (alleles) are DOMINANT over DWARF genes (these
are called RECESSIVE).
f. What are these determiners or factors now known as?
Genes!
Find out why Mendel’s ideas were not recognised/accepted until 1900.
He didn’t publish his results widely enough. They were
discovered by accident by British scientists after his death, &
they realised how important they were.
b. Key terminology
Subjects/science/subject data/PowerPoint presentations/
Boardworks/KS4 Biology/Inheritance.
There are quite a few new words to learn in genetics. Discuss with your
teacher/research what the following terms mean:
Genotype
A description of the pair of alleles present for a characteristic,
e.g.
BB, Tt, aa.
Phenotype
The physical expression of the alleles, e.g. tongue rolling, blue
eyes, red petals.
Allele (we have done this one!) A different form of a gene that codes for
a different version of a characteristic.
Dominant
- 13 -
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
An allele that will always be expressed even when there is only
one of these alleles present, represented by a capital letter.
Recessive
An allele that will only be expressed when both alleles are of this
type e.g. hh.
Homozygous
Pair of alleles that produce a characteristic are the same, e.g.
HH.
Heterozygous
Pair of alleles that produce a characteristic are different, e.g.
Hh.
In genetics we use capital letters for dominant alleles and lower case for recessive
alleles
Complete the following table. The first one has been done for you
Allele name
& letter
Homozygous
dominant
genotype
Tongue rolling
Can roll = T
Can’t roll = t
Eye colour
Brown = B
Blue = b
phenotype
TT
BB
Ear lobes
Free
=E
Attached = e
EE
Heterozygous
genotyp
e
(can roll)
phenotype
Tt
(can roll)
(brown eyes)
Bb
(free lobes)
Ee
Homozygous
recessive
genotype
phenotype
tt
(can’t roll)
(brown eyes)
bb
(blue eyes)
(free lobes)
ee
(attached
Lobes)
c. Monohybrid Inheritance
When a characteristic is determined by just one pair of alleles we can do a simple
genetics cross to investigate the mechanism of inheritance. This is called Monohybrid
Inheritance.
With the help of your teacher complete the following crosses involving eye colour:
i. 2 Homozygous parents, one with brown eyes, one with blue eyes.
Parents’ phenotype:
brown eyes
Parents’ genotype:
BB
Parents’ gametes:
(egg and sperm)
B
or
- 14 -
blue eyes
bb
B
b
or
b
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
All the offspring have
Biology workbook
ALL Bb
Offspring genotypes:
BROWN eyes
ii. 2 Heterozygous parents, both with brown eyes.
Parents’ phenotype:
brown eyes
brown eyes
Bb
Parents’ genotype:
B
Parents’ gametes:
(egg and sperm)
or
Bb
b
B
or
b
Offspring genotypes:
(Draw a Punnett square)
B
B
B
BB
Bb
b
Bb
bb
The genetics diagram shows a 3:1 ratio of brown to blue eyes.
This is called the MONOHYBRID RATIO
iii. A heterozygous brown eyed parent and a blue eyed parent.
Parents’ phenotype:
brown eyes
Bb
Parents’ genotype:
B
Parents’ gametes
(egg and sperm)
Offspring genotypes:
(Draw a Punnett square)
b
The diagram shows a 1
blue eyes
or
bb
b
b
B
b
Bb
bb
or
b
b
Bb
bb
: 1 ratio of Brown to Blue eyes.
Genetics Practice Questions.
The questions below get increasingly more difficult, have a go!
Answer on separate paper All the answers below give
only a summary
– make sure you answer in full to gain full credit!
1. The coat colour of mice is controlled by a single gene. This gene has 2 alleles B and
- 15 -
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
b.
a. A mouse with black fur (genotype BB) was crossed with a mouse with white fur
(genotype bb). All the baby mice were black. Use a genetics diagram to explain why
all the mice had black fur. All baby mice were Bb and B allele is
dominant
b. A different mouse with black fur (Bb) was crossed with a mouse with white fur (bb).
Use a genetics diagram to explain why some of the baby mice had black fur and some
had white fur. Half of the baby mice were Bb (black) and half were
bb (white).
2 .Draw genetics diagrams to predict the possible outcome when two heterozygous
tongue rolling people (Tt) have children. The tongue rolling allele is dominant to the
non- rolling allele. There should be a 3:1 ratio of tongue rolling to non-
rolling children (TT; Tt; Tt; tt – only tt will be non-rollers).
3. A homozygous long haired cat is crossed with a homozygous short haired cat and
produces a litter of 9 long haired kittens. Show the probable offspring which would be
produced if two of these kittens were mated, including genotype and phenotype ratios.
All of the long haired kittens would be Ll. If 2 of these were mated
there should be a 3:1 ratio of longhaired to short haired kittens –
only ll would be short haired.
4. In humans, the allele for wavy hair is dominant to that for straight hair. (In your
answer use H to represent the allele for wavy hair and h to represent the allele for
straight hair)
a. Explain the meaning of this statement:
“A person with wavy hair may be heterozygous or homozygous for this characteristic”.
Since the wavy hair allele is dominant, HH and Hh will produce
wavy hair.
b. Explain why a man with straight hair married to a woman with straight hair is unlikely
to have any children with wavy hair.
Both are homozygous – hh. Since neither carries a H allele,
NONE of their children should have wavy hair.
c. By means of a genetic diagram explain why a man with wavy hair who marries a
woman with wavy hair may have some children with straight hair.
If both are heterozygous Hh, then each of them could supply a h
allele in their egg/sperm, producing hh – a straight haired child.
5. Can you draw the genetics diagrams for Mendel’s experiment with pea plants?
Use T to represent the allele for tallness and t to represent the allele for dwarfness.
TT x tt all Tt – heterozygous tall
Tt x Tt ¼ TT; ½ Tt; ¼ tt – 3:1 ratio of tall to dwarf plan ts.
8
Genetic disorders
Discuss Genetic Disorders with your teacher.
Watch the PowerPoint or read your textbook on Genetic Disorders.
a. What is a Genetic Disorder?
A body malfunction caused by one or faulty alleles inherited from
one or both parents
b. What is a mutation?
A change in a gene, caused by a change in its DNA. It might or
might not be harmful.
c. Write down the name of 3 examples of Genetic Disorders.
Cystic fibrosis; Down’s syndrome; haemophilia; sickle-cell
- 16 -
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
anaemia; Huntington’s syndrome
The chance of children inheriting a genetic disorder from its parents depends on 2
things:
i. Whether one or both parents have one or more alleles for the genetic
disorder;
ii. Whether the genetic disorder is caused by a dominant or recessive allele.
d. In the disorder POLYDACTYLY, people have more fingers or toes than usual.
The disorder is caused by a dominant allele, P.
In the space below, carry out a genetics cross as indicated and find out the probability
of children inheriting the disorder:
i. A heterozygous man for Polydactyly has children with a normal-fingered woman.
Parents’ phenotype:
Polydactyl man
Normal woman
Pp
Parents’ genotype:
P
Parents’ gametes:
(egg and sperm)
Offspring genotypes:
(Draw a Punnett square)
The diagram shows a 1
: 1
or
pp
p
P
P
p
p
Pp
pp
p
Pp
pp
or
p
ratio of Polydactyl and normal children.
ii. How would the ratio of polydactyl to normal children differ if both parents were
heterozygous for Polydactyly?
There should be a 3 : 1 ratio of polydactyl to normal children
as shown in the Punnett square – only pp will be normal:
P
p
P
PP
Pp
p
Pp
pp
e. Cystic Fibrosis
Discuss Cystic Fibrosis with your teacher.
Watch video clips which show cystic fibrosis (CF) or read your textbook.
Answer the questions below:
i. Describe the main symptoms of CF.
Thick sticky mucus in lungs & intestine.
Breathing difficulties.
Problems digesting food.
ii. What kind of allele causes CF?
A faulty recessive allele.
iii. People who are heterozygous for CF are described as SYMPTOMLESS
CARRIERS. Explain what this means, & why they have no symptoms of CF.
They are Nn – they carry one normal dominant allele, N and one faulty
recessive allele,n. The dominant allele prevents the expression of the
recessive allele, so the carriers show no symptoms of CF.
- 17 -
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
iv. Can 2 normal people or one normal person and one symptomless carrier have
children with CF? Explain your answer in each case.
NO! 2 Normal people (NN) can only produce normal children.
A normal person (NN) & a symptomless carrier (Nn) can only produce
normal children or symptomless carriers.
v. Can 2 symptomless carriers have children with CF? YES!
Complete a genetic cross & calculate the % probability that a child will be born with CF.
(Label the dominant normal allele N and the recessive CF allele n)
Parents’ phenotype:
male symptomless
carrier
Nn
Parents’ genotype:
N
Parents’ gametes:
Offspring genotypes:
(Draw a Punnett square)
The diagram shows a 3
female symptomless
carrier
: 1
or
Nn
n
N
N
n
N
NN
Nn
n
Nn
nn
or
n
ratio of normal & symptomless carriers to CF children.
vi. Embryo screening can be used to test for defective alleles including the CF allele.
Explain what is meant by embryo screening.
Embryo screening checks an embryo to see if it's carrying any alleles for a
genetic disease (e.g. Huntington's disease or cystic fibrosis) so that the
parents would know how likely it would be for their offspring to have a
genetic disease. They could then decide whether to have a termination
because of the genetic disorder.
Read more: http://wiki.answers.com/Q/What_is_embryo_screening#ixzz1W9v4qvWe
vii. (If you have finished…. )
Approximately 1 in 25 people are symptomless carriers of CF. What is the likely
incidence of CF in the population? Show your working.
2 symptomless carriers would need to have children, so the
probability of any child being born is 1/25 x 1/25 = 1/625.
However, only 1 in 4 children are likely to have CF, so the
incidence is 1/625 x ¼ = 1/2500 – which is about what the
incidence is in the UK population.
Well done! You have now completed the Cell division & Inheritance topic.
Useful web links:
The student room
http://www.thestudentroom.co.uk/wiki/Revision:GCSE_Biology_-_Genetics__Variation,_Mitosis_and_Meiosis#Mitosis
s-cool
http://www.s-cool.co.uk/gcse/biology/genes-and-genetics
revision world
http://www.revisionworld.co.uk/gcse/biology/genetics-and-dna
bbc bitesize
- 18 -
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/celldivision/inheritance1.shtml
gcse-genetics
http://twig-it.com/gcse-genetics
purchon
http://www.purchon.com/biology/genetics.htm
biology resources
http://www.biology-resources.com/biology-questions.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Glossary (muddled – sort them out!)
Allele
Contains the genetic code, Shaped like a double helix
Chromosome Controls your characteristics. It is a section of DNA molecule
DNA
Found in the cell nucleus, X shaped, contains many genes.
Dominant
The alternative form of a gene e.g. Alleles for eye colour brown and
blue
Gene
Controls the development of a characteristic even when it is present
on only one of the chromosomes in a pair
Genetics
Paired chromosomes; only come together at the start of meiosis
Genotype
The chromosomes in a pair contain different alleles of a gene (Tt)
Homologous
The chromosomes in a pair contain the same alleles of a gene (TT or
tt)
Heterozygous The type of genes an organism has e.g. TT, Tt, tt
Homozygous
A study of the way parents pass characteristics on to their offspring
Meiosis
Cell division, produces cells for growth and repair
Mitosis
Unspecialised cells, capable of differentiating into other cell types
Phenotype
Controls the development of a characteristic only if a dominant gene is
not present
Polydactyly
Physical characteristics of an organism e.g. blue eyes, long tail
Recessive
Cell division, produces cells for sexual reproduction
Stem Cells
Condition of more than 5 toes or fingers, caused by a dominant allele
Questions.
- 19 -
3
5
2
1
4
8
9
10
7
6
12
16
15
13
11
14
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
- 20 -
Biology workbook
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
- 21 -
Biology workbook
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
- 22 -
Biology workbook
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
- 23 -
Biology workbook
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
AQA past paper question
3 In the 1850s, Gregor Mendel carried out breeding experiments using peas.
(a) The importance of Mendel’s work was not recognised until the early 1900s.
Explain why.
..........................................................................................................................................
..........................................................................................................................................
..........................................................................................................................................
..........................................................................................................................................
(2 marks)
(b) A student repeated one of Mendel’s experiments.
The flow chart shows her procedure.
- 24 -
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
Biology workbook
The diagram shows a representative sample of seeds produced by second generation
plants.
(i) Describe how the student could obtain a sample that is representative of seeds
produced by the second generation.
..................................................................................................................................
..................................................................................................................................
(1 mark)
(ii) What was the approximate ratio of yellow seeds to green seeds in the seeds
produced by the second generation?
..................................................................................................................................
(1 mark)
(iii) Seed colour in peas is controlled by a single gene which has two alleles.
Use a genetic diagram to show why this ratio of yellow seeds to green seeds was
produced by the second generation.
Use the symbol A to represent the dominant allele, and a to represent the recessive
allele.
(4 marks)
(H PAPER – JAN 2008)
- 25 -
Wellsway school science dept.
Curriculum 2011 – AQA Additional Science
- 26 -
Biology workbook