YEAR 10 SCIENCE
SEMESTER 2 – 2011
EXAM REVISION
GENETICS
1
UNIT 4.1 – INHERITANCE
REVISION
1.
In order to understand genetics questions in the exam, you need to understand
what each of the following words means.
For each word write a sentence explaining what it means:
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heredity
trait
dominant trait
gametes
deoxyribonucleic acid (DNA)
diploid cells
zygote
meiosis
dominant gene
genotype
heterozygous
punnett square
roan cattle
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genetics
true-breeding
recessive trait
gene
chromosomes
homologous pair
mitosis
alleles
recessive gene
homozygous
phenotype
codominance
incomplete dominance
2. You must know the following:
a). The structure of a cell.
b). The structure of a chromosome.
2
c). Number of chromosomes in the human body and other organisms.
d). How the process of MITOSIS works.
e). How the process of MIEOSIS works.
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f). The difference between Diploid cells and Haploid cells.
g). Mendel’s pea experiments.
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i).
Why did Mendel use true-breeding pea plants in his experiments?
ii).
Explain the difference between a dominant and recessive trait.
iii). Which pea pods were dominant
iii). Give an example of a dominant and recessive traits in a human.
iv). What conclusions did Mendel make from his pea experiments?
v). What do we call Mendel’s hereditary factors today?
h). Simple inheritance
The gene that controls pod colour in pea plants
comes in two forms : one codes for green pods;
the other for yellow pods.
i). What do we call these different forms of the
same gene?
ii). The allele for green pods is a dominant
gene. How would represent this allele?
iii). The allele for yellow pods is a recessive
gene. How would you represent this allele?
Each pea plant contains two genes for pod
colour, one received from the female, the other
from the male.
iv). What do we called the possible
combinations of parent’s genes?
v. Explain the difference between a
homozygous combination of genes and a
heterozygous combination of genes.
vi). What are the possible gene combinations for
pea pod colour?
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vii). What is the term used to describe the physical appearance of a gene combination?
viii). What are the two different possible phenotypes for pod colour?
ix). The diagram above shows one of Mendel’s pea experiments which relates to pod
colour in pea plants.
x).
In reference to the diagram above. Explain how the recessive yellow pea pod
colour appeared in the second generation of offspring, but not in the first generation
of offspring.
I. Punnett Squares
i). Explain what a punnet square is and why it is useful in genetics.
ii). Complete the following punnet squares for the pod colour experiment shown in the
diagram on the previous page.
Draw a punnet square for the first generation. Select two of the offspring from the
first generation and use them as the parents of the second generation.
Draw a punnet square for the second generation of offspring.
Punnett Square for Pod Colour
First generation
Second generation
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First Generation
Second Generation
Use the information in the table above to answer the following questions.
iii). In the first generation of offspring:
What is the probability that a pea plant will have a green colour?
What is the probability that a pea plant will have a yellow colour?
iv), In the second generation of offspring:
What is the probability that a pea plant will have a green colour?
What is the probability that a pea plant will have a yellow colour?
V). Draw a punnet square representing the crossing of two heterozygous pea plants.
Where “G” represents the dominant green pod colour and “g” represents the recessive
yellow pod colour.
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Other types of inheritance
In many ways Gregor Mendel was quite lucky in discovering his genetic laws.
He happened to use pea plants, which happened to have a number of easily
observable traits that were determined by just two alleles.
And for the traits he studied in his peas, one allele happened to be dominant for
the trait & the other was a recessive form.
Things aren't always so clear-cut & "simple" in the world of genetics, but luckily for
Mendel (& the science world) he happened to work with an organism whose genetic
make-up was fairly clear-cut & simple.
J. Codominance
The prefix “Co” means “together”, as in the word cooperate, meaning work together.
In COdominance, the "recessive" & "dominant" traits appear together in the phenotype
of hybrid organisms.
With Codominance, a cross between organisms with two different phenotypes
produces offspring with a third phenotype in which both of the parental traits
appear together.
Example 1 : Roan cattle
A very very common phenotype used in questions about Codominance is roan fur in
cattle.
Cattle can be :
 red
(RR = all red hairs).
 White (WW = all white hairs) or
 Roan (RW = red & white hairs
together).
Note that we are dealing with
genotypes that are both dominant,
there is no recessive gene.
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We can still use the Punnett Square to solve problems involving Codominance.
The only difference is that instead of using a capital letter for the dominant trait
& a lowercase letter for the recessive trait, the letters we use are both going to be
capital (because neither trait dominates the other).
i). Draw the punnet square for the crossing of a red homozygous bull with a white
homozygous cow.
ii). Draw the punnet square for the crossing of a roan heterozygous bull with a roan
heterozygous cow.
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iii). In the case above, what are the percentages of :
 Heterozygous roan offspring.
 Homozygous red offspring.
 Homozygous white offspring
iv) Draw the punnet square for the crossing of a homozygous white cow
with a roan bull.
v). What should the genotypes & phenotypes for parent cattle be if a farmer wanted
only cattle with red fur?
vi). A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur
together).
a) What pattern of inheritence does this illustrate?
b) What percentage of kittens would have tan fur if a tabby cat is crossed with a
black cat?
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K. Incomplete dominance
With incomplete dominance, a cross between organisms with two different
phenotypes produces offspring with a third phenotype that is a blending of the
parental traits.
It's like mixing paints, red + white will make pink. Red doesn't totally block (dominate)
the pink, instead there is incomplete dominance, and we end up with a new phenotype
in-between.
We can still use the Punnett Square to solve problems involving incomplete dominance.
The only difference is that instead of using a capital letter for the dominant trait & a
lowercase letter for the recessive trait, the letters we use are both going to be capital
(because neither trait dominates the other).
R = allele for red flowers
W = allele for white flowers
red x white ---> pink
RR x WW ---> 100% RW
The trick is to recognize when you are dealing with a question involving incomplete
dominance. There are two steps to this:
 Notice that the offspring is showing a 3rd phenotype. The parents each have
one, and the offspring are different from the parents.
 Notice that the trait in the offspring is a blend (mixing) of the parental traits.
i). A cross between a blue blahblah bird & a white blahblah bird produces
offspring that are silver. The color of blahblah birds is determined by just two
alleles.
 What are the genotypes of the parent blahblah birds in the original cross?
 What is/are the genotype(s) of the silver offspring?
 What would be the phenotypic ratios of offspring produced by two silver
blahblah birds?
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ii). The color of fruit for plant "X" is determined by two alleles.
When two plants with orange fruits are crossed the following phenotypic ratios
are present in the offspring:
 25% red fruit,
 50% orange fruit,
 25% yellow fruit.
What are the genotypes of the parent orange-fruited plants?
Additional Questions from the textbook (p136)
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What is a gene?
What is DNA and where do you find it?
What is meant by a true-breeding plant?
What is meant by a homozygous pair?
Describe the relationship between genes and chromosomes.
Identify how many chromosomes are contained in a human :
- Muscle cell
- Sperm cell
- Ovum
- Red blood cell
More practice questions
1. Compare mitosis and meiosis according to the following chart:
MITOSIS
MEIOSIS
Purpose of the process
Types of cells involved
(location in the body)
Number of cells produced
Name and general number
of chromosomes in the
parent cells
Name and general number
of chromosomes in the
daughter cells
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2.
Why can there be only two forms of a gene in any one cell?
3. How can you exhibit genetic characteristics that neither your mother nor father
show?
4. Explain what is meant by each of the following types of inheritance and give an
example:
a) dominant/recessive
b) incomplete dominance
c) co-dominance
5. Stem length in peas is caused by a single gene with two alleles :
: T (tall) is dominant.
: t (short) is recessive.
a). List all possible genotypes and their corresponding phenotypes for this
characteristic.
b). Use a Punnet square to show the expected offspring from a cross involving a TT
plant and a tt plant.
c) . Use a Punnet square to show the expected offspring from a cross involving a Tt
plant and a tt plant.
d). Use a Punnet square to show if it would be possible for a cross involving two tall
plants to produce any short-stemmed offspring.
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6.
In pea plants, the allele for green seeds (G) is dominant over the allele for yellow
seeds (g).
a)
b)
List all possible genotypes and their corresponding phenotypes for the
characteristic seed colour.
Using a Punnett Square, determine the phenotypic and genotypic ratios
for the types of seeds that will be produced if:
i). homozygous green seed plants are crossed with homozygous yellow
seed plants.
ii). heterozygous green seed plants are crossed among themselves.
7. In vinegar flies, red eye colour (R) is dominant over white eyes (r).
a). List all possible genotypes and their corresponding phenotypes for the
characteristic eye colour
b) Predict the colour of the eyes of the offspring from the following crosses:
i) homozygous red eyed and heterozygous red eyed flies.
ii) heterozygous red eyed and white eyed flies.
8. The ability to taste a bitter chemical called PTC is due to a dominant gene (P).
Predict the ability to taste PTC among children of parents who are:
a)
omozygous tasters (PP) and non-tasters.
b)
both heterozygous tasters .
9. In snapdragons, red flower colour allele (R) and white flower colour allele (R')
combine to produce pink flowers (RR').
a) What kind of inheritance is this? How do you know?
b. Why do you have to use the kinds of letters shown?
c). List all possible genotypes and their corresponding phenotypes for the
characteristic flower colour.
d). Determine the phenotype of the offspring from the following crosses:
i). red and pink
ii). both pink.
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10. The drawing shows some of the stages of reproduction in horses.
(a)
(i)
Name this type of reproduction .......................................................................
(ii)
Name the type of cell labelled A .....................................................................
(b)
Name the type of cell division taking place at the stage labelled:
(i)
B
(ii) C
(c)
How does the number of chromosomes in each cell of the embryo compare
with the number of chromosomes in cell A?
..............................................................................................................
(d)
When the foal grows up it will look similar to its parents but it will not be
identical to either parent.
(i)
Explain why it will look similar to its parents.
.....................................................................................................
......................................................................................................
(ii) Explain why it will not be identical to either of its parents.
......................................................................................................
......................................................................................................
......................................................................................................
11. The brown tapir can either have a Striped coat (S) or a Plain coat (s)
a.
Which coat type is dominant?
b.
A true-breeding striped tapir is crossed with a true-breeding plain tapir.
Write out the full cross for this mating.
c. What will be the genotype of the F1 offspring?
d. What will be the phenotype of the F1 offspring?
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12. Another of the F1 offspring from Q13 was crossed with another spotted tapir
from a different area of the rain forest. Four babies are produced, three have
striped coats and one has a plain coat.
a.
What genotype was the “foreign” tapir?
b.
What genotype would the plain tapir have?
13. Two of the F1 offspring from Q11 were crossed with each other.
a. Draw the punnet square for this mating.
b. Write out the full cross for this mating.
c. What will be the genotypes of the F2 offspring
d. What will be the phenotypes of the F2 offspring?
14. The photographs show a zorse and its parents, a zebra and a horse.
Zebra
Horse
Zorse
(a) Draw a ring around the correct answer to complete the sentence.
The zorse was produced by :
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cloning
asexual reproduction
sexual reproduction
.
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(b)
Explain the appearance of the zorse.
Use both words from the box in your explanation.
gametes
genes
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..............................................................................................................
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15. The diagram shows two patterns of cell division. Cell division type A is used in
gamete formation. Cell division type B is used in normal growth.
(a)
Name the two types of cell division, A and B, shown in the diagram.
Type A :
Type B :
(b)
Name the process in which an egg and sperm join together.
......................................................................................................
(c)
Cell 1 contains 46 chromosomes. How many chromosomes will there be in:
(i)
cell 10;
(ii) cell 14?.
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16. Complete the following passage:
Chromosomes carry genetic information. Chromosomes are made up of
……………………… . Human body cells contain …. Chromosomes. There are
twenty-two matching pairs but the final pair does not always match. It is these two that
determine yhe gender, or sex, of the human. If you are a ……………… the final pair of
chromosomes matches. If you are a ………………. The final pair of chromosomes
does not match.
17. (a) Mice with black fur can have the genotype BB or Bb, whilst mice with brown
fur have the genotype bb.
(b)
(i)
Use a genetic diagram to show what fur colours you would predict in
the F1 offspring produced by two mice who are both Bb.
(ii)
Why might your prediction of fur colour in the F1 generation not be
proved right?
......................................................................................................
......................................................................................................
Using the example in part (a) to help:
(i)
describe the difference between dominant and recessive alleles;
......................................................................................................
......................................................................................................
......................................................................................................
(ii) describe the difference between alleles and genes;
......................................................................................................
......................................................................................................
......................................................................................................
(iii)
describe the difference between homozygous and heterozygous
chromosomes.
......................................................................................................
......................................................................................................
......................................................................................................
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UNIT 4.2 – HUMAN INHERITANCE
REVISION
1. In order to understand genetics questions in the exam, you need to
understand what each of the following words means.
For each word write a sentence explaining what it means:
•
•
•
•
albinism
discontinuous variation
sex-linked conditions
carrier
•
•
•
continuous variation
pedigree chart
haemophilia
Simple human inheritance
In humans some characteristics are under the control of a single gene.
 Some of these characteristics are fairly trivial such as the ones listed in the table
below.
 Others such as right-handedness affect your everyday life.
 Some produce severe conditions such as albinism. Albinism is the inability to
make the pigment melanin that normally colours yor skin.
An albino has white hair and pink eyes. Normal colour (A) is dominant,
while lack of colour (a) is recessive.
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Suppose two people who are
heterozygous for albinism produce
offspring.
Using a punnet square method , the
chances that their offspring will be an
albino are one in four or 25%.
Blood Groups
There are two common ways to group blood.
 Rhesus factor
 ABO groupings.
Rhesus factor
Rhesus is a type of antigen.
 Blood that contains the Rhesus factor is classified
as Rhesus Positive – (Rh Positive).
 Blood without the Rhesus antigen is classified
as Rhesus Negative – (Rh Nrgative).
The Rh system is controlled by two alleles.
 Allele R – having the Rh factor – is dominant.
 Allele r – not having the Rh factor – is recessive.
The combinations that could come if both
parents were heterozygous Rh positive is
shown at left.
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A person may be :
Rh positive :
 Homozygous (RR)
 Heterozygous (Rr or rR)
Or
Rh Negative :
 Homozygous (rr)
ABO Blood Types
In the ABO blood grouping system :
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Type A blood contains : Antigen A
Type B blood contains : Antigen B
Type AB blood contains : Antigen A & B.
Type O blood contains : Neither Antigen A or B.
The most common blood type is Type O. This ABO system of blood types involves
three different alleles, identified as ;
 IA
 IB
 IO
Allelles IA and IB are codominant.
IO is recessive to both IA and IB.
Possible genotypes and phenotypes are shown below :
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The possible blood groups of a child can be determined if the blood groups of the parents
are known.
Alternatively, if the blood groups of the child are known, the possible blood groups of the
father can be determined.
The possible combinations of alleles produce blood types in the following way:
Eye Colour
In white skinned –people eye colour is determined by a single gene.
 Brown eyes (allele B) are dominant over blue eyes (allele b).
 If you have brown eyes, your genotype must be either BB or Bb.
 If you have blue eyes, it must be bb.
 Green and grey eyes are considered as forms of blue.
 Hazel and black are forms of brown.
Discontinuous vs Continuous Variation
 Being albino or not albino; right-handed or left-handed, are discontinuous
characteristics. It’s one or the other.
 Continuous variation relates to things like height, where there is a continuous
range of possible heights.
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Gender – Boy or Girl
 A human has 46 chromosomes arranged in 23 pairs.
 22 of these pairs have chromosomes the same size and shape.
 For pair 23, the chromosomes will either be the same or be different.
 This set of chromosomes determines your sex or gender.
 The chromosomes for pair 23 are known as the X and Y chromosomes.
Chromosomes
Chromosome Pair 23
 Males have the genotype XY
 Females have the genotype XX
 All eggs contain an X chromosome, sperm can have either and X or a Y
chromosome.
In humans, it is the type of sperm (X or Y)
from the father that determines the sex of
the offspring.
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Pedigrees
A pedigree is a pictorial family tree where individual who show a particular disease or
characteristic are marked on it. Pedigree are analysed to determine whether a particular
characteristic is dominant or recessive.
Symbols used in Pedigree
The symbols used in pedigree are shown below ;
Analysing a Pedigree
The pedigree chart below relates to the characteristic of night-blindness.
In generation IV, daughter 2 was not affected, even though both her parents had night
blindness. This suggests that night blindness is a dominant gene.
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If it were recessive the parents would have had to be homozygous to show the disease
and all their children would also show night blindness. The generation III parents must
have been heterozygous, and by chance produced a daughter who was not affected.
The pedigree below shows the incidence of a particular genetic characteristic, perhaps
a genetic disease.
In generation II, an unaffected male and an unaffected female (1) produce a child that
is afflicted by the characteristic. This indicated that the characteristic is caused by a
recessive gene and that the generation II parents are both heterozygous.
Sex-linked inheritance
 The Y chromosome is small and carries very few genes.
 The X chromosomes is longer and has many genes on it.
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 In males (XY) many of the genes on the X chromosomes do not have a matching
allele on the Y chromosome.
 A single gene on the X chromosome, regardless of whether it is dominant or
recessive, will therefore control the phenotype of the male.
 Over fifty diseases and conditions are caused by recessive genes on the X
chromosome. They are called sex-linked or X-linked conditions.
 They include colour blindness; some forms of haemophilia and some form of
muscular dystrophy. These conditions are far more common in males than
females.
Haeomophilia
Haemophilia is a recessive X-linked disease. The genotypes can be worked out by
using :


XH for the normal gene on an X chromosome.
Xh for the recessive gene for haeomophilia on an X chromosome.
Most males have the genotype XHY and don’t have haeomophilia.
Males that have the disease will have the genotype XhY
Females carry the recessive gene, but do not have the disease. Their genotype would
be XHXh. Although they carry the gene it is hidden by the dominant gene XH.
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 Females who have a hidden gene for a disease are called carriers of the
disease.
 Females 2 and 3 in generation II in the pedigree above are carriers XHXh
having inherited the Xh gene from their father.
 In generation II, male 1 must have inherited an Xh gene from his mother, and
a Y from his father.
 Female 1 in generation 1 must therefore have the genotype XHXh
Questions from your textbook – p146
1. Write the genotype of a :
i). Male
ii). A female.
2. Explain why the human population is approximately 50 per cent male and 50 per
cent female.
3. Explain why a carrier of a genetic disease is not affected by it.
4. Explain why males are more likely to be affected by genetic diseases and disorders
than females.
5. A child has blood group AB; the mother has blood group A.
i). identify the possible blood group genotypes of the father.
ii). Identify the possible blood groups of the father.
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Please complete the following practice problems:
2. The gene for human blood groups has three alleles, A, B and O.
Alleles A and B are co-dominant;
Allele O is recessive to both A and B.
Two parents, one blood group A and the other blood group B, had one child who
was blood group O and another child who was blood group AB.
Using a Punnett square or pedigree, explain the genotype of the parents in this
cross and the genotypes of the O and AB children.
2. The condition shown in the pedigree below is haemophilia, in which the gene, h, is
recessive and on the X-chromosome.
a)
What type of inheritance is this?
b)
Give the phenotype for each of the following genotypes :
c)
Work out the possible phenotypes and genotypes for the people in the
pedigree.
.
3. The black pigment in human skin and eyes is called melanin.
 A single gene controls the production of melanin.
 A person who is homozygous for the recessive allele of the gene has no
melanin and is said to be albino.
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The diagram below shows the inheritance of albinism in a family.
(a)
Use a genetic diagram to explain the inheritance of the albino allele by
children of parents P and Q.
(b)
R and S decide to have a child. What is the chance that this child will be
an albino? Use a genetic diagram to explain your answer
4. This is a family tree for a family suffering from a rare recessive blood disorder.
This medical condition is caused by a recessive gene (r). The father in this family does
not carry a copy of the recessive disease gene.
a. What is the genotype of the female A?
b What are the genotypes of the children.
5. The ability to taste a bitter chemical called PTC is due to a dominant gene (P).
Predict the ability to taste PTC among children of parents who are:
a)
b)
homozygous tasters (PP) and non-tasters.
both heterozygous tasters .
6. This is the tree of another family afflicted with the disease described in Q16.
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a. What are the genotypes of the parents?
b. If this family have another child, what are the chances that the new baby would
be affected by this blood disorder?
(Hint: You have to draw out the full cross to answer this question)
7. Complete each sentence by choosing the correct terms from the box.
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46
ADH
DNA
XX XY YY
dominant female male recessive strong 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.
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UNIT 4.3 – CHEMICAL CODE OF LIFE
REVISION
1.
In order to understand genetics questions in the exam, you need to
understand what each of the following words means.
For each word write a sentence explaining what it means:
•
•
•
•

•
•
•
double helix
nitrogen bases
thymine
guanine
replication
amino acids
mutation
sickle cell anaemia
•
•
•
•
•
•
•
sugar and phosphate units
adenine
cytosine
complementary base pairs
codons
gene expression
mutagen
2. Structure of DNA
Deoxyribonucleic acid (DNA) is a long molecule with two strands twisted together to
make what is called a double helix.
Between each strand are cross-links. Its basic structure is similar to a ladder that has
been twisted into a spiral.
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The ladder’s uprights are made of a chain of alternating sugar and phosphate units.
The ladders rungs are made of pairs of special molecules called nitrogen bases.
There are four nitrogen bases :




A - adenine
T - thymine
C – cytosine
G – guanine
Because of their chemical structure each base can pair only with one other.
The only possible complementary pairs are :
 A pairing with T
 C pairing with G
One upright of the ladder (one strand of DNA) might have the base sequence
ATTCGTC. The opposite strand would then have the complementary sequence,
TAAGCAG.
The sequence of these nitrogen bases along the strands of DNA is the basis for all
inherited characteristics.
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Copying DNA
When a cell reproduces by mitosis, the DNA is copied exactly in a process called
replication. The strands are first unzipped. An exact copy is then made by matching each
base with its complementary base. Once a section is copied, one old and one new starnd
are zipped together to produce the duplicate DNA.
Genetic Code
A segment of DNA with a sequence of up to 1000 bases forms a single gene.
The difference between one gene and another is its genetic code, which is established
by the order of its bases.






Sets of three bases form base sequences called codons.
Each codon describes the type and sequence of amino acids.
Cells use amino acids you make protein molecules.
Proteins are long molecules formed by joining amino acids together like beads
on a string.
It is these proteins that determine characteristics such as eye colour.
The order of the codons on a length of DNA “spells out” the order of the amino acids
on a length of protein.
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There are 64 different codons. From these, twenty different amino acids are coded for.
Different combinations of these twenty amino acids create thousands of different proteins.
Example
The codon :
 CGG codes for amino acid alanine.
 TTT codes for lysine
 CAA for valine
Some codes do not code for amino acids, bust instead code for “stop” or “start” instructions.
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Determining characteristics
Most proteins are enzymes that control chemical activities in the cell, affecting its very
nature.
Organisms function through hundreds of chemical reactions catalysed by hundreds of
enzymes. In this way, many characteristics are influenced by many genes.
Example
Tyrosine is normally a colourless amino acid.
In the presence of an enzyme called tyrosinase, however, it is converted into melanin, a
dark-coloured pigment.
The enzyme is not made if the gene for the production of tyrosinase is missing or defective.
Tyrosinase is then not converted to melanin and without melanin there is no pigment.
Albinism results.
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Gene expression
Every cell in your body contains the same type and quantity of DNA with exactly the same
code.
However, some cells grow into muscle cells, some into nerves, some into blood cells and
some into organs such as your lungs.
Other cells produce hormones such as insulin while others do not.
Gene expression is the appearance in the organism of the characteristic that the gene
codes for.
Genes contain information about where and when the gene is to act. Some genes are
switched off. Sometimes the environment switches genes on and off.
Example
Pigment formation in Himalayan rabbits.
The rabbits are normally white with black ears, nose and feet. They inherit a gene for an
enzyme involved in pigment formation. The emzyme is temperature sensitive.
In cold conditions the pigment is produced more actively, leading to the black colour seen
on the extremities of the rabbits. This gene expresses itself at low temperatures. If the
extremities are warmed, no black hair grows. If a coat section is removed and an icepack
added, the hair that grows back is black.
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Mutations
Accidents sometimes occur in the copying of the DNA strands during replication.
A mutation is any spontaneous change in a gene or chromosome that may produce an
alteration in the characteristic for which it codes.
Types of mutations
1. Single gene mutations
A section of DNA is incorrectly copied. Eg. Sickle cell anaemia.
Distorted red blood cells – victims die young.
2. Whole chromosome mutations
Parts of the chromosome break off and re-join, or whole chromosomes are lost or added.
Example, Down syndrome – individual has an extra chromosome nucleus number 21.
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Mutagens
Mutagens are the things in the environment that cause the mutations.
Eg. X-rays, gamma rays, nuclear radiation.
Some chemicals are also mutagens, such as mutagens.
Questions from your textbook. P.154 /5.
1. A base sequence in a particular DNA strand is :
CGGATAAGCTA
i). Write the complementary base sequence.
ii). Introduce a single mutation into the copying of this sequence.
2. Label the diagram and label the bases not already labelled.
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3. What is a codon?
4. Use a diagram to explain how then structure of DNA can be likened to a twisted ladder.
5. What is a double helix?
6. Identify the tree chemicals that make up the structure of DNA.
7. What is meant by “ complementary bases” in the structure of DNA.
8. What does the term “gene expression” mean?
9. List the names of the four nitrogen bases.
10. What do the letters DNA stand for?
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11. The “backbone” of the DNA molecule is made up of two
components, what are these?
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UNIT 4.4 – CONTROLLING INHERITANCE
REVISION
1.
In order to understand genetics questions in the exam, you need to understand
what each of the following words means.
For each word write a sentence explaining what it means:
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selective breeding
gene technology
plasmids
gene probe
Cloning
stem cells
human genome project
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genetic engineering
genetically modified (GM)
recombinant DNA
DNA fingerprinting
therapeutic cloning
gene cell therapy
2. Selective Breeding
Selective breeding is a common and simple process. Merino sheep produce more and
better quality wool than the breeds from which they were originally bred.
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Australian wheat was once attacked by fungal rust disease. Disease resistance, and a
higher yield, were gained when the wild rust-resistant relatives of wheat were crossed with
wheat plants to produce more seed than normal.
Keeping the seeds from only the best plants for next year’s crop is a simple example of
selective breeding.
3. Gene Technology
Scientists have been able to manipulate genes since the early 1970’s. Genetic engineering
and gene technology manipulate DNA within the organism.
Why change genes? Gene technology allows scientists to isolate a specific gene in one
organism, alter it, copy it and insert it into another organism. This done to provide :
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larger harvests
plants that are more disease resistant
plants that can grow in salty soil.
Fruit and vegetables that last longer.
4. GM
GM stands for genetically modified plants or animals.
Genetically modified cotton contains an inserted gene which produces a protein that kills the
caterpillars that would normally devour the plant.
Some plants have had genes inserted to make them to be tolerate to weed-killers such as
roundup. This means the whole paddock can be sprayed with roundup to kill the weeds
without killing the plants.
5. Manipulating the gene.
Enzymes and bacteria are used to cut out a gene and replace it with a desirable gene from
another organism. The mixed molecule that results when you insert a gene from another
organism into an organism is called recombinant DNA.
Circular pieces of DNA called plasmids are cut out of the DNA by enzymes or bacteria.
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6. Transgenics
It is possible to insert modified genes into plant and animal cells. The plant or animal with
the new gene is called transgenic.
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7. Using gene technology for testing
Prenatal testing can identify genetic defects or diseases before a baby is born.
A gene probe is a small piece of DNA with a base sequence identical to part of a
gene. This means the probe can “stick” to a specific gene. Probes are made to
recognise the base sequences of genes associated with diseases.
8. Using gene technology for cloning.
Cloning refers to production of an organism from a single cell. The first successful
cloning experiment was dolly the sheep.
9. Therapeutic cloning
Therapeutic cloning involves taking cells from a person, extracting the DNA, and
cloning the cells by inserting DNA into an egg. The egg grows and after a few days
the stem cells are removed from the egg. These stem cells are special, as they can
grow into any type of cell in the body, given the right conditions.
10. Gene cell therapy
Gene cell therapy involves removing the genetic material from some body cells,
manipulating it and reinserting it into the same person.
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11. Human Genome
The Human Genome is the genetic map of humans. It identifies every gene that
codes for a particular characteristic. 99.9% of the genetic code is the same for every
human. Only 6% of the DNA in person’s genetic code is unique to that person.
The Human Genome project will provide a lot of information to help find cures for genetic
diseases that cannot be cured using today’s medical knowledge.
Questions from your textbook. P.163.
1. Give two examples of selective breeding.
2. Explain the term gene technology.
3. What is recombinant DNA?
4. What is gene cell therapy?
5. Explain the difference between cloning and therapeutic cloning.
6. What evidence is there that cloning may be risky?
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