genetics
The basics
• Genetics is the study of heredity. The foundation for this branch of science
was laid by Gregor Mendel (1822-84).
• Hereditary traits or characteristics are determined by units called genes
(Mendel called these 'factors'). We know that genes are sections of DNA
that can direct the cell to make a particular protein.
• A gene for a particular trait (for example height) can have different results
or 'expressions'(for example tall or short).
• Alternative forms of a gene are called alleles. Letters of the alphabet can
be used to represent the alleles (for examples Tfor tall and I for short).
• In the body cells of an organism, chromosomes are found in homologous
pairs. Genes for a particular trait (feature) are found on both
chromosomes exactly at the same location (locus).
• When an organism produces gametes, each gamete receives only one
chromosome from the pair.
• When alleles are identical, we say the gene pair is homozygous (for
example TTfor tall or tt for short).
• When an organism has two different forms of the allele, we call the gene
pair heterozygous (for example Tt heterozygous tall).
• All true breeding or pure breeding organisms are homozygous for that
particular trait.
• In a heterozygous gene pair, the allele which
expresses itself is called the dominant allele and
the allele whose expression is hidden by the
dominant allele is called the recessive allele.
• The dominant allele is usually represented by an
upper-case letter and the recessive allele is
represented by the corresponding lower-case
letter.
• The genetic make-up of an organism (or simply
the genetic formula) is called its genotype (for
example TT, Tt, tt).
• The expression of an allele is called its phenotype
(for example tall, short).
• Across between two organisms of the same species
(male and female) to study the inheritance of a
particular trait is called a monohybrid cross.
• Below is an example of a simple monohybrid cross.
This is one of the experiments conducted by Mendel
on garden peas.
• Two parent plants were selected. One was true
breeding (homozygous) for round seeds and the other
parent was true breeding for wrinkled seeds. The
allele for round seed is R and the allele for wrinkled
seed is r. Mendel crossed these plants by dusting the
pollen from one plant onto the stigma of the other
plant. He then collected all the seeds and planted
them to study the first generation of offspring. He
called them the first filial generation or FI generation.
All the plants from this cross produced round seeds.
• This is one of the experiments conducted by Mendel
on garden peas.
• Two parent plants were selected.
• One was true breeding (homozygous) for round seeds
and the other parent was true breeding for wrinkled
seeds.
• The allele for round seed is R and the allele for
wrinkled seed is r.
• Mendel crossed these plants by dusting the pollen
from one plant onto the stigma of the other plant.
• He then collected all the seeds and planted them to
study the first generation of offspring.
• He called them the first filial generation or FI
generation.
• All the plants from this cross produced round seeds.
• He then allowed the F1 plants (heterozygous
round-seeded plants) to self-fertilise. The offspring
produced by this cross, the F2 generation,
contained round-seeded and wrinkled- seeded
plants in a definite ratio (3:1 ).
• The dark colour of this
mouse is due to a dominant
allele, C. The alternate
allele is c. In its
homozygous state cc
produces a white coat
colour.
• State the genotype of this
black mouse.
• State the phenotype of this
mouse
• State whether this mouse is
homozygous or
heterozygous
• This mouse has an allele for
black colour and an allele for
white colour but its coat
colour is black. Give a
reason for this.
• 5 State what the
genotype of a
homozygous black mouse
would be.
• 6 State what the
genotype of a white
mouse would be.
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What is the genotype of A
What is the genotype of A
Answer Dd. This is because the offspring of A
and B have both types of chin AND YET both
parents have only one type of chin, so the
parents must be heterozygous.
What is dominant? Shaded or clear? Why?
The grey is dominant because the offspring
of F and G are both shaded and clear AND
YET both parents are shaded so they must
be heterozygous and dominant.
• ans
• The allele for six fingers is probably dominant.
• All affected offspring have at least one affected
parent, for example D, E and F all have six fingers
and so did their mother/A from whom they
received their affected allele (or H, I, J, K and L
whose mother is D).
• in addition, it would not be possible for F and G
to have M and O if the allele was recessive
because M and O have 5 fingers and therefore,
must have at least one normal/dominant allele
which they cannot get from their parents who
have 6 fingers.
• .
answer
Copy
and
answer
Copy these and match with the terms
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a.Gamete
b Karyotype
c Genome
d Allele
e Autosomes
Each body cell of a mouse
contains 40 chromosomes.
Write down the number of
chromosomes found in each type
of cell shown below.
a A sperm cell
b An unfertilised egg
c A muscle cell
d a skin cell
• A mature sex cell having a single
set of unpaired chromosomes.
• A pair of alternative forms of a
gene that can occupy the same
location (locus) on a particular
chromosome and that control a
particular characteristic.
• a chromosome that is not a sex
chromosome.
• A complete set of an organism’s
genes; an organism’s genetic
material.
• A diagram of chromosomes of a
cell arranged by size and
centromere position
The
answer
Copy these and match with the terms
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a.Gamete
b Karyotype
c Genome
d Allele
e Autosomes
Each body cell of a mouse
contains 40 chromosomes.
Write down the number of
chromosomes found in each type
of cell shown below.
a A sperm cell
b An unfertilised egg
c A muscle cell
d a skin cell
• A mature sex cell having a single
set of unpaired chromosomes.
• A pair of alternative forms of a
gene that can occupy the same
location (locus) on a particular
chromosome and that control a
particular characteristic.
• a chromosome that is not a sex
chromosome.
• A complete set of an organism’s
genes; an organism’s genetic
material.
• A diagram of chromosomes of a
cell arranged by size and
centromere position
answer
Copy and match these:
• Define ‘chromosome’.
• Define the term ‘gene’.
• Describe the link between
chromosomes and genes
• A structure found in the
nucleus of a cell made up of
DNA and protein carrying
genes in a linear order.
• A section of DNA that acts as a
unit of heredity. It directs the
cell to manufacture a specific
protein.
• sections of chromosomes. A
long strand of DNA with
special proteins holding it
together. So, chemically both
chromosome and +++++are
the same, both made up of
DNA.
Copy and match these:
• Define ‘chromosome’.
• Define the term ‘gene’.
• Describe the link between
chromosomes and genes
• A structure found in the
nucleus of a cell made up of
DNA and protein carrying
genes in a linear order.
• A section of DNA that acts as a
unit of heredity. It directs the
cell to manufacture a specific
protein.
• sections of chromosomes. A
long strand of DNA with
special proteins holding it
together. So, chemically both
chromosome and genesare the
same, both made up of DNA.
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Give the meanings of the following words.
1 Gene
2 Allele
3 Genotype
4 Phenotype
5 Dominant allele
6 Recessive allele
7 Homozygous
8 Heterozygous
Print this
• A gardener bought some purple seeds from
the garden centre. Sometimes these types of
seed are yellow. Explain what he could do to
determine whether the corn seed he bought
was homozygous or heterozygous.
• Use punnett squares to explain your answer.
answer
• Cross the purple seed corn
plant with yellow seed
(homozygous recessive) corn
plant. Collect all seeds and
grow them to see the colour of
seeds they bear.
• If all corn plants from this
cross produced purple plants,
the original corn plant will be
homozygous dominant (PP).
See Cross 1 next slide. If any of
the corn plants from this
• cross produced yellow seeds,
then the original corn plant
will be heterozygous purple
(Pp). See Cross 2.
• Ming breeds budgies as a hobby. In her pet shop she has budgies of several
colours - green, blue, grey and white. She keeps pedigree charts for each bird
she sells.
• The following is a pedigree chart showing the inheritance of feather colours.
• State which characteristic, green or blue, is the dominant characteristic.
• Give two reasons for your answer.
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Answer:
Green feather colour is dominant. If green feather is a recessive trait, there would be no chance of
producing a blue budgie when two green budgies were crossed.
Here, the two parent birds A and B both are green and two of its offspring 6 and 7 are blue
feathered.
There is a mixture of colours in the offspring so parents must be heterozygous .
The diagrams below show two fruit flies A and B. Fly A is
heterozygous for its eye colour. Fly B is homozygous for its eye
colour. Fly A has red eyes while fly B has brown eyes. The allele
for red eye colour is R and the allele for brown eye colour is r.
Copy and complete
FLY A
Phenotype
Genotype
True breeding
Yes/No
Shows dominant/
recessive trait?
Alleles in gametes
R or r
(50%, 50%)
FLY B
• Olo, who has normal hair, married Hessa.
• Complete the punnett square diagram to show the
chance of different coloured hair in their children.
• What is the chance that Hessa and Olo’s first child will
have white forelock?
Answer
½ or 50% chance
• Qn: Cystic fibrosis is a hereditary disease that affects many glands
in the body. This is a recessive condition in humans.
• The allele for cystic fibrosis can be represented by a and the allele
for unaffected people is A. Pita and Tania have one normal child
but their second child was affected by this condition.
• With the help of a punnett square diagram, explain how this
occurred.
• Ans
• Pita and Tania are heterozygous for cystic fibrosis. They both have
an allele for this condition that they can pass on to their offspring.
Qn : There are several varieties of tomatoes available, including round and
oblong tomatoes.
The allele for oblong shape O is dominant over the allele for round shape o.
In an experiment, a gardener pollinated the flower of a round tomato plant with
the pollen from an oblong tomato plant.
When the fruit ripened he collected 80 seeds. He planted these seeds and
obtained the results as shown below.
State the genotypes of the parent plants.
a Round tomato
b Oblong tomato
2 Complete the punnett square to show the probability of different offspring in such a
cross.
answer
Queston:
1. In meiosis explain why the number of
chromosomes is halved.
Answer
The type of cell division is meiosis, the one
responsible for the formation of gametes. During
meiosis, homologous pairs of chromosomes pair up
and then separate. As a result, the resulting cells
receive only one set of chromosome.
2. Cell division by meiosis produces genetic variation.
Explain how meiosis increases genetic variety and
its significance.
Answer.
Since meiosis produces sex cells which are not all
genetically identical, offspring produced by sexual
reproduction or by the joining of two sex cells that
have different genetic make up, shows variation.
Variation plays a major role in natural selection
and evolution. Variations increase the chance
of survival of a species in a constantly changing
environment.
The diagram below shows the life cycle of cattle. They have 60 chromosomes in their body
cells. The circles represent the cells and the boxes represent the process involved in the
production of cells.
1 Complete the diagram by writing down the numbers and words from the list below. Place
chromosome numbers in each circle and the name of a process in each box.
Discuss the importance of processes A, B and C in the life cycle of cattle.
Answer:a . Meiosis produces gametes. Meiosis reduces the number of chromosomes to 30.
Exchange of genetic material occurs during meiosis produces variation among offspring.
b. Fertilisation joins the male and female gametes. The original number of chromosome is thus
regained.
c.Mitosis occurs at this stage produces growth. Zygote is a single cell. This single cell divides to
form an embryo.
All cells in an embryo must have the same genetic material. This is achieved by mitosis at this
stage.
• In male bears, black ears B
is dominant over white
ears b.
• The genotype of a male
bear is Bb. His mother has
white ears. W h a t is the
genotype of his mother?
• The male bear above
mates with a w h i t e eared female.
• Copy and complete the
following cross diagram
(Punnet Square) t o show
the genotypes of the
offspring that could result.
• In male bears, black ears B
is dominant over white
ears b.
• The genotype of a male
bear is Bb. His mother has
white ears. W h a t is the
genotype of his mother?
• Ans
• bb
• The male bear above
mates with a w h i t e eared female.
• Copy and complete the
following cross diagram
(Punnet Square) t o show
the genotypes of the
offspring that could result.
bb
b
Bb
bb
b
Bb
bb
• The table below shows
information about two pure bred
houseflies.
• W h a t is the genotype of t h e
curly-winged fly?
• The phenotype brittle bones in
people is inherited.
• Brittleness is dominant (B) over
non - brittle bones(b)• Four children have a father with
non - brittle bones (genotype bb)
and a mother with brittle bones
(genotype Bb).
• a) What is the probability that
any of t h e four children has
brittle bones?
• b) In reality, none of the children
have brittle bones. How could
this be so given your answer in
a)?
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The table below shows information
about two pure bred houseflies.
W h a t is the genotype of t h e curlywinged fly?
Ans
nn
The phenotype brittle bones in people is
inherited.
Brittleness is dominant (B) over non brittle bones (b)
-Four children have a father with non brittle bones (genotype bb) and a mother
with brittle bones (genotype Bb).
a) What is the probability that any of t h
e four children has brittle bones?
Ans
50%
b) In reality, none of the children have
brittle bones. H o w could this be so
given your answer in a)?
ans
This result is just a statistical result but in
reality it can be different for a small
sample
•
Some dogs bark when following a scent,
others are silent
Barking (B) is dominant to non-barking (b)
• A hunter owns a barker which he wants
to use for breeding purposes but he
wants to be sure it is a BB barker.
• What is the genotype of the bitch he
should mate with this dog?
• The ability to roll the tongue is inherited.
• In the following family t r e e the which
characteristic is dominant?
What evidence supports your answer to a).
• If the dominant allele has the symbol R
and the recessive allele has t h e symbol
r, give the genotype of individual G.
• The children of A and B are heterozygous.
• W h i c h individual in the family t r e e
shows that the n o n - roller allele exists
in t h e children? Give a reason.
• F and G have another offspring. What is
the probability of i t being a non-tongue
roller?
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Some dogs bark when following a scent,
others are silent
Barking (B) is dominant to non-barking (b)
A hunter owns a barker which he wants to use
for breeding purposes but he wants to be sure
it is a BB barker.
What is the genotype of the bitch he should
mate with this dog?
Ans
He should mate it with a non-barking bb and if
there are any offspring that can bark then he
knows that his dog is Bb, but if there are all
barking dog offspring then he knows that his
dog is PROBABLY BB.
The ability to roll the tongue is inherited.
In the following family tree the which
characteristic is dominant?
Ans
Tongue rolling is dominant
What evidence supports your answer to a).
Ans
Because all the children of parents A and B are
going to be heterozygous and so will show the
dominant characteristic and that is rolling.
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If the dominant allele has the symbol R and the
recessive allele has t h e symbol r, give the genotype
of individual G.
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Ans
rr
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The ability to roll the tongue is inherited.
In the following family tree the which
characteristic is dominant?
Ans
Tongue rolling is dominant
What evidence supports your answer to a).
Ans
Because all the children of parents A and B are
going to be heterozygous and sho will sow the
dominant characteristic and that is rolling.
If the dominant allele has the symbol R and the
recessive allele has t h e symbol r, give the genotype
of individual G.
Ans
Rr
The children of A and B are heterozygous.
W h i c h individual in the family t r e e shows that
the n o n - roller allele exists in t h e children? Give a
reason.
Ans
Individual I
This is because I has 2 different alleles and so got one
from I .
F and G have another offspring. What is the
probability of i t being a non-tongue roller?
Ans
50%
• The following pedigree (called a
'family tree') shows the result of
crossing two white animals. They
produced a litter of six young ones,
five white and one black. In these
animals, coat colour is coded by a
single pair of alleles (alternative
forms of a single gene).
• The allele for white coat colour is
represented by A.
• The allele for black coat colour is a.
• a) Give the genotype for each parent
• b) If the above cross was repeated
many times, what would you expect
to be the ratio of white and black
individuals?
• c) If the black individual was crossed
with a white homozygous individual,
give the genotype and phenotype of
their offspring.
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The following pedigree (called a 'family
tree') shows the result of crossing two
white animals. They produced a litter of
six young ones, five white and one black.
In these animals, coat colour is coded by
a single pair of alleles (alternative forms
of a single gene).
The allele for white coat colour is
represented by A.
The allele for black coat colour is a.
a) Give the genotype for each parent
Ans
Aa and Aa
b) If the above cross was repeated many
times, what would you expect to be the
ratio of white and black individuals?
ans
1:3
c) If the black individual was crossed with
a white homozygous individual, give the
genotype and phenotype of their
offspring.
Ans
All would be Aa and would be white.
• Two parents were known to be right-handed.
They had three children. Sheila and Mary
(identical twins) and Maurice.
• Right-handedness (R) is dominant to lefthandedness (r).
• a) If Maurice is left-handed, what were the
genotypes of the parents?
• b) What is the probability that Sheila is right
handed?
• c) If Mary is right-handed, predict the
handedness of Sheila.
• Two parents were known to be right-handed. They had
three children. Sheila and Mary (identical twins) and
Maurice.
• Right-handedness (R) is dominant to left-handedness (r).
• a) If Maurice is left-handed, what were the genotypes of
the parents?
• Ans
• Rr
• b) What is the probability that Sheila is right handed?
• Ans
• 75%
• c) If Mary is right-handed, predict the handedness of
Sheila.
• Ans
• Righthanded also
• In cats, short tails (S) are dominant over long tails
(s).
• A short-tailed female cat mates with a long-tailed
male and produces a litter of I long-tailed and 7
short-tailed kittens.
• a) Give the genotype of each parent
• b) Out of 8 kittens, how many of them would you
predict by theory to be long-tailed?
• c) The gene for short-tailed is said to be the
'dominant' gene. Explain what this means.
• A plant breeder crossed purebred red-flowered
plants with purebred white flowered plants. All the
offspring were red-flowered.
• D)The gene for red flowers is most likely to be: is it
dominant or recessive?
• In cats, short tails (S) are dominant over long tails (s).
• A short-tailed female cat mates with a long-tailed male and produces a
litter of I long-tailed and 7 short-tailed kittens.
• a) Give the genotype of each parent
• Ans
• Female is Ss and male is ss
• b) Out of 8 kittens, how many of them would you predict by theory to
be long-tailed?
• Ans
• 50% so that would be 4 kittens
• c) The gene for short-tailed is said to be the 'dominant' gene. Explain
what this means.
• ans
• There only needs to be one allele for the phenotype to be expressed.
• A plant breeder crossed purebred red-flowered plants with purebred
white flowered plants. All the offspring were red-flowered.
• What is gene for red flowers is most likely to be:
• Ans
• Dominant.
• In the garden-pea, white flower (w) is recessive to
purple flower (W).
• a) What is the phenotype of plant Ww?
• b) What is the genotype of a plant with white
flowers?
• c) What is the genotype of a plant with purple
flowers?
• In the garden-pea, white flower (w) is recessive to
purple flower (W).
• a) What is the phenotype of plant Ww?
• Ans
• purple
• b) What is the genotype of a plant with white
flowers?
• Ans
• ww
• c) What is the genotype of a plant with purple
flowers?
• ans
• Ww or WW
• Colour blindness is a sex linked character. This
means that it is caused by a gene which:
• A) is present in only one sex.
• B) causes secondary sexual features but is not
found on t h e sex chromosome.
• C) does not cause sexual features but is found
on t h e sex chromosome.
• D) causes primary sexual features.
• Colour blindness is a sex linked character. This
means that it is caused by a gene which:
• A) is present in only one sex.
• B) causes secondary sexual features but is not
found on t h e sex chromosome.
• C) does not cause sexual features but is found
on t h e sex chromosome.
• D) causes primary sexual features.
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Colour-blindness is said t o be a sex-linked characteristic in people. The following is
a chart of t h e pedigree of the inheritance of colour-blindness in a family. Complete
t h e following chart with the genotypes of the given phenotypes.
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Colour-blindness is said t o be a sex-linked characteristic in people. The following is
a chart of t h e pedigree of the inheritance of colour-blindness in a family. Complete
t h e following chart with the genotypes of the given phenotypes.
ans
B
b
BB
Bb
bb
• In pea plants, flowers can be produced at the tip of the stem (called
terminal flowers) or further down the stem (called axial flowers).
• Breeding between a plant with axial flowers and a plant with terminal
flowers always produces offspring with axial flowers. Both parents are purebreeding.
• Describe what pure-breeding means in terms of the type of alleles present.
• In pea plants, flowers can be produced at the tip of the stem (called
terminal flowers) or further down the stem (called axial flowers).
• Breeding between a plant with axial flowers and a plant with terminal
flowers always produces offspring with axial flowers. Both parents are purebreeding.
• Describe what pure-breeding means in terms of the type of alleles present.
• Ans
• Has the same type of allele/is homozygous for one allele
• Explain why all the offspring have axial flowers
using a fully labelled Punnett square and the
symbols F and f to correctly represent the
alleles.NB both parents are pure bred
• Explain why all the offspring have axial flowers
using a fully labelled Punnett square and the
symbols F and f to correctly represent the
alleles.
• Ans
• Has parents as FF – axial, ff – terminal, offspring
as Ff axial correct in Punnett square
• Plants with an FF genotype and an Ff genotype
will both have axial flowers.
• (c) Discuss how the genotype of a plant with
axial flowers could be proven by doing a simple
breeding experiment …..(terminal flowers)
• Ans
• Use a pure breed ff and if genotype of plant is
FF all the (many) offspring will be plants with
axial flowers. However, if Ff genotype then
some offspring would show terminal flowers.
• You Should use a fully labelled Punnett Squares
• (a) Define the term homozygous.
• (b) Using the letter L, give the genotype for a
homozygous recessive individual
• (c) German shepherd dogs have 78
chromosomes. How many would they have in
an egg cell?
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(a) Define the term homozygous.
Ans
A genotype when the two alleles are the same
(b) Using the letter L, give the genotype for a
homozygous recessive individual
Ans
ll
(c) German shepherd dogs have 78 chromosomes.
How many would they have in an egg cell?
Ans
39 chromosomes
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A homozygous short haired male German shepherd was mated with a long haired
female German shepherd and all the pups were short haired.
Complete the
Punnett square
for the cross
showing the
genotype of the pups
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A homozygous short haired male German shepherd was mated with a long haired
female German shepherd and all the pups were short haired.
Complete the
Punnett square
for the cross
showing the
genotype of the pups
• ans
• In pea plants, the allele for yellow seeds (G) is
dominant to that for green seeds (g).
• Describe what is meant by the terms
dominant and recessive.
• Ans
• Dominant shows in phenotype and masks
recessive:
• recessive only expressed if two copies of the
allele are present.
• A heterozygous yellow-seed pea plant is
crossed with another heterozygous yellowseed pea plant.
• Draw a Punnett square to show the genotypes
of the offspring.
• A heterozygous yellow-seed pea plant is
crossed with another heterozygous yellowseed pea plant.
• ans
• Explain the difference between genotype and
phenotype. Use examples from your
Punnett square in the last slide to help in
your explanation.
• Ans
• Genotype is Gg/GG/gg:
• phenotype is yellow seeds/green seeds
• Kathleen is heterozygous for a condition that
is carried on the X chromosome as a recessive
trait.
• (a) Describe the term heterozygous.
• Ans
• the 2 chromosomes carried have two different
alleles for the gene.
• What is meant by the term recessive trait?
• Ans
• a recessive trait is a characteristic that is
expressed only if both alleles are recessive.
• Gottron’s Syndrome is a rare recessive condition
which causes people to develop small hands and
small feet and age prematurely (early). Kathleen’s
cousin has Gottron’s Syndrome but her Uncle and
Aunty do not have it. You have been asked to give a
talk to your class to explain how this family could
have produced a daughter with Gottron’s Syndrome
from parents who do not have it.
• Discuss how you would tell your class how this
family ended up having a daughter with this
condition. What is the chance of their next child
also having this rare disease? Use a Punnet square
to help with your answer
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•
Gottron’s Syndrome is a rare recessive condition which causes people to develop small
hands and small feet and age prematurely (early). Kathleen’s cousin has Gottron’s
Syndrome but her Uncle and Aunty do not have it. You have been asked to give a talk to
your class to explain how this family could have produced a daughter with Gottron’s
Syndrome from parents who do not have it.
Discuss how you would tell your class how this family ended up having a daughter with
this condition. What is the chance of their next child also having this rare disease? Use
a Punnet square to help with your answer
• Ans
• Both parents were heterozygous and so masked a hidden gene so
there is a 25% chance of next child getting it.Each parent gives 1
recessive allele which in the zygote is expressed as this disease .
• Also need the punnet square
The diagram below shows that DNA is made
up of repeating units called nucleotides.
Write the complementary base pairs
A
G
C
A
T
The diagram below shows that DNA is made
up of repeating units called nucleotides.
Answer TCGTA
A
G
C
A
T
• Gregor Mendel was a monk who studied the inheritance of
different characteristics of pea plants. One characteristic he
looked at was pea shape. He found round peas were dominant
over wrinkled peas.
• Discuss the relationship between DNA, genes, alleles and
chromosomes in the pea plants. You should:
•
• Explain the difference between genes and alleles.
• Ans
• A gene is a section of DNA/part of a chromosome that carries the
instructions for a particular feature/to make a protein.
• An allele is the different/alternate form of a gene.
• In the example given, the gene for pea shape has two alleles,
smooth and wrinkled.
• Gregor Mendel was a monk who studied the inheritance of different
characteristics of pea plants. One characteristic he looked at was pea
shape. He found round peas were dominant over wrinkled peas.
• Discuss the relationship between DNA, genes, alleles and chromosomes
in the pea plants. You should:
•
• Explain how the information in the DNA determines the characteristics
(eg. pea shape) and the different forms (variations) of that characteristic
(eg. smooth and wrinkled peas). You may support your answer with a
diagram.
• Ans
• The order of the DNA bases within a gene determines which protein is
made.
• The differences in the base sequences of the alleles, for a particular gene,
results in different features and variation shown in offspring.
• The order of the bases in the allele that codes for the “smooth”
phenotype is different to the order of bases in the allele that codes for
the “wrinkled” phenotype.
The following pedigree chart shows the inheritance of wool
colour in sheep:
Explain which characteristic, white or black wool, is recessive.
Refer to specific sheep in the pedigree chart to support your
1
2
answer.
11
3
4
12
13
6
5
7
14
8
15
Key:
female
male
White wool
Black wool
9
10
16
17
18
The following pedigree chart shows the inheritance of wool
colour in sheep:
Explain which characteristic, white or black wool, is recessive.
Ans
Black wool colour is recessive because:
Sheep 1 and 2 had 6 offspring and some were white number 9 had
black wool so 1 and 2 must be heterozygous white so black is
recessive.
1
11
3
4
12
13
5
2
6
7
14
Key:
female
male
White wool
Black wool
8
9
10
15
16
17
18
Using ‘A’ to represent the dominant allele, and ‘a’ to represent
the recessive allele, give the probable genotype of sheep
number 1.
Ans
1
2
Sheep 1 = Aa
11
3
4
12
13
6
5
7
14
8
15
Key:
female
male
White wool
Black wool
9
10
16
17
18
• hSheep have 54 chromosomes in their body (somatic)
cells. These cells are produced by a process called
mitosis. Gametes are produced during meiosis, which
is a type of cell division that produces genetic
variation.
• Discuss how the sheep gametes produced are
different to normal body cells. Your discussion should
include:
•
– The number of chromosomes in sheep gametes compared
with body (somatic) cells
– An explanation of 2 ways meiosis results in cells showing
genetic variation.
• You may use diagrams to support your answer.
• Sheep have 54 chromosomes in their body (somatic) cells.
These cells are produced by a process called mitosis. Gametes
are produced during meiosis, which is a type of cell division that
produces genetic variation.
• Ans
• The number of chromosomes in sheep gametes is half the
number in sheep somatic cells the gametes of sheep have 27
chromosomes.
• Two processes in meiosis that result in gametes showing genetic
variation are independent assortment and crossing over.
• During meiosis, the homologous chromosomes are randomly
separated into gametes.
• It is random which daughter cell each homologue ends up in.
• Crossing over occurs when homologous chromosome pairs line
up during meiosis and segments of homologous chromosomes
are swapped resulting in chromosomes with different allele
combinations.
•
•
•
•
•
•
Spider lamb syndrome is a disease found •
in certain breeds of sheep in countries
including New Zealand. It is caused by a
recessive mutation and results in lambs
with defects including abnormally long,
spider-like legs, twisted spines and
underdeveloped muscles. Lambs born
with the condition do not normally
survive to full maturity.
Discuss how spider lamb syndrome is
inherited even when both parents do
not have the syndrome. You should:
Complete the punnet square below
showing the genotypes of the lambs
produced when a ram and a ewe are
mated, which are both heterozygous for
the spider lamb syndrome alleles. Use
‘A’ to represent the dominant allele and
‘a’ to represent the recessive allele.
Give the phenotype ratio of the lambs
produced.
Use the results of the punnet square to
explain how lambs can inherit spider
lamb syndrome.
The lamb on the left has spider
lamb syndrome and has abnormally
long legs, compared with the
normal lamb on the right.
• Discuss how spider lamb syndrome is • The lamb on the left has spider
inherited even when both parents do
lamb syndrome and has
not have the syndrome.
abnormally long legs, compared
• Ans
with the normal lamb on the
• Phenotype ratio: 3 normal lambs:I
right.
lamb with spider lamb syndrome.
• Spider lamb syndrome is caused by a
recessive mutation.
• This means lambs will only be born
with the syndrome if they inherit two
recessive alleles (aa).
• When two heterozygous parents mate
(Aa) there is a 25% chance of a lamb
being born with the syndrome.
• Both parents do not have the
Aa
AA
syndrome because even though they
possess a recessive allele, (a) it is
masked by the dominant allele (A).
aa
Aa
• Meiosis results in increased genetic variation within a
population. Mutations can also result in increased genetic
variation. Define mutation and explain why mutations can be
considered the ultimate source of variation compared with
variation resulting from meiosis.
• Ans
• A mutation is a (permanent) change/mistake in the DNA
sequence.
• Mutations are the ultimate source of variation because they
result in new alleles being added to a population. Mutations
change the order of the DNA bases.
• Processes in meiosis result in genetically unique gametes but
these processing are only reshuffling of existing genetic
material.
• Independent assortment produces gametes with new
combinations of chromosomes whereas crossing over produces
new combinations of alleles on chromosomes.
• Aphids are one of the main
organisms that live on rose bushes.
They mainly feed on young buds
and leaves. Aphid eggs live
through the winter in protected
nooks and crannies on the plant. In
the spring, eggs hatch into females
that are capable of reproducing
without mating (asexual
reproduction). In autumn, winged
sexual forms (males and females)
are produced. They mate, and the
females lay eggs for overwintering.
Aphids feed by piercing the plant
and sucking up the liquids. They
mainly feed on young buds and
new leaves.
• Explain the advantages of
asexual reproduction in aphids
• Explain the advantages of asexual
reproduction in aphids.
• Ans
• One advantage of asexual
reproduction is that it is a fast
method of reproduction.
• There is no need to find a mate
and when favourable conditions
arise in Spring, when food supply is
optimal, many offspring can be
produced to take advantage of the
favourable environment.
• Another advantage of asexual
reproduction is that offspring are
genetically identical to the parent.
• Because the parent successfully
exploited the rose bush habitat, the
offspring are also likely to thrive.
•
• Aphids are one of the main organisms
that live on rose bushes. They mainly
feed on young buds and leaves. Aphid
eggs live through the winter in
protected nooks and crannies on the
plant. In the spring, eggs hatch into
females that are capable of
reproducing without mating (asexual
reproduction). In autumn, winged
sexual forms (males and females) are
produced. They mate, and the females
lay eggs for overwintering. Aphids feed
by piercing the plant and sucking up
the liquids. They mainly feed on young
buds and new leaves.
• Sexual reproduction results in
aphids that show genetic
variation. Discuss why genetic
variation is important for
survival in a population of
aphids.
• Sexual reproduction results in
aphids that show genetic
variation. Discuss why genetic
variation is important for
survival in a population of
aphids.
• Ans
• Genetic variation is important in
a population for species survival
in a changing environment.
• If the environment changes, or
for example, a pesticide spray is
used to kill the aphids, if there is
variation in the population it
increases the chance that some
members of the population will
survive and reproduce therefore
ensuring survival of the species.
• The following is a pedigree of
myopia (nearsightedness).
• Individuals with myopia are
shaded.
• Use the key below to classify
the genotypes of individuals
I- 6.
• Key:
• A Heterozygous
• B Homozygous
• C Not sufficient evidence to
determine genotype
• Is the gene which produces
myopia dominant or
recessive.