Key area 2: Plant and animal
breeding by manipulation of
heredity
Manipulating heredity
LI:
1. Give examples of characteristics that
have been selected for by breeders and
explain their benefits.
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Selective breeding
Breeders of crops and livestock have been
manipulating heredity (passing on of
traits to offspring) for hundreds of
thousands of years.
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Watch this clip on Supercows
Selective breeding is done in order to
produce new and improved cultivar of a
plant or breed if an animal that will
provide a sustainable source of food for
humans.
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Heritable
characteristic
Higher yield
Example
Increase in mass of food produced
by wheat crop
Higher nutritional Increase in mass of protein produced
value
by soya bean crop
Resistance to pests Resistance of tomato to eelworm
Resistance to
disease
Resistance of potato to late blight
Possession of
Growth of cereal crop to uniform
useful physical
height suited for mechanical
characteristics
harvesting
Ability to thrive in Ability of maize to grow in cold, damp
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a particular
climate.
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and
environment
interdependence
Plant field trials
LI:
1. Explain why plant field trials are
carried out.
2. In plant field trials explain the
importance of: selection of
treatments, replicates and
randomisation of treatments.
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Plant field trials
A plant field trial is a type of
investigation, set up to:
1. Compare the performance of two
different plant cultivars (e.g.
conventional versus GM) under the
same set of experimental conditions
2. Find out the effect of different
environmental conditions on a new
cultivar of crop plant.
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Designing a plant field trial
Once you have decided what it to be
investigated the following factors
must be considered:
1. Selection of treatments to be used
2. Number of replicates to be included
3. Randomisation of treatments
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Assume we are conducting a field trial to
investigate the effect of the
concentration of nitrogenous fertiliser
on a new cultivar of cereal plant.
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1. Selection of treatments
For each equal sized crop only one
variable should be altered e.g.
concentration of fertiliser.
All other variables should remain constant
to ensure a valid comparison can be
made (fair comparison).
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2. Number of replicates
If only one treatment of each condition
of fertiliser were carried out the
results would be unreliable.
Differences in each plot and differences
in how the experiment was carried out
would occur – this is called experimental
error.
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To minimise experimental error then a
minimum of three replicates must be
set up.
The more replicates are set up the more
reliable the results.
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3. Randomisation of treatments
If the plots in a field
were treated in an
orderly basis then
bias could exist.
e.g. In this field there
are 4 treatments (a,
b, c and d) being
investigated being
repeated 3 times.
Plot
Block
A
B
C
1
2
3
4
a
b
c
d
a
b
c
d
a
b
c
d
Non-randomised field trial
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This bias could be due
to conditions in the
soil.
Allocating the plot
treatments randomly
helps to minimise
this bias.
Plot
Block
A
B
C
1
2
3
a
d
b
c
b
a
c
d
d
b
a
c
Randomised field trial
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4
Selecting and breeding
LI:
1. State the difference between
inbreeding and outbreeding
2. Describe the effects on inbreeding:
elimination of heterozygotes and
inbreeding depression
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Outbreeding
Outbreeding involves the fusion of two
gametes from unrelated members of
the same species.
Wild animals and cross-pollinating plants
are naturally outbreeding.
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Inbreeding
Inbreeding involves the fusion of two
gametes from close relatives.
Inbreeding is naturally occurring in some
species of self-pollinating plants e.g.
peas, wheat and rice.
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Effects of inbreeding
Inbreeding ensures that the members of
each generation of a selectively bred
strain receive alleles for the desired
characteristic. But can also lead to:
1. Loss of heterozygosity
2. Inbreeding depression
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The Vadoma tribe
of Zimbabwe
have
Ectrodactyly
(two toed feet).
This mutation is
more common
due to
inbreeding.
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1. Loss of heterozygosity
Heterozgosity
Aa (selfed)
P
F1
AA
F2 AA AA AA AA
F3
Aa
AA Aa Aa aa
100%
aa
Aa
AA Aa Aa aa aa aa aa aa
50%
25%
4AA 4AA 4AA 4AA 4AA AA AA 4aa 4AA AA AA 4aa 4aa 4aa 4aa 4aa 12.5%
2Aa 2Aa
2Aa 2Aa
aa aa
aa aa
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Continuous inbreeding leads to a loss of
heterozygosity and development of
homozygosity.
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2. Inbreeding depression
If a natural
outbreeder is
forced to inbred
inbreeding
depression can
occur.
Inbreeding
depression in maize
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This is due to the accumulation of
homozygous recessive alleles which can
be deleterious (harmful).
This appears as a decline in vigour, size,
fertility and yield of the plant or animal.
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Cross breeding and F1 Hybrids
LI:
1. State that cross breeding can be used
to produce individuals with desired
characteristics of both parents.
2. Explain how a new cross breed can be
maintained.
3. Define hybrid vigour
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Hybrids
An F1 hybrid is an individual resulting from
a cross between two genetically
dissimilar parents.
Breeders will cross members of one variety
of a species that have a desired
characteristic with members of another
variety that have another desired
characteristic in the attempt to produce
a hybrid that has both desirable
characteristics.
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Crossbreeding
Because of the problems associated with
inbreeding. New alleles can be
introduced into a plant or animal species
by crossbreeding with a strain
exhibited a different but desired
genotype.
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Savannah cats
Watch this
video
introducing
the savannah
cat
Produced by
breeding a
serval with a
domestic cat
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Wild Serval
cat
X Domestic cat
F1 hybrid
X
Looks like
serval,
retains
some wild
character
F2 hybrid
Looks like
serval,
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milder
temprement
Domestic cat
This type of
breeding is
called a
Sustainability
backandcross
interdependence
Hybrid vigour
Hybridisation (mating) of two different
inbred homozygous cultivars of plant
species produces offspring who are
uniformly heterozygous.
Parents
gametes
F1
AABBccdd X
All ABcd
aabbCCDD
All abCD
AaBbCcDd
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They also display increased:
• Vigour
• Yield
• Fertility
This is called
hybrid vigour.
Poorer recessive genes
are masked by superior
dominant ones.
Parent 1
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F1 hybrid Parent 2
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However, if F1 hybrids are allowed to
interbreed with one another, the F2
generation can be too genetically
diverse and many will lack the improved
characteristics.
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Inheritance
LI:
1. Explain what is meant by “true
breeding”
2. Work the inheritance of a gene using a
Punnet square.
3. Explain when a test cross would be
used and work out the genotypes
produced.
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Single gene inheritance
Variation in a population can be defined as
either:
• Continuous (varying from extreme to
another) e.g.
• Discrete (divides members of a species
on to two or more groups) e.g.
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A characteristic that shows discrete
variation is normally controlled by alleles
of a single gene.
The alleles can either have be dominant or
recessive.
Characteristic
Plant height
Dominant Trait
Tall
Recessive Trait
Dwarf
Pod shape
Inflated
Constricted
Pod colour
Seed shape
Flower colour
Green
Round
Purple
Yellow
Wrinkled
White
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White Mouse (bb)
White Mouse (bb)
Black Mouse (BB) Black Mouse (BB)
Black Mouse (BB)
X
X
White Mouse (bb)
White Mouse (bb)
Black Mouse (BB)
Black Mouse (BB)
X
X
Black Mouse (BB)
White Mouse (bb)
True
Breeding
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Black Mouse (BB)
White Mouse (bb)
X
Black Mouse (Bb)
Black Mouse (Bb)
Single gene
inheritance
Black Mouse (BB)
X
Black Mouse (Bb)
Black Mouse (Bb) White Mouse (bb)
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3 Black Mice
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:
1 white mouse
Test cross
What genotype could this
brown mouse have?
How can you tell if this
Black mouse is BB or Bb?
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Test cross
A test cross is a cross between an
organism whose genotype for a certain
trait is unknown and an organism that is
homozygous recessive for that trait.
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Black Mouse (?)
White Mouse (bb)
Black Mouse (?)
X
X
½ Black Mouse
All Black Mice
b
White Mouse (bb)
b
½ White Mouse
b
b
B
Bb
B b
B
B b
B b
B
Bb
B b
b
b b
b b
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The genotype of theCFE
unknown
The genotype
of the
unknown
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and
mouse must have been BB
mouse must
have been Bb
interdependence
Genetic technology
LI:
1. Explain how genetic technology can be
used to improve cultivars or breeds
for commercial gain.
2. Give examples of crops produced by
genetic transformation, explain how
they were created and give their
potential benefits.
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Genetic technology
As well as enhancing cultivars and breeds
through breeding strategies, plants
and animals can be enhanced by use of
genetic technologies:
1. Genetic sequencing (remember from Unit 1)
2. Genetic transformation (remember from
Unit 2)
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Genetic sequencing
Genetic sequencing techniques can be
used to identify organisms that possess
particular alleles for a desired
characteristic.
This organism can then be selected for
use in a breeding programme.
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Genetic transformation
Genetic transformation is the transfer
of genetic information from one
organism to another (think back to GM
crops in Nat 5).
This technique can be used to enhance a
crop species which can then be used in a
breeding programme.
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Case study
Using the internet and other available
resources complete the following table.
Description of
Potential
how the crop was benefits
created
Bt toxin for
pest resistance
Golden rice
Glyphosateresistance for
herbicide
tolerance
1.
What gene was
inserted?
2.
Where did it
originally come
from?
3.
What does the
protein made
from this gene
do? Biology
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1.
What are the
economic
benefits
2.
Or the health
benefits
3.
Or the
environmental
benefits
Sustainability
(Note: it may be only
and
one benefit)
interdependence