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Higher Biology
Unit 3
3.2- Plant and Animal Breeding
Selective Breeding
• Breeders of crop plants and livestock
attempt to manipulate an organism’s
heredity to obtain desirable
characteristics
• This is done to produce a new and
improved cultivar or breed that are
beneficial to humans
Heritable
Characteristic
Example
Increase in Yield
Increase is mass of food
produced by wheat crop
Increase in nutritional
value
Resistance to pests
Increase in mass of protein
produced by soya bean crop
Resistance of tomato to
eelworm
Resistance to disease
Resistance of potato to
late blight
Growth of cereal crop to
uniform height suited to
mechanical harvesting
Ability of maize to grow in
cold, damp climate
Possession of useful
characteristic
Ability to thrive in a
particular environment
Plant Field Trials
• A plant field trial is an investigation set
up to
1. Compare the performance of 2
different plant cultivars in the same
environmental conditions
2. Find out the effect of different
environmental conditions on a new
cultivar of a crop plant
3. Evaluate GM crops
Plots and Treatments
• The area of land used for a field trial is
divided into equal sized portions called
plots
• A field trail involves treatments
• Treatments are the different ways in
which each plot is treated, for example
one given a high concentration of
fertiliser and another a low
concentration
Designing a field trial
• After establishing what will be
investigated the next stage is to design
the trial
• A number of factors need to be taken
into consideration when designing the
field trial
Selection of treatments
• For each treatment only one variable
should be changed
• All other variables should remain the
same
• This allows for a fair comparison to be
made between treatments
Number of replicates
• It is impossible to ensure that
treatment application and plots are
exactly identical each time a treatment
is applied
• This is known as experimental error
• To reduce the impact of experimental
error several replicates must be set up
of each treatment
• This makes the results more reliable
Randomisation of treatments
• If plots are treated in an orderly
sequence a bias may occur
• This could be due to other
environmental factors
• To avoid this plots should be treated
randomly
Repeats in other environments
• Plant field trials may be repeated in
different environments
• This is to ensure that the conclusions
drawn from the trial are valid across
different environments
• For example a plant that grows well in
sandy soil may not in a more temperate
climate
Selecting and breeding
• Breeders select parents with the
characteristics they desire and breed
them to produce superior offspring
• The aim is to ensure that offspring
possess the desirable alleles and
express the desirable traits
• This occurs over many generations
Outbreeding
• Outbreeding involves the fusion of 2
gametes from unrelated member of the
same species
• Animals and cross pollinating plants are
naturally outbreeding
• Cross pollinating plants often possess
features that prevent self pollination from
occurring
• Outbreeding plants include tomato, sugar
beet and maize
Inbreeding
• Inbreeding involves the fusion of
gametes from two closely related
individuals
• Some species of plants are self
pollinating and are therefore inbreeders
• Peas, wheat and rice are natural
inbreeders
Effects of Inbreeding
• Inbreeding ensures that all members of
each generation receive the same alleles
•
• This can be beneficial if they are bred
for desirable characteristics such as
increased yield or disease resistance in
plants
Inbreeding
• Some of the problems associated with
inbreeding include
1. Loss of heterozygosity
2. Inbreeding depression
Loss of Heterozygosity
• Continuous inbreeding leads to a
development of homozygosity and a
decrease in heterozygosity
• This isn’t an issue for self pollinating
plants because harmful alleles have been
lost due to natural selection
Inbreeding depression
• Inbreeding depression occurs when a
natural outbreeder is forced to inbreed
• When this happens genotypes emerge
that are homozygous for recessive
alleles that are deleterious (harmful)
• This can result in a loss of vigour,
smaller size, and reduction in yield
The first plant represents the parent plant forced to
inbreed. The next 3 plants are the 3 generations
produced.
Crossbreeding
• Inbreeding will bring about the
improvement of a desired trait but will
also result in the build up of harmful
recessive alleles
• To avoid this breeders will cross breed
with a strain possessing a different but
desirable characteristic
Savannah cats
Watch this
video
introducing
the savannah
cat
Produced by
breeding a
serval with a
domestic cat
Wild Serval
cat
X Domestic cat
F1 hybrid
X
Looks like
serval,
retains
some wild
character
F2 hybrid
Looks like
serval,
milder
temprement
Domestic cat
This type of
breeding is
called a
back cross
Hybrid vigour
Hybridisation (mating) of two different
inbred homozygous cultivars of plant
species produces offspring who are
AABBccdd X aabbCCDD
Parents heterozygous.
uniformly
gametes
F1
All ABcd
All abCD
AaBbCcDd
They also display increased:
• Vigour
• Yield
• Fertility
This is called
hybrid vigour.
Poorer recessive genes
are masked by superior
dominant ones.
Parent 1
F1 hybrid Parent 2
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.