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Page 1 of 17 CM 2017 Unit 3: Sustainability and Interdependence Sub-topic 3.2 Plant and Animal Breeding
Page 1 of 17 CM 2017
Unit 3: Sustainability and Interdependence
Sub-topic 3.2 Plant and Animal Breeding
Page 2 of 17 CM 2017
On completion of this sub-topic I will be able to:
understand that plant and animal breeding involves the manipulation of
heredity;
know that this manipulation will allow the development of new and improved
organisms to provide sustainable food sources;
understand that breeders seek to develop crops and stock with higher
yields, higher nutritional values, resistance to pests and diseases;
know that this is known as artificial selection;
be able to outline the facts that improvements to physical characteristics are
important which are suited to rearing and harvesting as well as those that
can thrive in particular environmental conditions;
know that continuous variation may be due to polygenic inheritance;
understand that polygenic inheritance refers to the interaction of multiple
genes and/or environmental influences;
be able to describe inherited characteristics that show discrete variation and
that these are usually controlled by a single gene;
understand the need for plant field trials;
know that trials are carried out in a range of environments to compare the
performance of different cultivars or treatments;
be able to explain the factors which have to be taken into account when
designing field trials;
know that these factors include:
1. the selection of treatments to ensure fair comparison,
2. the number of replicates to take account of the variability within the
sample,
3. the randomisation of treatments to eliminate bias when measuring
treatment effects;
be able to explain that animals and cross pollinating plants are naturally
outbreeding;
be able to describe how, in inbreeding, selected plants or animals are bred for
several generations until the population breeds true to the desired type;
know that this true breeding is due to the elimination of heterozygotes;
be able to explain how test crosses can be used to identify unwanted
individuals with heterozygous recessive alleles;
understand that inbreeding depression is the accumulation of recessive,
deleterious homozygous alleles;
know that inbreeding depression can lead to organisms with reduced vigour
and health;
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be able to describe how self-pollinating plants which are naturally inbreeding
deal with inbreeding depression;
know that self-pollinating plants are less susceptible to inbreeding depression
due to the elimination of deleterious alleles by natural selection;
understand that outbreeding is breeding with a non-related individual;
know that in outbreeding species, inbreeding depression is avoided by
selecting for the desired characteristic while maintaining an otherwise
genetically diverse population;
understand that new alleles can be introduced to plant and animal lines by
crossing a cultivar or breed with an individual which possesses a different,
desired genotype;
be able to describe how, in animals, individuals from different breeds may
produce a new crossbreed population with improved characteristics;
know that the F2 generation will have a wide variety of genotypes;
understand that a process of selection and backcrossing is required to
maintain the new breed;
be able to explain that, as an alternative, the two parent breeds can be
maintained to produce crossbreed animals for production;
understand that, in plants, F1 hybrids, produced by the crossing of two
different inbred lines, creates a relatively uniform heterozygous crop;
understand that F1 hybrids often have increased vigour and yield;
be able to explain why the F2 generation is genetically variable and of little
use for further production;
understand that the F2 generation can provide a source of new varieties;
be able to describe how as a result of genome sequencing, organisms with
desirable genes can be identified and then used in breeding programmes;
be able to describe how a desired gene can be cut from a chromosome using
enzymes;
be able to explain that genetic transformation techniques allow a single gene
to be inserted into a genome;
be able to describe how this reprogrammed genome can be used in breeding
experiments.
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Prior Learning
Unit (2.5) Inheritance
Variation of characteristics exists within populations. Combining genes from separate parents contributes to variation within a
species.
The meaning of the terms discrete and continuous variation. Examples of characteristics which can be described as discrete or continuous
variation. Alleles are different forms of a gene. The majority of features of an individual phenotype are polygenic and show
continuous variation. Polygenic inheritance is caused by the interaction of the alleles of several
different genes and results in a large range of phenotypes. The meaning of the words phenotype and genotype. The meaning of the terms dominant and recessive.
The meaning of the terms homozygous and heterozygous. I can identify the P, F1 and F2 generations in a monohybrid cross. I understand that the phenotypes of the F1 produced from a homozygous
cross are all uniform (the same).
I can explain monohybrid crosses in terms of the genotypes produced.
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Plant and animal breeding by manipulation of heredity
Breeders of crops and livestock have been manipulating heredity (passing on of
traits to offspring) for thousands of years. Selective breeding is the process by which
selected individuals are bred together to produce offspring with desirable features
e.g. improved cultivars of plants or breeds of animals.
These improvements support sustainable food production. Farmers and breeders
select plants and animals with the required characteristics to be parents of the next
generation. This brings together desired alleles so that the offspring are more useful
than the parents.
All the plants below are derived from one wild species, the wild cabbage, Brassica oleracea. Humans have taken this wild plant and selectively bred it into these very different kinds of foods. This form of selection in which humans have improved characteristics in organisms is known as artificial selection.
Other examples are summarised in the table below:
Desirable feature Example
Higher yield Wheat
Higher protein content Soya bean
Disease resistance Potato (to blight)
Pest resistance Tomato (to eelworm)
Frost resistance Strawberry
High milk yield Dairy cattle
High meat yield Beef cattle
Useful physical characteristic Uniform height
Ability to thrive in certain environment Maize in cold, damp climate
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Inheritance
Variation in a population can be defined as either:
• Continuous (varying from extreme to another) e.g. height, weight
Continuous variation is the combined effect of many genes, known as polygenic
inheritance. The effect of the genes involved is additive. The greater the number
of genes involved, the greater the number of intermediate phenotypes produced.
Many traits showing polygenic inheritance are influenced by the environment.
• Discrete (divides members of a species on to two or more groups) e.g. eye
colour, wing shape
A characteristic that shows discrete variation is normally controlled by alleles of a
single gene. The alleles can be dominant or recessive.
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True breeding:
Where the characteristic of the
parent is always passed on to the
offspring – because both parents
are homozygous dominant or
homozygous recessive. This
usually happens through self-
pollination or inbreeding.
Single gene inheritance:
This involves looking at only one
difference in inherited
characteristics. The F1 generation
are always uniform i.e. show the
same phenotype. However, in the
F2 generation there is a ratio of
3:1 of dominant to recessive
phenotypes.
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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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Key Words Revision
Define the following:
Phenotype
____________________________________________________________________
Genotype
____________________________________________________________________
Allele
____________________________________________________________________
Heterozygous
____________________________________________________________________
Homozygous/True Breeding
____________________________________________________________________
Parents (P)
_______________________________________
Monohybrid Cross
____________________________________________________________________
Back Cross/Test Cross
____________________________________________________________________
F1
_______________________________________
F2
_______________________________________
Punnet Square
___________________________________________________________________
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Selecting and breeding
Outbreeding involves the fusion of two gametes from unrelated members of the same species and promotes heterozygosity. Wild animals and cross-pollinating plants are naturally outbreeding. Recessive alleles are often present but masked by dominant alleles. Inbreeding involves the fusion of two gametes from close relatives and promotes homozygosity. It is naturally occurring in some species of self-pollinating plants e.g. peas, wheat and rice. Effects of Inbreeding: Desired effect: selected plants or animals are bred for several generations until the population breeds true to the desired type. Negative effects:
Loss of heterozygosity (not a problem for naturally inbreeding plants) Inbreeding depression
Inbreeding results in homozygosity, leading to an accumulation of harmful (deleterious) homozygous alleles and increases the chances of offspring being affected by recessive traits. This generally leads to a decreased fitness of a population, which is called inbreeding depression. Inbreeding depression can result in a decline in vigour, size, fertility and yield of the plant or animal. Inbreeding depression can be avoided by selecting parent plants that are homozygous for desired characteristics but heterozygous for others.
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Inbreeding is naturally occurring in
some species of self-pollinating plants
e.g. peas, wheat and rice.
Crossbreeding
Inbreeding is not usually carried out indefinitely because of the problems associated
with it. New alleles can be introduced into a plant or animal species by crossbreeding
with a strain that has a different but desired genotype.
Back Crossing
A back cross involves the crossing of an F1 hybrid with one of its parents or with a
genetically identical individual. Back crossing may be used to incorporate a required
gene from a parent while maintaining other desired features e.g. cultivated tomatoes
are crossed with eelworm-resistant wild tomatoes; the F1 are back crossed with the
cultivated parent for several generations until most wild genetic material (apart from
resistance to eelworm) has been eliminated.
F1 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.
Such a cross between two different homozygous parents creates a uniform F1
generation. F1 hybrids have to always be produced from true-breeding parents
therefore the parent breeds have to be maintained. An F1 self-cross will produce a
genetically diverse F2, usually unsuitable as a crop but useful for production of new
varieties. Selection and backcrossing may be used to maintain a required breed.
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Hybrid Vigour
F1 hybrids have increased vigour, yield and fertility because recessive alleles are
masked by superior dominant alleles. In the example below the maize cob in the
centre is the F1 from the parents on either side.
What is cross breeding?
____________________________________________________________________
How does cross breeding affect an organism’s genotype?
____________________________________________________________________
How can cross breeding be used to introduce characteristics?
____________________________________________________________________
How can new varieties that arise from cross breeding be maintained?
____________________________________________________________________
What effect does crossing 2 inbred lines have on the offspring?
____________________________________________________________________
Why does the F2 from crossing 2 inbred lines have little use for further production?
____________________________________________________________________
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Plant Field Trials:
A plant field trial is a type of investigation allowing to test the new breeding
characteristics, 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.
Designing a field trial:
Since humans first began to cultivate soil, crops have been improved by selecting plants of desired characteristics. More recently, crop improvement has involved interbreeding plant varieties or closely related species. A vast range of varieties and hybrids have been created to maximize various genetic characteristics, such as timing and size of yield, tolerance to pests and diseases, and colour, taste, and shape of fruit. The development of new plants is progressing rapidly and research moves from the laboratory or greenhouse to the field where small scale field trials are carried out. These field trials have to be carefully and scientifically monitored to ensure accurate results are obtained from them and there are no adverse effects on the environment. Plant field trials are carried out in a range of environments to compare the performance of different cultivars or treatments. In designing plant field trials account has to be taken of:
1. the selection of treatments (to ensure fair comparisons);
2. the number of replicates (to take account of the variability within the sample);
3. the randomisation of treatments (to eliminate bias when measuring
treatment effects).
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Designing a field trial
- E.g. investigating the effect of the concentration of nitrogenous fertiliser on a
new cultivar of cereal plant.
We must consider:
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).
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. 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.
3. Randomisation of treatments - If the plots in a field were treated in an orderly
way then bias could exist e.g. in this field there are 4 treatments (a, b, c and
d) being investigated each repeated 3 times.
Bias could result – due to, for example, soil conditions (one side of the field
may be wetter) so allocating the plot treatments randomly helps to eliminate
this bias.
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Genetic Technology Plants and animals can also be enhanced by the use of genetic technologies such as genome sequencing and genetic transformation.
1. Genome sequencing
Genomic sequencing can be used to identify organisms that possess alleles
for a desired characteristic. These organisms can then be used in breeding
programmes.
2. Genetic Transformation
Genetic transformation can be used to enhance a crop species which can then
be used in a breeding programme e.g.
Gene(s) added Host Organism Benefit
Genet for Bt toxin (kills insects) taken from soil bacterium
Crop plants e.g. maize Crop resistant to insect pests; yield increased.
Genes for vitamin A Rice ‘Golden’ rice that provides vitamin A; better nutrition.
Genes for herbicide resistance (from naturally resistant plants)
Soya, maize, sorghum Herbicide kills weeds without damaging crops; yield increased.
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I can Traffic
light
Plant and animal breeding involves the manipulation of heredity to develop new and improved organisms to provide sustainable food sources.
Various crop and domesticated animal species have been created by artificial selection for example the cabbage (Brassica oleracea) and the dog.
Selective breeding is the process whereby new varieties of species are produced as a result of humans choosing individuals possessing desirable characteristics, and using those individuals for breeding.
Selective breeding aims to enhance desirable characteristics by choosing individuals showing these characteristics as parents.
Examples of desirable characteristics are: – higher crop yield; – higher nutritional value; – resistance to pests and diseases; – physical characteristics suited to rearing and harvesting; – ability to thrive in particular environmental conditions.
Selective breeding has been used to produce different varieties of crop plants, for example, the cabbage (Brassica oleracea) and domesticated animals, for example dogs.
Selective breeding requires many generations of breeding to produce the new improved varieties.
Hybridization involves cross-breeding two separate species to allow the combination of their desirable characteristics in their offspring.
Continuous variation may be due to polygenic inheritance which refers to the interaction of multiple genes and/or environmental influences.
Inherited characteristics that show discrete variations are usually controlled by a single gene.
Plant field trials are carried out in a range of environments to compare the performance of different cultivars or treatments.
Animals and cross pollinating plants are naturally outbreeding
Inbreeding is the reproduction from the mating of two genetically related parents.
• Inbreeding depression is the accumulation of recessive, deleterious homozygous alleles.
A genetically diverse population must be maintained to provide the continued health of the organisms
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New alleles can be cross bred into plant and animal lines.
These new individuals may have improved characteristics.
A process of selection and backcrossing is required to maintain the new breed.
Genetic technology (engineering) has contributed to the development of new varieties of organisms to human advantage.
The genotype of an individual can be determined by carrying out a backcross and examining the phenotype of the progeny
• In genetic engineering , the genes from one species are combined into the genome of another.
Organisms like bacteria and yeast can be genetically engineered to accept the genes from another organism such as a human.
These host organisms can then be cultured and the products of the activity of the inserted genes isolated and purified.
Examples of genetically engineered products include human growth hormone and insulin.
Hybridisation is used by animal and plant breeders to produce individuals which possess more than one improved characteristic.
• It involves taking one individual with one desired characteristic and allowing it to breed with another individual with a different desired characteristic.
• Because it works at the gene level, new varieties can be produced in just one generation.
