Unit 3: Sustainability and Interdependence

Transcription

1 Unit 3: Sustainability and Interdependence Sub-topic 3.2 Plant and Animal Breeding Page 1 of 17

2 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; Page 2 of 17

3 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. Page 3 of 17

4 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. Page 4 of 17

5 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 Higher yield Higher protein content Disease resistance Pest resistance Frost resistance High milk yield High meat yield Useful physical characteristic Ability to thrive in certain environment Example Wheat Soya bean Potato (to blight) Tomato (to eelworm) Strawberry Dairy cattle Beef cattle Uniform height Maize in cold, damp climate Page 5 of 17

6 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. Page 6 of 17

7 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 selfpollination 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. Page 7 of 17

8 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. Page 8 of 17

9 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 Page 9 of 17

10 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 selfpollinating 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. Page 10 of 17

11 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. Page 11 of 17

12 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? Page 12 of 17

13 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). Page 13 of 17

14 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. Page 14 of 17

15 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. Page 15 of 17

16 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 Page 16 of 17

17 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. Page 17 of 17