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Sustainability and Interdependence Sub-Topic 3.2 Plant and Animal

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Sustainability and Interdependence Sub-Topic 3.2 Plant and Animal Unit 3: Sustainability and Interdependence Sub-topic 3.2 Plant and Animal Breeding Page 1 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; Page 2 of 17 CM 2017 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 CM 2017 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 CM 2017 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 Page 5 of 17 CM 2017 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 CM 2017 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. Page 7 of 17 CM 2017 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 CM 2017 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 CM 2017 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
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