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List the different Types of cultivars:
- Homozygous and homogeneous (pure-line cultivars)
- Heterozygous and homogeneous (hybrid cultivars)
- Homozygous and heterogeneous (mixed landrace types)
- Heterozygous and heterogeneous (synthetic and composite breeding)
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Mass selection of self-pollinating plants:
- Population improvement through increasing the gene frequencies of desirable genes
- Improvement of the average performance of the base population
- Limited to the genetic variation that exists in the population
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Application of mass selecting self-pollinating plants:
- Maintain purity of an existing or soon-to-be released cultivar
- Develop a cultivar from a base population, hybrid population, or introduced population Used to breed horizontal resistance—apply low density of disease inoculum so that quantitative genetic effects can be assessed
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Mass selection
- Desirable for both self and cross-pollinated species
- Selection can be positive or negative
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Mass selection advantages:
- Rapid, simple, and straightforward
- Large populations can be handled
- Inexpensive to conduct
- Phenotypically uniform and stable
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Mass selection disadvantages:
- Traits need high heritability
- Selection based on phenotypic values— need uniform environments
- With dominance, heterozygotes can’t be distinguished and can segregate in subsequent generations
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Pure-line selection:
- Lines that are genetically different may be isolated from within a mixed genetic population
- Highly homozygous selection usually from a single plant—variation is caused by the environment
- Useful when breeding for a specific trait (flower color, processing quality)
- In theory, has identical alleles at all loci
- Have a very narrow genetic based and tend to be uniform for traits of interest
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Application of Pure-line selection:
- Cultivars for mechanized production
- Cultivars with a premium on visual appeal or processing quality
- Advancing ‘mutants’ in the population
- A part of other methods such as pedigree and bulk population selection
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Genetic issues with Pure-line selection:
- With narrow genetic base are less likely to be stable across environments
- Variants arise through mutations and outcrossing
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Pure-line selection overview:
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Pure-line selection advantages:
- Rapid and inexpensive
- Population size can be variable
- Applicable to low heritability traits because selection is based on progeny performance
- Only the best lines are advanced
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Pure-line selection disadvantages:
- Narrow genetic base—variants arise
- Promotes genetic erosion—only the superior lines are advanced and multiplied
- Progeny rows take up more resources
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Pedigree selection:
- Ancestry of the cultivar well documented
- Continuous individual selection after hybridization
- Must have an effective and easy to maintain system of record keeping
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Applications of Pedigree selection:
- Must allow plants to be observed, described, and harvested separately
- Used for readily identifiable qualitative traits of the target for improvement
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Pedigree selection overview:
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Advantages of Pedigree selection:
- Provides a catalog of genetic information
- Selection based on phenotype as well as genotype (progeny row)
- High degree of genetic purity
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Disadvantages of Pedigree selection:
- Record keeping is slow, tedious, and expensive (modern equipment is making this easier)
- Must be able to individually select plants More suited for qualitative rather than quantitative disease resistance
- Selecting early for yield traits may not be effective
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Bulk Population Breeding:
- A strategy of crop improvement in which the natural selection effect is solicited early and stringent selection solicited late
- Usually suitable for crops which are normally closely spaced in production and need a competitive advantage
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Key feature of Bulk Population Breeding:
- Natural selection in early generations, stringent selection in later generations
- Natural selection eliminates less fit genotypes
- Stringent selection delayed until homozygosity is reached
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Bulk Population overview:
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Genetic issues with Bulk Population:
- Promotes homozygosity
- Mean population performance will be increased
- Inferior genotypes may out-compete superior lines
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Advantages of Bulk Population breeding:
- Simple and convenient to conduct
- Less labor intensive in early generations Compatible with mass selection
- Allows for large amounts of materials to be handled
- Well adapted lines through natural selection
- High degree of homozygosity
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Disadvantages of Bulk Population breeding:
- Superior genotypes can be lost to natural selection
- Selecting in off-season nurseries not useful Genetic characteristics are hard to ascertain from generation to generation
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Single-seed descent:
- F2 population is generated to maximize genetic variation
- Each plant is kept as an individual unit and a single seed is harvested and replanted Done until homozygosity is reached
- Little selection done in previous generations
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Genetic issues of Single-seed descent:
- Wide genetic diversity is carried on to advanced generations
- Only one seed is selected from each plant, regardless of the number of seeds produced
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Advantages of Single-seed descent:
- Easy and rapid way to attain homozygosity (depending on the crop cycle)
- Small spaces are required in early generations
- Duration of breeding program can be reduced
- Great genetic diversity exists
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Disdvantages of Single-seed descent:
- Plants are selected based on individual performance and not progeny performance
- Selecting a single seed runs the risk of losing desirable genes from the population
- Natural selection has no benefit
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Backcross Breeding
- Used to replace a specific undesirable gene with a desirable alternative genes, while preserving all other qualities (both good and bad)
- Uses a recurrent parent and a donor parent
- Most useful for simply inherited genes
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Backcross breeding overview:
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Backcross breeding Genetic issues:
- Number of backcrosses will determine the proportion of donor genes present
- 1-(1/2m-1) where m is the number of generations
- Linkage drag an issue
- Heritability of the trait important
- Genetic distance between parents should be considered
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Advantages of Backcross breeding:
- Reduce the number of field testings needed It is a conservative method; does not permit new recombination to occur
- Useful for introgressing specific genes from wide crosses
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Disadvantages of Backcross breeding:
- Not effective for transferring quantitative traits
- Linkage drag an issue
- Recessive traits are more time consuming to transfer
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Multiline and Cultivar Blends:
- Planned seed mixture of cultivars or lines
- Can be done by the breeder or the grower Usually used for disease protection
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Advantages of Multiline and Cultivar Blends:
- Provides protection to a broad spectrum of races of a disease/pest
- Phenotypically uniform (except blends)
- Provide greater yield stability (over many years)
- Can be readily modified
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Disadvantages of Multiline and Cultivar Blends:
- It takes a long time to develop all of the isolines
- Most effective in areas where there is a specialized disease of major importance
- Maintaining the isoline is labor intensive
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