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Classical breeding methods June 10 th 2008. Source: Bioethics. Few terms (1).
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Source: Bioethics Few terms (1) Dominance: Effects of different versions of a gene on a trait. Many plants have two copies of each gene, one inherited from each parent. Genes are located on specific locations called alleles. If the two copies of a gene are different, and their combined effect is determined by one of the genes, than we call that gene dominant. Recessive: The other gene, which does not determine the outcome, is said to be recessive. Source: Model horse reference Meiosis Source: University of Central Arkansas
Few terms (3) Qualitative trait: A qualitative trait is expressed qualitatively, which means that the phenotype falls into different categories. The pattern of inheritance for a qualitative trait is typically monogenetic, which means that the trait is only influenced by a single gene. Quantitative trait: A quantitative trait shows continued variation. If several gene effects are present, the phenotype values for a population will typically have a normal distribution. Source: Knud Christensen
Source: University of Guelph Few terms (2) Homozygous: In genetics, zygosity describes the similarity or dissimilarity of DNA between homologouschromosomes at a specific allelic position or gene. Homogeneous Heritability: Proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. Variation among individuals may be due to genetic and/or environmental factors.
Few terms (4) Polyploidy: occurs in cells and organisms when there are more than two homologous sets of chromosomes. Polyploid types are labelled according to the number of chromosome sets in the nucleus. Most organisms are normally diploid; polyploidy may occur due to abnormal cell division. It is most commonly found in plants. Haploidy may also occur as a normal stage in an organism's life. A haploid has only one set of chromosomes.
Overview: Cultivars Pure inbred lines (species that are highly self-pollinated) homogeneous, homozygous series of self- pollinations Open-pollinated populations (species that are naturally cross-pollinated) heterogeneous, heterozygous recurrent selection Hybrids (crossing inbred lines) homogeneous, heterozygous Clones (asexually propagation) homogeneous, heterozygous
Note: You often have a combination between different breeding methods depending on the breeding objective and the stage of your breeding program!! Often limited to the environment in which is was developed. As a general rule modifications of breeding methods are possible.
Mass selection Key features: Oldest method for self-pollinated species Variable population Self- and cross-pollinated species Solely selection (no variability) Recurrent Based on phenotype Applications: Maintain purity of a cultivar Develop cultivar from base population Initial inexpensive screening
Generalized steps in breeding by mass selection for (a) cultivar development, and (b) purification of an existing cultivar Mass selection Procedure: Plant heterogeneous population negative mass selection positive mass selection Disadvantages: should have > heritability Uniform environment Cannot distinguish homozygous from heterozygous Advantages:?
Pure line selection Key features: Lines that are genetically different may be isolated from within a population of mixed genetic types. Variation is due to environmental factors and ??? Applications: Cultivars Parents for hybrid production (uniform etc.) Generalized steps in breeding by pure-line selection
Mendel’s laws of inheritance (1) The Law of DominanceIn a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype. (2) The Law of SegregationDuring the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.
Pedigree selection Key features: Records of the ancestry of the cultivar Hybridization is used to generate variability Followed by selection of a segregating population Applications: Mainly crop plants, Improvement of quantitative traits
Pedigree selection Procedure: Establish base population Space plant progenies of selected plants Keep records of generations Generalized steps in breeding by pedigree selection
Pedigree selection • Advantages and disadvantages: • + Selection is not only on phenotype but also genotype (plants are usually observed and described over several generations) • +/- High degree of genetic purity • Expensive • Takes 10 – 12 years
Increasing homozygosity in each subsequent generation via self- hybridization Broman W. 2005
Bulk population breeding Key features: Delay artificial selection to allow natural selection pressure Pure line cultivars Mostly self-pollinated crops (soybean) Generalized steps in breeding by bulk selection
Bulk population breeding Key features: Delay artificial selection to allow natural selection pressure Pure line cultivars Mostly self-pollinated crops (soybean) Advantages and disadvantages: + Simple and convenient + Less labor intensive and less expensive + Natural selection may increase frequency of desirable traits - Selecting in off-season may favor genotypes that are undesirable in the production region
Backcross breeding Key features: ? Replace a specific undesirable gene with a desirable alternative, while preserving all other qualities of an adapted cultivar Backcrossing to ‘recurrent parent’ Applications: Improvement of established cultivars Introgress genes from wild relatives Isogenic lines
Backcross breeding Procedure: Cross between donor parent and recurrent parent Repeated backcrossing to recurrent parent MOVIE!!!!!
Backcross breeding • Advantages and disadvantages: • + Reproducible • + Predictable (no new recombination events) • + Allows introgressions from wild relatives • + Applicable to breeding self and cross- pollinated species • Only qualitative traits • Quantitative traits (major genes) • Linkage
Breeding cross-pollinated species Focus on the improvement of plant populations Change the gene frequency such that desirable genotypes predominate Maintain genetic variability for future improvements.
Recurrent selection Key features: One of the oldest methods in plant breeding Gradually concentrating desirable alleles in a population The concept of recurrent selection
Concept of recurrent selection Cyclical and systematic technique in which desirable individuals are selected from a population and mated to form a new population Improvement of a population without losing the variability Parents should be highly diverse Applications: Establish broad genetic base, add new germplasm Since possible intermating between multiple families add new germplasm Used in Legumes and grasses
Emasculating Zinnias: Collecting Pollen: Images kindly provided by Goldsmith Seeds
Fertilization Images kindly provided by Goldsmith Seeds
Breeding vegetatively propagated crops Maintenance of desired genotypes
Through: • bulbs and corms (tuplips, gladioli) • Budding or grafting (?) • Cuttings (pelargoniums, carnations) • Division (hardy herbaceous perennials)
“Grafted before planting” “Field grafting”
Mass selection • Polyploidy breeding • Hybrid breeding
