Estimation of heterotic effects

Consider a cross between two inbred lines, A and B, with population means of XP1 and XP2, respectively. The phenotypic variability of the F1 is generally less than the variability of either parent. This indicates that the heterozygotes are less subject to environmental influences than the homozygotes. The heterotic effect resulting from the crossing is roughly estimated as This equation indicates the average excess in vigor exhibited by F1 hybrids over the midpoint  between the means of the inbred parents. K.R. Lamkey and J.W. Edward...

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Methods for developing heterotic groups

A number of procedures may be used by breeders to establish heterotic groups and patterns. These include pedigree analysis, geographic isolation inference, measurement of heterosis, and combining ability analysis. Some have used diallel analysis to obtain preliminary information on heterotic patterns. The procedure is recommended for use with small populations. The technology of molecular markers may be used to refine existing groups and patterns or for expediting the establishment of new ones, through the determination of genetic distances....

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Dominance theory

The dominance theory assumes that vigor in plants is conditioned by dominant alleles, recessive alleles being deleterious or neutral in effect. It follows then that a genotype with more dominant alleles will be more vigorous than one with few dominant alleles. Consequently, crossing two parents with complementary dominant alleles will concentrate more favorable alleles in the hybrid than either parent. The dominance theory is the more favored of the two theories by most scientists, even though neither is completely satisfactory. In practice,...

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Concept of heterotic relationship

Genetic diversity in the germplasm used in a breeding program affects the potential genetic gain that can be achieved through selection. The most costly and time consuming phase in a hybrid program is the identification of parental lines that would produce superior hybrids when crossed. Hybrid production exploits the phenomenon of heterosis, as already indicated. Genetic distance between parents plays a role in heterosis.In general, heterosis is considered an expression of the genetic divergence among cultivars.When heterosis or some of its...

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Hybrid vigor

Hybrid vigor may be defined as the increase in size, vigor, fertility, and overall productivity of a hybrid plant over the mid-parent value. It is calculated as the difference between the crossbred and inbred means: The synonymous term, heterosis, was coined by G.H. Shull. It should be pointed out immediately that, as it stands, heterosis is of no value to the breeder, if a hybrid will only exceed the mid-parent in performance. Such advantageous hybrid vigor is observed more frequently when breeders cross parents that are genetically diverse. The...

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What is a hybrid cultivar?

A hybrid cultivar, by definition, is the F1 offspring of a planned cross between inbred lines, cultivars, clones, or populations. Depending on the breeding approach, the hybrid may comprise two or more parents. A critical requirement of hybrid production isthat the parents are not identical. As will be discussed next, it is this divergence that gives hybrids their superior performance. The outstanding yields of certain modern crops, notably corn, owe their success to the exploitation of the phenomenon of heterosis, which is high when parents...

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Factors affecting performance of synthetic cultivars

Three factors are key in determining the performance of a synthetic cultivar. Number of parental lines used. Synthetic cultivars are maintained by open pollination. Consequently, the F2 yield should be high to make it a successful cultivar. The Hardy– Weinberg equilibrium is reached in syn-2 for each individual locus and, hence, should remain unchanged in subsequent generations. It follows then that the F3 should produce as well as syn-2. Some researchers have even shown that F3 and F4 generations yielded as much or slightly better than F2...

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Highest yield performance is obtained in the syn-1 generation

The highest yield performance is obtained in the syn-1 generation, hybrid vigor declining with subsequent generations. It is generally estimated that a synthetic forage cultivar of cross-fertilized diploid or polyploidy species will experience a maximum yield decline of 10–12% from syn-1 to syn-2 generation, as previously stated. The yield decline is less in subsequent generations. Sewall Wright proposed a formula to predict the F2 yield of a group of inbred lines as follows: where F2 is the expected performance of the F2, F1 is the mean F1...

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Primary steps in the development of a synthetic

There are three primary steps in the development of a synthetic: Assembly of parents. Assessment of GCA. Random mating to produce synthetic cultivars. The parents used in synthetics may be clones or inbred lines. Whereas forages can be increased indefinitely by clonal propagation, inbred lines are needed to perpetuate the genotypes used in hybrid production. The parental materials are reproducible and may be substituted with new genotypes as they become available, for some improvement in the synthetic cultivar. The parents are selected...

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Development of synthetic cultivars

1. Synthetic cultivar versus germless composites There are two basic types of open-pollinated populations of crops – those produced by population improvement, and synthetics. As previously discussed, population improvement methods can be categorized into two – those that depend on purely phenotypic selection and those that involve selection with progeny testing. A synthetic cultivar may be defined as an advanced generation of crossfertilized seed mixture of parents that may be strains, clones, or hybrids. The parents are selected based on...

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2 Full-sib reciprocal recurrent selection

Developed by Hallauer and Eberhart as modifications of the method by Comstock and colleagues, the fullsib method requires at least one of the populations to be prolific. The recombination units are half-sibs. Developed for maize, full-sib families are produced by pairing plants from two populations, A and B. The top ear of a plant from population A is crossed with a plant from populations B. The lower ear is selfed to be saved as remnant seed. The same is done for the reciprocal plant from population B, if they have two ears, otherwise, they are...

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2 Half-sib selection with test cross

Another way of evaluating genotypes to be composited is by conducting a test cross. This variation of half-sib selection allows the breeder to more precisely evaluate the genotype of the selected plant by choosing the most suitable test cross parent. The half-sib lines to be composited are selected based on a test cross evaluation not progeny performance.The tester may be an inbred, in which case all the progeny lines will have a common parental gamete. Like half-sib selection with progeny test, this procedure is applicable to cross-pollinated...

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1 Half-sib selection with progeny test

Half-sib or half-sib family selection is so-called because only one parent in the cross is known. In 1899, C.G. Hopkins first used this procedure to alter the chemical composition of corn by growing progeny rows from corn ears picked from desirable plants. Superior rows were harvested and increased as a new cultivar. The method as applied to corn is called ear - to- row breeding. There are various half-sib progeny tests, such as, topcross progeny test, open-pollinated progeny test, and polycross progeny test. A half-sib is a plant  with...

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2 Full-sib family selection

Full sibs are generated from biparental crosses using parents from the base population. The families are evaluated in a replicated trial to identify and select superior full-sib families, which are then recombined to initiate the next cycle. Steps Cycle 0 _ Season 1. Select random pairs of plants from the base population and intermate, pollinating one with the other. Make between 100 and 200 biparental crosses. Save the remnant seed of each full-sib cross. _ Season 2. Evaluate full-sib progenies in multilocation replicated trails....

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Family selection methods

Family selection methods are characterized by three general steps: Creation of a family structure. Evaluation of families and selection of superior ones by progeny testing. Recombination of selected families or plants within families to create a new base population for the next cycle of selection. Generally, the duration of each step is one generation, but variations exist. 1 Half-sib family selection methods The basic feature of this group of methods is that half-sib families are created for evaluation and recombination, both...

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