Topics: Population genetics, Breeding, Genetics Pages: 9 (3110 words) Published: December 21, 2012

Selective Breeding Terms
* When you breed plants, the results are known as cultigens, cultivars or varieties. When there is a cross of animals, the results are referred to as crossbreeds, while a cross of plants results in hybrids. Similar methods are used in animal and plant breeding. When animals with desirable traits are selected, they are bred through the process of culling for particular for traits. Culling is the process of selecting livestock based on desired criteria, and destroying the others. This is how purebreds are produced. Purebreds with a recognizable lineage are known as pedigreed, while a mix of two separate purebreds will produce crossbreeds. The three methods of selective breeding are outcrossing, inbreeding and line breeding. Line Breeding

* Line breeding is the process of breeding animals or plants that are closely related so as to "fix" or "set" desirable traits. For example, if a horse has some qualities that the breeder likes, the breeder could breed that horse with another relative so as to reinforce the desirable traits through a "pooling" of the genes. The idea is that if one animal has desirable qualities, mating it with a genetically related animal will increase those desirable traits. In human terms, linebreeding is like mating two close, but one-step-removed relatives, like first cousins, grandparent and grandchild, or uncle and niece. Inbreeding

* Inbreeding is the mating of very closely related animals in the hopes of increasing the desired traits in the next generation. Inbreeding is similar to linebreeding only that in inbreeding, the animals would be as close as parents and offspring, or siblings. Inbreeding has some serious flaws because, while it may intensify the desired traits, it will also intensify any faults in the parents. Linebreeding is a little better, because the parents are one step removed. Still, it has similar drawbacks to inbreeding, because it is still that same small gene pool that is being passed from one generation to the next. Outcrossing

* Outcrossing is the breeding of two animals or plants that are not related to each other. This means that the animals do not have any related ancestors in their pedigree for four generations or more. Outcrossing introduces new traits that are missing in the limited gene pools available in linebreeding and inbreeding. It can also "dilute" the effects of inbreeding by reducing the concentration of undesirable genes. SYSTEMS OF BREEDING

Random breeding is easy to define, but difficult to achieve. In principle, random breeding requires that the product rule of probability be satisfied. This means that the chance of choosing any one male out of m males must be 1/m and of choosing any one female out of f females be 1/f and that the chance of mating any two specific mice be 1/mf. Tables of random sampling numbers or other equivalent randomizing devices must be used to unsure random choices and random matings. Indiscriminate or nonsystematic ways of making up matings are likely to permit unknown biases to occur in the choices of mates. The effect of random breeding on the genetic structure of a stock of mice has to be considered separately for theoretical and actual populations. In theoretical or large populations, random breeding will preserve the gene and genotype frequencies generation after generation. Discrete traits usually determined by one or a few pairs of genes will be retained at stable frequencies in the population, assuming there is no selection and no mutation. The means and variances of continuous or metrical traits will likewise remain constant, under the same assumptions, provided the nongenetic components of variances do not change. These assertions about stability of gene frequencies, genotype frequencies, means, and variances follow from the Hardy-Weinberg principle ( Li, 1955). In actual populations of small size the expected results of a random breeding system...
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