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Mutations (continued)
Mutations are small changes in DNA that affect a gene. When this affects the performance of a gene and the gene product is lost (e.g. a pigment) they are known as "loss of function" mutations. The defective gene is usually masked when there is a normal copy of the gene present so it is called "recessive". It still can be passed on to a future generation in a manner in which one can predict the outcome of crosses using the mutation. If a mutation causes a new gene product to be formed where it normally isn't, or a new trait to appear (e.g. frills) that is not in the wild population, it is sometimes classified as a "gain of function" mutation. The mutants I will mention will sort out in ratios known as Mendelian ratios. They are either recessive, dominant, or codominant traits. Some of these can give very interesting affects when present on the same bird.

Unlike singe-gene mutations, strains are "breeds" of birds where, over a long period of selection, a trait that is coded for by many genes (i.e. a quantitative trait), or influenced by many other genes, is fixed in a population. Strains must be maintained with careful selection and not randomly crossed because they will revert quickly to less desirable birds. A dog example would be German Shepherd. There is no single gene for "German Shepherd". It is a breed or strain that took many generations to create by combining a host of genes that give the dog the specific appearance. If you cross a German Shepherd to a French poodle you would not get German Shepherds that in the next generation would segregate into shepherds and poodles. Instead, you would mix lots of traits together and lose the strain that you desire. In birds, some example of traits normally controlled by a multitude of genes are size, conformation, feather quality, and intensity of color.

It is not always this simple. Some mutations are highly influenced by other genes and their level of expression, therefore, can be modified and different in different genetic backgrounds. So, in some lines a single gene mutation will appear more strongly than in others since it is the other specific genes present in that bird that modify the single gene's expression. If you intend to obtain a specific goal when breeding , it would be helpful to get a working knowledge of Mendelian genetics, mutations, and strains.