![]() ![]() ![]() Molecular biological methods of gene transfer alleviate some of these problems by allowing the process to be manipulated at a more fundamental level. Such are the difficulties and limitations of classical breeding. Sometimes undesired traits derived from one parent of a new, improved variety persist whereas the desired traits are lost. Even then, the progeny usually have to be crossed back to the parental variety to ensure stable adoption of the new trait. In addition, because the traits desired usually come from only one parent and may be controlled by one or a few genes, many crosses are necessary before the ''right" chance recombination of genes results in expression of the trait in the offspring. Through the fusion of sperm and egg, each parent contributes half of its genome (an organism's entire repertoire of genes) to its offspring, but the composition of that half varies in each parental sex cell and hence in each cross. When traditional, or classical, breeders cross two sexually reproducing plants or animals, they mix tens of thousands of genes in the hope of obtaining progeny with the desired trait or traits. The major differences between traditional agricultural breeding and molecular biological methods of gene transfer lie neither in aims nor in processes, but rather in speed, precision, reliability, and scope. ![]() In both cases, human beings manipulate a naturally occurring process to produce varieties of organisms that display desired traits, for example, food animals with a higher proportion of muscle to fat, or disease-resistant corn. This fact underlies all attempts to improve agriculturally important species, whether through traditional agricultural breeding or through the techniques of molecular biology. The transfer of genes from one organism to another is a natural process that creates variation in biological traits. ![]()
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