The notion of genetic modification of foods and livestock is a contentious issue. The purposeful insertion or deletion of a gene into a plant or animal's genome to yield specific traits, such as herbicide resistance, nutritional composition, or size, prompted the Codex Alimentarius Commission, an international effort to regulate the safety of foods, to issue guidelines concerning genetically-modified foods.

The committee is aware of the concept of unintended effects, i.e., effects that were not part of the original gene insertion or deletion design. In their report, last updated in 2009, they state that:
Unintended effects can result from the random insertion of DNA sequences into the plant genome, which may cause disruption or silencing of existing genes, activation of silent genes, or modifications in the expression of existing genes. Unintended effects may also result in the formation of new or changed patterns of metabolites. For example, the expression of enzymes at high levels may give rise to secondary biochemical effects or changes in the regulation of metabolic pathways and/or altered levels of metabolites.
They make the point that food crops generated using techniques without genetic modification are released into the food supply without safety testing:
New varieties of corn, soybean, potatoes and other common food plants are evaluated by breeders for agronomic and phenotypic characteristics, but generally, foods derived from such new plant varieties are not subjected to the rigorous and extensive food safety testing procedures, including studies in animals, that are typical of chemicals, such as food additives or pesticide residues, that may be present in food.
In other words, conventional plant breeding techniques, such as hybridization, backcrossing, and introgression, practices that include crossing parental plants with their progeny over and over again or crossing a plant with an unrelated plant, yield unique plants that are not subject to any regulation. This means that unintended effects that arise are often not identified or tested. Plant geneticists know that, when one plant is crossed with another, approximately 5% of the genes in the offspring are unique to that plant and not present in either parent. It means that offspring may express new characteristics, such as unique gliadin or gluten proteins in wheat, not expressed in either parent and with new immunological potential in consuming humans.

Dr. James Maryanski, the FDA's Biotechnology Coordinator, stated during Congressional testimony in 1999 that:
The new gene splicing techniques are being used to achieve many of the same goals and improvements that plant breeders have sought through conventional methods. Today's techniques are different from their predecessors in two significant ways. First, they can be used with greater precision and allow for more complete characterization and, therefore, greater predictability about the qualities of the new variety. These techniques give scientists the ability to isolate genes and to introduce new traits into foods without simultaneously introducing many other undesirable traits, as may occur with traditional breeding. [Emphasis mine.]
Efforts by the Codex Alimentarius and FDA are meant to control the introduction and specify safety testing procedures for genetically modified foods. But both organizations have publicly stated that there is another larger problem that has not been addressed that predates genetic modification. In other words, conventional methods like hybridization techniques, the crossing of different strains of a crop or crossing two dissimilar plants (e.g., wheat with a wild grass) have been practiced for decades before genetic modification became possible. And it is still going on.

In other words, the potential hazards of hybridization, often taken to extremes, have essentially been ignored. Hybridized plants are introduced into the food supply with no question of human safety. While hybridization can yield what appear to be benign foods, such as the tangelo, a hybrid of tangerines and grapefruit, it can also yield plants containing extensive unintended effects. It means that unique immunological sequences can be generated. It might be a unique gliadin sequence in wheat or a unique lectin sequence in beans. None are tested prior to selling to humans. So the world frets over the potential dangers of genetic modification while, all along, the much larger hazard of hybridization techniques have been - and still are - going on.

Imagine we applied the hybridization techniques applied by plant geneticists to humans, mating an uncle with his niece, then having the uncle mate again with the offspring, repeating it over and over until some trait was fully expressed. Such extensive inbreeding was practiced in the 19th century German village of Dilsberg, what Mark Twain described as "a thriving and diligent idiot factory."