In nature, most new organisms arise through sexual reproduction. New gene combinations accompany each reproduced organism. For instance, a tan cow may mate with a yellow cow to produce a calf of an entirely new color, but the number of new gene combinations is limited by reproductive mechanisms. Cattle can only breed with other cattle or with other variations within a species if the offspring are to be viable. To create a hypothetical violet cow, the breeder would have to locate a cow that already had the necessary violet genes. Genetic engeneering removes this reproductive restriction to some extent by allowing scientists to obtain genes from other organisms.
Theoretically, a genetic engineer could obtain genes that encode violet coloration from an iris or a sea urchin and use them to create violet cows. In practice, genes cannot always fit in the new organism’s genome, nor are the complex genetic codes that determine phenotypes like color readily transferrable, but genetic modification does provide a means to create combinations not found in nature.
Although the prospect of creating violet cows may be enticing, genetic engeneering is still an imprecise technology that may disrupt other genes that are essential to the organism’s life.
Some genetic engineers aim to create engineered seeds that exhibit desirable traits such as durability, productivity and abundant yields. While these traits are desirable, genetically engineered seeds share an identical gene structure. If a fungus or virus specific to that genome arises, the result could be widespread crop failure. The seeds can also be carried by insects, other plants or the wind into nearby fields and lands. The natural plants growing in these areas could theoretically cross-pollinate with the engineered crops.
The health effects of consuming genetically engineered foods are still not widely understood because the process of genetic engeneering has not been part of the food supply long enough to permit long-term studies. The new foods may potentially produce unknown allergens or trigger allergic reactions in people who would otherwise safely eat an unmodified version of the food. Although current studies of genetically modified foods show no discernible difference between these foods and conventionally cross-bred foods among animal or human populations, testing continues.