To understand the risks posed by genetically modified pest-protected plants, it is important to understand that, in addition to the direct effects of the pest-protective gene (section 2.4.1), breeding can lead to indirect effects, such as the effects of extraneous genes linked to the protective genes and pleiotropic effects caused by the protective genes.
The potential for inadvertent changes caused by the addition of extraneous genes that are physically linked to protective genes depends on the breeding method used and the source of the protective gene. The breeding method and the source of genes used for breeding determine the amount of new DNA moved into the cultivar and the number of novel genes linked to the pest-protective gene. Therefore, genetic modification methods, both conventional and transgenic, are discussed below with regard to their potential for adding novel extraneous genes and their potential for causing unanticipated pleiotropic effects.
The choice of parents used in the crosses and the mating structure of the plant species are important in determining the potential for inadvertent health effects associated with the progeny (hybrid, inbred line, or population).
In crosses, or sexual hybrids, the amount of DNA transferred to the progeny can be immense. Depending on the mating design, a parent 's contribution can range from very small (less than 1% for the donor parent in backcrossing) to very large (over 99% for the recurrent parent in backcrossing). In a cross between two parents that have been previously cultivated (that is, cultivars), each parent contributes one half its DNA to the progeny. In bread wheat, each parent contributes 16 billion base pairs to the progeny (see chapter 1, table 1.1). To put that into perspective, each wheat parent contributes roughly 55 times the total amount of DNA found in Arabidopsis thaliana. At first glance, the potential for inadvertent changes that could create new allergens or toxic compounds might seem high. However, through the long history of wheat improvement, wheat has remained a staple food consumed safely by much of the world (people suffering from celiac disease constitute a notable exception). The same is true of most major food crops.
In most conventional plant breeding, the goal is to create new genotypes that combine the favorable alleles from two cultivars into a superior