Traits which are of the most interest to us belong to the category of
complex traits. Complex traits are controlled by many interactive components,
including developmental-environmental factors, and many genes. In the
biomedical field most complex traits, such as body weight, cancer susceptibility,
diabetes, blood pressure, depression, alcoholism, etc., have heritable
Historical Background. Focusing on single genes and qualitative traits, Bartlett and Haldane (Bartlett and Haldane, 1935) indicated that the object of an experiment may be to introduce a certain known gene "into a pure line, so as to study its effects against a standard genetic background." In the early 1980s, it was generally thought that the introgression of (unknown) genes for complex traits was limited to qualitatively distinguishable phenotypes. For example, Earl. L. Green (Jackson Laboratory), suggested that "To be successful, the method must be limited to discrete traits or to continuous traits that exhibit clear bimodality in the segregating strains" (Green, 1981) (p.117).
The RQI concept. The RQI principle posits that a set of unidentified polygenes (or QTLs) which define the expression of a quantitative trait with continuous distribution can be transferred together to a another genetic background. The transfer can be done for various reasons, e.g., for