Improvement of More Than One Character
Often in a variety development program, the breeder wants to add more than one gene to a variety. This is known as gene stacking. It is possible to introduce several different characters in the course of a backcross program (Fig. 10). Usually the other characters being added have already been introduced into the recurrent parent in other backcross programs. For example, one breeder wanted to develop rust and bunt (another disease) resistant Baart 38 wheat. He did this by backcrossing rust resistance into Baart and by backcrossing bunt resistance into a second line of Baart. He then crossed the rust resistant Baart and the bunt resistant Baart and selected Baart that was both rust and bunt resistant.
Currently, backcrossing is often used to develop improved gene pools or selection populations without trying to return completely to the recurrent parent’s phenotype. In some cases, the variety has a good phenotype and the breeder wants to improve it without having a clearly defined trait (for example, yield).
In order to meet this objective, the breeder can use different numbers of backcrosses (1 to 6) to add as much or as little of the recurrent parent as he/she/they think is necessary. A backcross one population (A x (A x B)) is preferred by many breeders to a simple three-way cross (A x (B x C)), which may have too much variation. In this case A, B, and C represent elite lines or varieties.
Strengths and Weaknesses
There are both strengths and weaknesses of the backcross breeding method. Properly executed, the backcross breeding methods allow all the desirable characteristics of the recurrent parent to be recovered with the possible exception for characters governed by genes tightly linked to the gene(s) being transferred. This may be considered a strength. The backcross method provides a certain and precise way of making predictable gains with little possibility that uncontrolled segregation will produce subtle weaknesses in the variety.
This same characteristic may also be considered a weakness. The method sets an upper limit which is equal to the recurrent parent’s phenotype. This limit will often be lower than the progress that is possible with other lines when segregation is not rigidly controlled. This phenomenon is known as yield lag and is graphically depicted in Figure 11. Series 1 shows the gradual percent improvements reached over time for a given breeding program that is active for 6 years. In contrast, Series 2 is put through a backcross program at year 4, where its percent improvement plateaus.