COST-EFFECTIVE TRANSFER OF RECESSIVE TRAITS VIA THE BACKCROSS PROCEDURE

The backcross breeding method is often used to transfer recessive trai ts controlled by one or a few genes from one pure line to another. Gui delines are needed to help plant breeders to use backcrossing cost-eff ectively. In a backcrossing program, a plant breeder may choose betwee n methods with two or three stages per cycle. The two-stage method uti lizes alternate backcrossing and selfing to identify BCiSj plants with the recessive trait while the three-stage method uses two sequential crosses followed by selfing. In the three-stage method, n(min), the mi nimum number of plants required to recover at Least one or two BCiSj p lants in the i(th) cycle with a given probability (1 - alpha), is obta ined by growing only one BCiSij plant for each of n(min), BCiSo. Alter native values for the number of BCiSo plants (n(b)) and BCiSj plants p er BCiSo (n(s)) are present for alpha = 0.05 and alpha = 0.01 for trai ts controlled by one, two, three, or four recessive genetic loci. A me thod to compare the cost-effectiveness of alternative values is presen ted for cases where the relative costs of crossing, selfing, and evalu ation of S-j progeny are known. Unless time is the paramount concern, it is only in cases where the ressive trait is controlled by a single locus that it can be more cost-effective to make two sequential crosse s to the recurrent parent before selfing than to cross and self and th en only if the cost of evaluating BCiSj plants is high relative to the cost of producing BCiSo plants. When using the three-stage backcross method, the breeder can reduce the n(b) and increase n(s) if the cost of evaluating BCiSj plants is low relative to the cost of producing BC iSo plants.