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Cost-Effective Transfer of Recessive Traits via the Backcross Procedure

Department of Crop Science, N.C. State Univ., Raleigh, NC 27695-7629

The backcross breeding method is often used to transfer recessive traits controlled by one or a few genes from one pure line to another. Guidelines are needed to help plant breeders to use backcrossing cost-effectively. In a backcrossing program, a plant breeder may choose between methods with two or three stages per cycle. The two-stage method utilizes alternate backcrossing and selfing to identify BC_iS_j 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 minimum number of plants required to recover at least one or two BC_iS_j plants in the i_th cycle with a given probability (1 — ), is obtained by growing only one BC_iS_j plant for each of n_min BC_iS_o. Alternative values for the number of BC_iS_o plants (rib) and BC_iS_j plants per BC_iS_o (n_s are present for = 0.05 and = 0.01 for traits controlled by one, two, three, or four recessive genetic loci. A method to compare the cost-effectiveness of alternative values is presented for cases where the relative costs of crossing, selfing, and evaluation 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 crosses to the recurrent parent before selflng than to cross and self and then only if the cost of evaluating BC_iS_j plants is high relative to the cost of producing BC_iS_o plants. When using the three-stage backcross method, the breeder can reduce the n_b and increase n_s if the cost of evaluating BC_iS_j plants is low relative to the cost of producing BC_iS_o plants.

Received for publication February 13, 1996.