test material that is homozygous for the known determinant a tester plant is crossed with material that is homozygous for the genetic determinant that is to be tested this latter plant is referred to as the donor plant the donor plant to be tested should be or should be made homozygous for the genetic determinant to be tested the skilled person knows how to obtain a plant that is homozygous for the genetic determinant to be tested when in the f2 of the cross between a donor plant and a tester plant no segregation for the phenotype related to the genetic determinant is observed the genetic determinants of the donor plant and the tester plant have been proven to be equivalent or the same the invention also relates to a brassica plant that may comprise a genetic determinant conferring resistance to thrips tabaci wherein plants of first generation progeny f1 of a cross of the said plant with a tester plant that may comprise the said genetic determinant and of which representative seed was deposited with the ncimb under accession number ncimb 41760 or a progeny plant thereof that may comprise the said genetic determinant or a plant derived therefrom and which may comprise the said genetic determinant show a 1 0 segregation for the resistance against thrips tabaci in both the tester plant and the plant of the invention the genetic determinant is present in homozygous form plants of the second and further generations if obtained by selfing also show a 1 0 segregation for the said resistance pattern.