ABSTRACT
The incidence of some lesions in F1 rats was intermediate with respect to the incidence of the same lesions in the two parent strains; that is, one parent strain had a high incidence of a certain lesion, the second parent a relatively low incidence, and the F1 had an incidence somewhere in-between. This chapter discusses the differences observed between the F1 hybrid and the parent strains. Several variations in the incidence patterns of neoplastic and nonneoplastic lesions were recognized in F1 rats compared to the parent strains. Another variation was the higher incidence of certain other lesions in the F1 rats as compared to either parent strain. This variation is illustrated by the finding of severe paresis and paralysis, which occurred in less than 2% of the Brown Norway (BN)/Bi or Wistar-derived (WAG)/Rij rats. Similarly, it was uncommon in F1 females.
