A Recessive Mutation (fer) Causing Germ Cell Deficiency and a High Incidence of Congenital Testicular Teratomas in 129/Sv-fer Mice1,2,3

ABSTRACT-The genetic basis of a germ cell deficiency, ac-companied by a high incidence of testicular teratomas, was studied in strain 129/Sv-ter mice (formerly designated "129/terSv"). Germ cell deficiency became more severe with advancing age in males, and they were sterile whether or not they had bilateral teratomas. Germ cell-deficient testes were smaller than normal except when the testes had teratomas. In females the ovaries were smaller than normal, but the germ cell deficiency did not progress and most were fertile. The germ cell deficiency resulted from the homo-zygous state of a recessive mutant gene designated "teratoma (ter). " Matings between females with small ovaries and homozygous normal males produced no germ cell-deficient offspring. When F1 offspring with normal gonads were mated together, germ cell-deficient F2animals appeared at a frequency close to 1 in 4. When females with small ovaries (ter/ter) were mated with heterozygous males (ter/+), half of the offspring were germ cell deficient. It was concluded that the genetic factor is a single recessive gene. The incidence of teratomas in +/+ strain 129/Sv-ter males was 1.4% (3/216), and all teratomas were unilateral. Seventeen percent (20/117) of heterozygous males had teratomas, of which 18 were unilateral and 2 were bilateral. Ninety-four percent (167/178) of homozygous ter/ter males had teratomas, of which 75 % were bilateral. Introduction of the mutant gene ter onto the C57BL/6 genetic background resulted in germ cell deficiency in homozy-gotes, but it reduced considerably the teratoma incidence in tertte