Fig. 1.

Mutagenesis screen. A behavioral test can be used to detect mutations in either first-or third-generation offspring of N-ethyi-N-nitrosourea (ENU)-treated males. (A) Genetic screen for dominant or semi-dominant mutations in first-generation (G1) offspring. Male mice were injected intraperitoneally with ENU to intervene at the stage of spermatogonia and enter a period of sterility. Upon recovery of fertility (12 to 16 weeks after treatment), they were bred with multiple females to produce G₁ progeny that were heterozygous for any ENU-induced mutation (indicated by an asterisk). (B) Genetic screen for recessive mutations in third-generation (G₃) offspring. In this scheme, G₁ male progeny are bred with wild-type females to produce second-generation (G₂) progeny. The G₂ female progeny are then bred with their G₁ father to produce G₃ progeny. Compared to intercrossing G₂ females and males, backcrossing G₂ females to the G₁ father doubles the likelihood of producing G₃ progeny that are homozygous for a mutation.