fertile male rabbit with a minute Y chromosome

Nineteen of 33 or 58% of six‐day blastocysts, recovered from four female rabbits demonstrated an apparent male chromosomal complement with a minute Y chromosome. The remaining 42% of the blastocysts had a normal female XX chromosomal complement. All four female rabbits, two California and two New Zealand White, were inseminated by a particular California male rabbit. Chromosome studies of peripheral blood cultures from the male, which had normal gonadal and genital anatomy as well as normal fertility, revealed a karyotype identical to that seen in the blastocysts with the apparent male chromosomal complement. Autoradiographic studies from blood cultures of the male indicated that the minute chromosome was late‐replicating and was most likely the Y chromosome. The suggestion is made that the rabbit may be another mammal with Y chromosome polymorphism.