A Spontaneous Mouse Chimera Formed from Separate Fertilization of Two Meiotic Products of Oogenesis

Abstract

A female mouse, having about equal proportions of yellow and black fur, arranged, roughly, in broad transverse bands, arose in a cross of +krAy un we/kr ax + un + we ♀ by kr a + un + we/kra +un+we ♂. This female was healthy, normal size, and fully fertile. Separate cell cultures derived from left pinna, right pinna, yellow-hair-bearing skin and black-hair-bearing skin yielded 40 chromosomes in almost all metaphases counted. The yellow-black female transmitted mainly three types of Linkage-Group-5 chromosomes, namely +krAy un we, kr a + un + we, and kr ax+un +we, as well as certain informative crossover products. From these results it was possible to rule out several origins that might have been postulated on the basis of phenotype and parentage alone. It is concluded that the female was a 2n/2n chimera of genotypes +krAy un we/kra +un +we and kr a× +un +we/kr a +un +we, these two genotypes being present in the ratio of about 1:1 in the skin and 3:1 in germinal tissue. Such a chimera could come about through: (1) participation of both secondary oocytes (in lieu of first-polar-body abstriction), each being separately fertilized by an X-bearing sperm; or (2) participation of both products of the second meiotic division (in lieu of second-polar-body abstriction), each being separately fertilized by an X-bearing sperm, following a crossover proximal to the most proximal of the four marked loci; or (3) fusion of two originally separate fertilized eggs or early embryos of the proper genotypes. Close parallelisms with two recently described human cases are discussed.