The phenotypic expression of a t6/t6/t6 genotype.

In vivo, embryos which are homozygous for the t6 mutation die during egg cylinder development (gestation days 5.5--6.5). In vitro, these mutant embryos can be distinguished from their wild-type littermates by their developmental arrest and by the failure of their trophoblast cells to transform to giant cells. We have investigated the nature of this lethality by constructing triploid embryos with varying combinations of the t6 mutant chromosome. The phenotypes of outgrowths from these triploid embryos were examined and compared with the phenotypes of outgrowths from +/+, +/t6, and t6/t6 embryos. The data show 1) that +/t6/t6 embryos are phenotypically wild-type, while t6/t6/t6 embryos are phenotypically mutant and 2) that t6/t6/t6 and t6/t6 embryos are developmentally arrested at the same stage of outgrowth.