Knobbly, FuKb, a dominant mutation on chromosome 17 of the mouse, causes a kinked tail in heterozygotes and embryonic lethality at about nine days in homozygotes. Abnormal organization of the embryonic ectoderm is first apparent at about embryonic day 7, and the retarded and malformed embryos die by mid 9 days.
Kink, FuKi, a dominant gene that maps in the same region, is also an embryonic lethal in homozygotes; heterozygotes have kinked tails and often a circling gait. We have shown FuKb and FuKi are allelic; the FuKb / FuKi compound dies at the same time as FuKb / FuKb embryos with the same morphological syndrome.
The FuKb phenotype is not seen in offspring in expected ratios. The FuKb males may have a low transmission ratio or penetrance may be incomplete.
An incidental finding of our histological studies is a high incidence of complete twinning when FuKb heterozygotes mate inter se or when FuKb × FuKi matings are made.