a1 The Wistar Institute of Anatomy and Biology, 36th Street at Spruce, Philadelphia, Pennsylvania 19104 USA
The various lethal t-haplotypes are divided into several complementation groups defined by lethality of tx/ty heterozygotes, and only partial complementation is usually observed in heterozygotes between t complementation groups. Several effects of the t region have been found to be preferentially or exclusively detectable in males. This paper describes results showing that the viability of t6/tw5 heterozygotes is influenced by the sex of the embryo; female viability is only slightly reduced (viability ratio approximately 0·80), while male viability is far below normal (0·25).
(Received July 24 1981)
(Revised August 20 1981)