The lethal t-haplotypes of the mouse fall into several distinct complementation groups, and varying degrees of partial complementation occur when haplotypes from different groups are crossed. The haplotype th20 arose as a mutant from the haplotype t6, and is thought to include a small deletion covering loci near the t-lethals. In this paper the complementarity of th20 was compared with that of its parent haplotype t6. th20 showed weaker complementation than t6 of the haplotypes tw5, tw32 and tw1. Hence the loci of these lethals were probably exposed by the deletion in th20. However, th20 showed full complementation of tw18 and showed full viability in compound with the dominant brachyury allele hairpin-tail, Thp. Therefore, the th20 deletion probably does not overlap the presumed deletion in Thp, nor extend to the locus of the tw18 lethal. The significance of the weak complementation of tw5, tw32 and tw1 is discussed.

A viable t-haplotype, t38, has been shown to distort linkage relations in the T−tf region of mouse chromosome 17. Separate experiments were done with two different interstitial markers, Low and qk, which map respectively about 5 and 4 centimorgans (cM) from the locus of T in female T + tf/ + (Low or qk)+ heterozygotes. In females that carried t38, however (i.e. t38+ + / + Low tf or t38+ + / + qk tf), there was virtually no recombination between t38 and either interstitial marker, although the t38−tf distance was normal. These observations suggest that t38 suppresses recombination in the lefthand part of that region and strongly enhances it further to the right.

A sex-linked mutation with a phenotype similar to that of an allele of the Mottled series has been discovered in a mosaic male. This mutation can be normally transmitted through the sperm of the original male but not through the oocytes. Several interpretations of this abnormal transmission are discussed.

For one-, two- and n-locus models of a quantitative trait assumed to be determined by overdominant loci, it is shown that the mean of an inbred line will be equal to or larger than the mean of the base population if the original gene frequencies satisfy a condition which is generally a function of the degree of overdominance and of the genotypic values.

The semi-dominant X-linked gene tabby (Ta) in the mouse and its recessive autosomal mimic crinkled (cr) produce the same mutant syndrome involving abnormalities of the hair, teeth and certain exocrine glands. Previous work has provided some indication of interaction between these loci in terms of vibrissa number. The results of the present study demonstrate that, for tooth size, mice doubly heterozygous for tabby and crinkled show a more extreme phenotype than either heterozygotes for tabby or crinkled alone.

The recessive lethal T/t-complex haplotype tw75 was isolated from Jena, East Germany, prior to 1973. It was at first thought to be a standard member of the tw5 complementation group because matings of T/tw75 × T/tw5 produced tailless offspring only (Dunn, Bennett & Cookingham, 1973). Cross tests with all other complementation groups and embryological studies have recently been completed, with results demonstrating that tw75 does not complement the members of either the tw5 or the tw1 complementation groups. Histological studies show that compound embryos tw75/tw5 and tw75/tw1 die with symptoms indistinguishable from tw5/tw5 and tw1/tw1 homozygtes respectively. Thus tw75 represents the first confirmed isolation of a t-haplotype that overlaps two complementation groups.

Assortative mating is generalized to include social homogamy and phenotypic homogamy as two special cases. This generalization, called mixed homogamy, enables tests of hypotheses on the nature of assortment. Cultural inheritance is also extended to include two components: transmitted from parental environments, and non-transmitted sibship environment. Familial correlations are derived for a variety of relationships under mixed homogamy.