Derivation of mutantt-haplotypes of the mouse by presumed duplication or deletion

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Abstract
The genetic properties of some new mutant t-haplotypes derived from the naturally occurring haplotype t6 were investigated. Several mutant haplotypes gave taillessness with brachyury, T, were viable when homozygous, and when in compound with t6, and all these expressed tf and appeared to have arisen by meiotic crossing-over between T and tf in the region of crossover-suppression of t6. The haplotype th17 arose by crossing-over in a stock of t6 carrying the translocation T(1:17)190Ca, in which the translocation break in chromosome 17 is within the crossover-suppressing region; th17 could not be separated from T190. It was of the complementary crossover type in that it did not interact with T, and was lethal in compound with t6; its male segregation ratio was normal with heterogeneity. A further haplotype, th20, expressed tf, gave taillessness with T, and was lethal when homozygous and in compound with t6. The expression of tf was attributed to a deletion, which also covered the loci of the nearby lethals knobbly, Kb, and tw5, th20 showed weak complementation of tw5. The haplotype th7 were previously interpreted as carrying a duplication; this duplication was semi-lethal when homozygous. Lethal mutant t-haplotypes commonly arise by duplication or deletion but such structural changes are not necessarily at the end of the haplotype, and need not arise by unequal meiotic crossing-over.