MUTATION AND RECOMBINATION IN THE VICINITY OF A COMPLEX GENE

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Abstract
The genetic relationships of a series of mutations at locus T have been examined in breeding experiments. Combinations of T (a lethal) with other members of the series (t alleles) yield a tailless phenotype. When the t allele is lethal, matings T/t X T/t constitute a balanced lethal system. In general such matings produce only tailless offspring at birth but 46 living exceptions with normal tails, out of 25,751 were observed. All 28 exceptions analysed proved to be tn/tx, tn being the parental t-allele, tx a new allele. No instance of recombination between T and any t allele was found either in the above or in 4399 offspring from T/t X T/t in which the t allele was viable. By use of a marker gene (t f) near locus T, it was shown that in general lethal t alleles (with one exception) suppress recombination in the T-tf interval, (8-10 crossover units). Seven exceptional offspring arising from such marked balanced lethal lines (T tf/tn +) or from + tf/tn + were tn +/tx tf, that is, recombination between T and tf was involved in the origin of a new t allele which in six of these cases was viable, in one case lethal. Seven viable t alleles appear to be nonsuppressors of recombination in the T-tf interval. The hypothesis is discussed that lethal t alleles which are also suppressors of recombination are connected with structural changes in the region of locus T, different lethals differing in the extent of a structural change such as inversion or deficiency; that the allelic behavior of lethals at this locus is due to this fact; and that "mutations" from such lethals to viable nonsuppressors involve restitution of normal structure by recombination. Cytogenetic evidence for or against this hypothesis is not available at present.