Single locus heterosis

Summary Experimental evidence is reported in favor of superdominance in over-all vigor inArabidopsis thaliana in connection with two x-ray induced mutants. The tests adopted (outcrossing to unrelated tester, repeated recombination for 15 generations) failed to reveal any additional genetic variation. The genetic and physiolgical mechanism of superdominance is discussed in light of recent information on gene structure and function.Contribution from the Missouri Agricultural Experiment Station. Journal Series Number 2388. Approved by the Director.     

Genetic linkage analysis places locus DXS250 between locus DXYS1 and locus DXS3 in Xq21.3.

The locus DXS250, which is linked to the Allan-Herndon type of X-linked mental retardation, maps between DXS3 and DXYS1 in a panel of 40 families established by the Centre d'Etude du Polymorphisme Humain, Paris.     

Prealbumin locus in chickens

Three different phenotypes of a weakly staining prealbumin migrating near the borate line during starch gel electrophoresis were found in chicken sera and egg yolks. It is shown that the synthesis of these phenotypes is determined by one autosomal locus with two alleles, Pa ^A and Pa ^B.
The paper also presents the results of partial isolation of polymorphic prealbumins from cock sera. Rivanol®– ammonium sulphate precipitation and chromatography on Sephadex G-200® were used for this purpose.     

The cystic fibrosis locus

The identification of the cystic fibrosis locus (CF) provides a model for the study of single gene defects where the biochemical lesion is not known. Using families each of which has several affected siblings, it was possible to exclude a number of 'candidate genes' which had previously been proposed as possible sites of the CF mutation. Exclusion mapping of the genome using polymorphic protein and DNA markers showed that CF is on the long arm of human chromosome 7. The most closely linked flanking markers were identified, and human chromosome fragments containing them (and therefore the CF locus) were isolated in rodent cell lines by chromosome-mediated gene transfer. The transgenome was then analysed using cosmid contig mapping, pulse-field gel electrophoresis, HTF island identification and linkage disequilibrium. In this way, a candidate coding sequence has been identified which always segregates with CF.     

Agrobacterium tumefaciens virulence locus pscA is related to the Rhizobium meliloti exoC locus.

Agrobacterium tumefaciens and Rhizobium meliloti carry related genetic loci which have important roles in virulence and symbiosis. Previously, it was shown that two virulence loci of A. tumefaciens, chvA and chvB, are related to two R. meliloti symbiosis loci, ndvA and ndvB, respectively (T. Dylan, L. Ielpi, S. Stanfield, L. Kashyap, C. Douglas, M. Yanofsky, E. Nester, D. R. Helinski, and G. Ditta, Proc. Natl. Acad. Sci. USA 83:4403-4407, 1986). Here we show that these two phytobacteria possess additional related virulence/symbiosis genes. Results of genetic complementation and DNA hybridization experiments indicate that the pscA virulence locus of A. tumefaciens is structurally and functionally related to the exoC symbiosis locus of R. meliloti. Thus, A. tumefaciens and R. meliloti bear at least three related genetic loci that have crucial roles in establishing the interactions that each bacterium has with its respective host plants.     

The effect of positive assortative mating at one locus on a second linked locus

Summary Considerations proceed from a model of positive assortative mating based on genotype at one locus, with an arbitrary number of alleles, assuming no selection, mutation, or migration, hypothetically infinite population size, and discrete non-overlapping generations. From these conditions, inferences are made about the genotypic structure at a linked locus, as well as about the corresponding 2-locus gametic structure.The following main results are presented: in the course of the generations, the genotypic structure at the second locus and the 2-locus gametic structure always tend to a limit responsive to the initial conditions concerning the joint genotypic structure at the two loci and the degree of assortativity and linkage. A complete, analytical representation of the limits is given. In particular, if assortative mating is only partial and at the same time linkage is not complete, a population is not able to maintain a permanent deviation of the gametic structure from linkage equilibrium, and thus the genotypic structure at the second locus tends to Hardy-Weinberg proportions. On the other hand, if initial linkage disequilibrium is combined with partial assortative mating and complete linkage (or with complete assortative mating and unlinked loci) the population maintains this disequilibrium and thus the genotypic structure at the second locus need not tend to Hardy-Weinberg proportions. It turns out that the conditions not only of complete linkage, but also of unlinked loci together with complete assortativity, imply no change in gametic structure from the initial structure.In order to demonstrate the influence of several parameters on the speed of convergence to and the magnitude of the respective limits, several graphs are included.     

The effect of positive assortative mating at one locus on a second linked locus

Summary A model for positive assortative mating based on genotype for one locus is employed to investigate the effect of this mating system on the genotypic structure of a second linked locus as well as on the joint genotypic structure of these two loci. It is shown that the second locus does not attain a precise positive assortative mating structure, but yet it shares a property that is characteristic of positive assortative mating, namely an increase in the frequency of homozygotes over that typically found in panmictic structures. Given any arbitrary genotypic structure for the parental population, the resulting offspring generation possesses a structure at the second locus that does not depend on the recombination frequency, while the joint structure of course does. In case assortative mating as well as linkage are not complete, there exists a unique joint equilibrium state for the two loci, which is characterized by complete stochastic independence between the two loci as well as by Hardy-Weinberg proportions at the second locus. For the second locus alone, Hardy-Weinberg equilibrium is realized if and only if gametic linkage equilibrium and an additionally specified condition are realized.