Loose Linkage Research Articles

A theoretical appraisal of the relative merits of 50 % hybrid and synthetic varieties of grasses

SUMMARYThe expected merits of the semi-hybrid variety, produced by bulk interpollination of two populations (Foster, 1971a, 6, c), are examined in relation to those of the synthetic variety. With diploid inheritance, the mean expression of both types of variety is seen to be the same. Any postulated difference in productivity must depend not only on the preferential establishment and survival of a small fraction of the total population, but on the existence of a correlation between characters determining productivity and establishment. For two populations producing a heterotic hybrid, a correlation due to loose linkage or gene association without linkage may result in the superiority of the semi-hybrid, but tight linkage or pleiotropy are more likely to favour the synthetic variety. It is concluded that superiority of the semi-hybrid mode of varietal production can only be established by means of a direct comparison with its synthetic counterpart, and not solely by the demonstration of upper-parent heterosis for productivity.

Demonstration of a sex difference in recombination fraction in the loose linkage, Rh and PGM.

Demonstration of a sex difference in recombination fraction in the loose linkage, Rh and PGM.

Linkage between the MN- and Hb beta-loci?

Other authors have suggested that the MN-locus is on the long arm of either chromosome 2 or 4 and that the hemoglobin β-locus is on the long arm of a B group chromosome. Ananalysis of new pedigree data together with previously reporteddata hints loose linkage between the hemoglobin β- and MN-loci.

A New Factor for Resistance to Common Bunt in Hexaploid Wheats1

A new gene, designated Bt10, conferred resistance to races T‐l and T‐15 of common bunt (Tilletia caries and T. foetida) in the wheat cross, (Triticum αestivum L.) PI 116306 (resistant)✕ ‘Elgin’ (susceptible) and to races T‐l and L‐9 in the cross PI 116301 ✕ Elgin. Bt10 confers resistance to all races of common bunt found in the Pacific Northwest States with the exception of race X‐l. The possibility of a loose linkage between Bt10 and the locus for glume color on chromosome IB is discussed.

SOME PROPERTIES OF TWO-LOCUS SYSTEMS WITH EPISTATIC SELECTION

WHEN natural selection acting on two loci contains nonadditive interactions, that is, when selection is epistatic, the properties of the system cannot necessarily be predicted from the behaviour of single loci. LEWONTIN and KOJIMA (1960) and others since have noted that there is frequently permanent gametic excess (linkage disequilibrium) at equilibrium, that there may be alternative stable equilibria, and that the amount of gametic excess, and the equilibrium gene frequencies (and even the stability of the equilibrium) are dependent upon the recombination fraction between the loci. With weak epistasis and loose linkage most of these complexities can be ignored (KIMURA 1965). There are four properties of such systems which do not seem to receive as much attention as they deserve: the analogy between one- and two-locus systems in the properties of some of the derivatives of mean fitness; a similar analogy between the way in which dominance and epistasis affect the position of equilibria; the similarity between multiplicative selection for one and two loci; and the conditions leading to gametic equilibria whose stability varies with the recombination fraction. This paper deals briefly with these properties. Random mating is assumed throughout.

Linkage and selection: new equilibrium properties of the two-locus symmetric viability model.

In the two-locus symmetric viability model with recombination, conditions for the existence and stability of a new class of polymorphic equilibria are given. These "unsymmetric" equilibria, which exist for relatively loose linkage, can be explicitly obtained, in contrast to the previously known symmetric equilibria which are generally given by the solutions of a cubic equation. For sufficiently tight linkage, there is always at least one stable symmetric equilibrium. Seven is the precise maximum for the number of interior equilibria. The qualitative consequences of these results are briefly examined. In particular it is pointed out that since for some linkage values no stable polymorphic equilibria exist, there is no "overdominance theorem" comparable to the single-locus case.

WHY DOES THE GENOTYPE NOT CONGEAL?

If it can be shown that natural selection favors closer linkage between loci, as is argued by the existence of supergenes, why do we still have chromosomes which cross over ? Fisher (1930) pointed out in a verbal proof that if natural selection favors gametes AB and ab, and disfavors Ab and aB, then at equilibrium the frequency of the favored gametes is decreased by recombination in double heterozygotes (AaBb), so that selection will favor closer linkage between the loci; this has been shown explicitly for special cases by Kimura (1956), Lewontin and Kojima (1960), Haldane (1962), Jain and Allard (1965), Nei (1964), Parsons (1963), and Bodmer (in Bodmer and Parsons, 1962). Recently I have tried to combine the various approaches to the problem of two interacting loci into a single model, mostly without explicit equations, in which Fisher's proof is confirmed; increased linkage is always favored provided that the population is at a stable equilibrium or under particular conditions of change; although sometimes when the population is changing under natural selection, it is looser linkage which is favored. This model includes the case in which the two types of double heterozygote, AB/ab and Ab/aB, have different fitnePsses aorin reduiiced reromhinatinn is favored at the stable equilibrium, but not necessarily while the population is changing (Turner, 1967a). Fisher considered also the problem of why, if interacting loci tend to become more closely linked (as populations will tend to remain near to stable equilibria), the whole genotype does not condense, coagulate, or congeal into one huge pair of chromosomes. His own explanation seems to fail because it cuts both ways-that a new favorable mutation has little chance of establishing itself if closely linked in repulsion with a polymorphic locus does not imply that the conditions most favorable to its spread are looser linkage, but that it should be tightly linked in coupling, or so it seems from deterministic models (Bodmer and Parsons, 1962). However, Dr. W. J. Ewens has kindly shown me some of his unpublished work demonstrating that with a stochastic model the conditions favorable to the spread of a new mutation may be close linkage or loose linkage, according to the circumstances. Thus the gain in fitness given to those lines in which a new mutation is spreading may produce selection for looser linkage; on the other hand, sometimes it will cause selection for tight linkage. Until we know whether one kind of selection is commoner than the other, we are no nearer to solving our problem; therefore we will

Linkage relationships of egg albumen loci in the domestic fowl

Synopsis The linkage relationships between egg albumen loci Ov, II, III and Tf are examined in segregation data from backcross matings. The suggestion of close linkage between the Ov locus and locus II is confirmed. Loci II, III and Tf show evidence of very loose linkage but the data are also compatible with the loci located on different chromosomes.

Variation inMicrosporum gypseum. I. A genetic study of pleomorphism

Studies on the nature of pleomorphism were made possible by the rediscovery of sexual reproduction in the dermatophyte, Microsporum gypseum (Bodin) Guiart & Grigorakis. Ascus analysis of progeny resulting from crosses between certain pleomorphic cultures and wild-type revealed a 1:1 segregation of mutant to wild-type. Several other variants not considered pleomorphic by conventional definition, showed a similar segregation pattern. Crosses between mutant cultures revealed linkages among several loci. Most of the white, fluffy, aconidial cultures were sexually sterile; however, five such cultures capable of crossing with wild-type were found to have resulted from two gene mutations. Ascus analysis of crosses between these double mutants and wild-type have indicated loose linkage between the two mutant loci in one culture, and the absence of linkage in the other four cultures. Pleomorphism is thus shown to be the result of one or more gene mutations. It is suggested that the term pleomorphism be discarded i...

The selection of double heterozygotes

Selection in favour of double heterozygotes leads to different types of equilibrium according to the linkage between the loci concerned. For slight selection or loose linkage coupling and repulsion should be equally frequent. When the selection or the linkage becomes sufficiently intense there is an equilibrium when almost all or almost none of the double heterozygotes are in coupling.

A new linkage in the house mouse; vestigial and Rex.

IN 1947, Wright1 (Mrs. M. E. Wallace) reported the remarkable discovery of recombination frequencies exceeding 50 per cent in linkage group VII of the house mouse. Such a phenomenon had been predicted as a theoretical possibility by Jennings2 in 1923 for substantially the right reasons, although he based his argument on a theory of two-strand crossing-over and a simplified discontinuous theory of interference. Mrs. Wallace found that the genetic markers sh2 (shaker-2) and wv2 (waved-2), which show loose linkage with each other of about 30 per cent recombination frequency, both recombined with sex with a frequency of about 56 per cent. This finding was the first case of partial sex-linkage to be reported in any mammal other than man. Rex (symbol Re), which was known to be about 20 per cent from sh2 and about 40 per cent from wv2, assorted independently of sex in other data from the same department.

Order of genes in the 5th linkage group of the house mouse.

WORK carried out in this Department during the past two years on the fifth linkage group in Mus musculus has established the order circler (kr), agouti (A), undulated (un), wellhaarig (we) and pallid (pa)1. Recent work also suggests that a gene (pl) modifying the frequency of manifestation of polydactyly is located in this region, possibly between A and pa. Mrs. Wallace2, also working in this Department, determined a linkage of the order of 25 per cent recombination between fidget (fi) and Danforth's short tail (Sd). About the same time, Carter and Gruneberg3 ascertained a linkage of about 33.5 per cent recombination between fidget and agouti. Earlier work, both at Cambridge and at the Jackson Memorial Laboratory, Bar Harbor, showed indications of a loose linkage (about 40 per cent recombination) between A and Sd. Thus there was clear evidence of a second series of loci in the order A—fi—Sd.

GENETIC LINKAGE IN MAN, WITH PARTICULAR REFERENCE TO THE USEFULNESS OF VERY SMALL BODIES OF DATA

1. The term genetic linkage frequently proves misleading. It is often supposed that when two genes are linked they tend to stick together in hereditary transmission. What is actually observed is that, instead of the random distribution seen when the genes are situated on different chromosome pairs, they either tend to stick together unduly often, or, just as frequently, tend to separate unduly often. 2. The consequences of genetic linkage are explained for the case of two rare dominant traits. 3. An examination is made of a pedigree recently recorded by Penfold and Lipscomb, in which elliptocytosis and multiple telangiectasia occurred together in the same family group. The pedigree is a very small one, but it nevertheless provides strong evidence against very close linkage (a cross-over value of five per cent. or less) and good evidence against somewhat looser linkage (a cross-over value of 10 per cent. or less). 4. Observers who encounter the simultaneous occurrence of two inherited conditions in the same family group should on no account omit to place on record as full a description as possible. It may easily happen that a single small pedigree may be decisive in regard to whether or not there is close or moderate linkage between the genes, thus making possible a solid contribution to the task of mapping the human chromosomes, and, if the chance is missed, so rare an opportunity may not recur for centuries.