Showing papers on "Crossover published in 1976"

Evolution of repeated DNA sequences by unequal crossover

Abstract: It is often supposed that highly repetitious DNA's arise only as a result of unusual mechanisms or in response to selective pressure. My arguments and simulations suggest, by contrast, that a pattern of tandem repeats is the natural state of DNA whose sequence is not maintained by selection. The simulations show that periodicities can develop readily from nonreptitious DNA as a result of the random accumulation of random mutations and random homology-dependent unequal crossovers. The lengths of these periodicities, and the patterns of subrepeats within them, would fluctuate in evolution, with the probability of a given pattern being dependent on the unknown exact nature of the crossover mechanism. Qualitatively, then, unequal crossover provides a reasonable and uncontrived explanation for the prevalence of highly repeated sequences in DNA and for the patterns of periodicity they evince.

Active Crossover Networks for Noncoincident Drivers

Abstract: The spatial separation between drivers in a loudspeaker system affects the radiation pattern over the frequency range where more than one driver contributes to the total acoustic output. An analysis of conventional crossover networks shows that the main lobe of the radiation pattern shifts in direction and increases in amplitude. A new network transfer function, which can easily be realized with operational amplifiers, eliminates this problem. Additional active delay networks are used to compensate for offsets in the acoustical planes from which the individual drivers radiate. The audibility of phase distortion is investigated with the conclusion that it is undetectable for the proposed types of networks.

Crossover behaviour and effective critical exponents in isotropic and anisotropic Heisenberg systems

Abstract: Renormalisation group methods are used to determine, to second order in epsilon =4-d, the scaling function describing the crossover from Gaussian to Heisenberg behaviour in the susceptibility of an isotropic n-component spin system. The results are used in conjunction with an earlier Feynman graph calculation to obtain an O( epsilon 2) representation of the n-to-m-component susceptibility crossover function, and the corresponding effective exponents, for an anisotropic n-component system.

The quantum-classical crossover critical behaviour of the Ising model in a transverse field

Abstract: The quantum-classical crossover critical behaviour of the Ising model in a transverse field Gamma is first studied in one dimension where exact results are obtained. The correspondence between the quantum-classical crossover behaviour of this d-dimensional quantum model and the finite-size crossover behaviour of an equivalent (d+1)-dimensional classical model exact for d=1 is extended to higher dimensions. The crossover depends strongly on the relative magnitude of the crossover exponent phi and of the critical line-shift exponent Psi . In particular, if Pi > Psi approaches the critical line along a path of constant transverse field less than the critical field Gamma c and a low temperature, only classical behaviour is seen, quantum effects do not show up. There is some controversy about the value of Psi . Experiments are suggested to give an answer.

Field theoretic approach to Gaussian-Heisenberg crossover behavior

Abstract: The renormalization prescription of t'Hooft and Veltman (see Nucl. Phys., vol.B44, p.189 (1972)) is used to construct crossover scaling functions for the susceptibility and free energy in an isotropic n vector model. Some difficulties in interpreting the epsilon expansion in this context are discussed, and the formalism is illustrated by a calculation of the expansion factor alpha 2 of a polymer in dilute solution.

Crossover phenomena in isotropic dipolar ferromagnets

Abstract: Crossover behaviour in dipolar ferromagnets is investigated using renormalization group methods. The scaling functions describing the evolution from short-range to dipole-dominated behaviour are calculated for the susceptibility and specific heat, to leading order in epsilon =4-d. The resulting effective critical exponents are found to exhibit striking behaviour in the crossover region.

Renormalization group equations with several length scales and crossover scaling functions for axial anisotropy

Abstract: The critical behaviour of axially anisotropicn-vector models is characterized by two distinct length scales, the correlation lengthsξ∥ andξ⊥ for the easy and hard axes. In order to handle the full range of anisotropics fromξ∥≈ξ⊥ toξ∥ partial differential renormalization group equations are derived, depending onξ∥ andξ⊥. The anisotropicX-Y model is studied in detail near four dimensions. The crossover scaling functions for the susceptibilities are calculated to first order ine=4−d. Two distinct crossover regions are found for weak and dominant anisotropy, respectively.

Clustering of specific crossovers in maize microsporocytes.

Abstract: Analysis of systematic scan data from microsporocyte smear preparations suggests that cells with a crossover in a specific region tend to be geographically clustered to some extent within anthers. Crossover classes observed include single crossovers within a heterozygous inversion, three-strand double crossovers within and proximal to the inversion, and four-strand double crossovers within the inversion. Evidence for clustering within scans of cells of the first two classes is reported, but results are inconclusive for the third class. No evidence for within-scan correlation of frequencies of any two of the crossover classes was found. It is inferred that a large part of the variability in crossover frequency observed is probably due to factors which alter crossover interference. Implications related to the frequency and distribution of crossover sites are discussed.

Rail crossover arrangement

Abstract: Rail crossover arrangement characterized in that wheels can pass through the crossover without shock. To this end flanged portions of the wheels are supported on the crossover plate provided in the intersections of the rails when the wheels pass through the intersections.

Transfer and Recombination of Genetic Material

Abstract: “Recombination” means the formation of new genotypes by exchange of genetic material from two parents differing in two or more inheritable properties. In the genetics of higher organisms we differentiate between a) Recombination of “linked” genes, which lie on homologous chromosomes and can only be recombined by a crossover, and b) Recombination of “non-linked” genes, which lie on non-homologous chromosomes and therefore can be freely combined without a crossover.