首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   705篇
  免费   52篇
  国内免费   20篇
  2024年   1篇
  2023年   6篇
  2022年   6篇
  2021年   14篇
  2020年   9篇
  2019年   23篇
  2018年   10篇
  2017年   19篇
  2016年   18篇
  2015年   18篇
  2014年   72篇
  2013年   95篇
  2012年   50篇
  2011年   41篇
  2010年   16篇
  2009年   28篇
  2008年   32篇
  2007年   37篇
  2006年   40篇
  2005年   32篇
  2004年   26篇
  2003年   25篇
  2002年   18篇
  2001年   17篇
  2000年   17篇
  1999年   10篇
  1998年   10篇
  1997年   14篇
  1996年   9篇
  1995年   10篇
  1994年   4篇
  1993年   5篇
  1992年   6篇
  1991年   7篇
  1990年   2篇
  1989年   4篇
  1988年   3篇
  1987年   4篇
  1986年   8篇
  1985年   2篇
  1983年   2篇
  1982年   1篇
  1981年   2篇
  1980年   2篇
  1979年   1篇
  1978年   1篇
排序方式: 共有777条查询结果,搜索用时 15 毫秒
1.
2.
Molecular techniques provide powerful tools for studying the geographic structure of hybrid zones and the dynamics of gene exchange between incipient species. We examined allozyme variation at five loci (PGM, GPI, MDH-1, MDH-2, and LDH) for 27 populations of Palaemonetes kadiakensis from the central, coastal, and eastern regions of Texas. Central Texas populations of P. kadiakensis exhibited highly significant linkage disequilibrium and departures from Hardy-Weinberg genotype proportions. In populations with linkage disequilibrium, allelic differences at GPI defined two types of P. kadiakensis, designated A and B. Both types existed in central Texas with little or no evidence of interbreeding, whereas the populations from all other localities showed complete introgression of type B alleles into the type A gene pool. We also examined ribosomal DNA (rDNA) and mitochondrial DNA (mtDNA) variation in a subset of populations, chosen to cover a range of geographic locations and levels of linkage disequilibrium. Two groups of mtDNA haplotypes and two restriction fragment patterns for the rDNA corresponded to allozyme type A and B individuals in populations exhibiting linkage disequilibrium. In populations with ongoing hybridization, all hybrid animals (N= 15) exhibited type A mtDNA. Exhibition of type A mtDNA indicated that type A females had mated successfully with type B males, but type B females had not mated successfully with type A males. Genotype distributions suggest reduced reproduction by hybrid offspring in central Texas populations. These patterns are consistent with a mosaic model of hybrid zone dynamics.  相似文献   
3.
Summary The effect of selection intensity and population size on the response to selection for percent oil in the grain of maize (Zea mays L.) was evaluated in a replicated experiment over ten cycles of selection. An open-pollinated variety, Armel's Reid Yellow Dent, was divided into subpopulations of 6,10 and 50 plants. Selection proportions of 17% and 5% were imposed upon each subpopulation. Selection was based on the percentage of oil in individual kernels as determined by wide-line nuclear magnetic resonance spectroscopy. As expected, total response to selection increased with larger population sizes and selection intensities. The concave shape of the response curves suggested that an appreciable part of the genetic variance can be attributed to additive genes at high initial frequencies, dominance genes at low initial frequencies, or to the generation of negative linkage disequilibrium due to selection. The consistently greater loss of vigor experienced by the more intensely selected populations reflects the enhancement of inbreeding due to artificial selection, an effect that increases with the intensity of selection. The results indicate that combined selection, based on kernels and using within- and amongfamily information, will be more efficient than other conventional selection procedures, including the normal combined scheme where selection is based on plants.Deceased  相似文献   
4.
The utilization of inorganic carbon by three species of marine diatom, Skeletonema costatum (Grev.) Cleve. Ditylum brightwellii (West) Grun., and Chaetoceros calcitrans Paulsen was investigated using an inorganic carbon isotopic disequilibnum technique and inorganic carbon dose-response curves. Stable carbon isotope data of the diatoms are also presented. Observed rates of photosynthetic oxygen evolution were greater than could be accounted for by the theoretical rate of CO2 supply from the uncatalyzed dehydration of HCO3? in the external medium, suggesting use of HCO3? as an inorganic carbon source. Data from the isotopic disequilibrium experiment demonstrate the use of both HCO3? and CO2 for photosynthesis. Carbon isotope discrimination values support the use of HCO3? by the diatoms.  相似文献   
5.
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.  相似文献   
6.
The effect of selection and linkage on the decay of linkage disequilibrium, D, is investigated for a hierarchy of two-locus models. The method of analysis rests upon a qualitative classification of the dynamic of D under selection relative to the neutral dynamic. To eliminate the confounding effects of gene frequency change, the behavior of D is first studied with gene frequencies fixed at their invariant values. Second, the results are extended to certain special situations where gene frequencies are changing simultaneously.A wide variety of selection regimes can cause an acceleration of the rate of decay of D relative to the neutral rate. Specifically, the asymptotic rate of decay is always faster than the neutral rate in the neighborhood of a stable equilibrium point, when viabilities are additive or only one locus is selected. This is not necessarily the case for models in which there is nonzero additive epistasis. With multiplicative viabilities, decay is always accelerated near a stable boundary equilibrium, but decay is only faster near the stable central equilibrium (with = 0) if linkage is sufficiently loose. In the symmetric viability model, decay may even be retarded near a stable boundary equilibrium. Decay is only accelerated near a stable corner equilibrium when the double homozygote is more fit than the double heterozygotes. Decay near a stable edge equilibrium may be retarded if there is loose linkage. With symmetric viabilities there is usually an acceleration of the decay process for gene frequencies near 1/2 when the central equilibrium (with = 0) is stable. This is always the case when the sign of the epistasis is negative or zero.Conversely, the decay ofD is retarded in the neighborhood of a stable equilibrium in the multiplicative and symmetric viability models if any of the conditions above are violated. Near an unstable equilibrium of any of the models considered,D may either increase or decay at a rate slower than, equal to, or faster than the neutral rate. These analytic results are supplemented by numerical studies of the symmetric viability model.  相似文献   
7.
Selection reduces additive genetic variation by generating gametic-phase disequilibrium, a phenomenon largely ignored when predicting response in plant breeding programs. The development of gametic-phase disequilibrium is here taken into account when predicting the response to selection for various schemes of recurrent selection applicable to plant populations. A general program permits prediction of response to selection from schemes of recurrent selection in which two or more rounds of selection occur in each cycle. An example from Sugar Beet, with alternate rounds of half-sib and S1 family selection, is illustrated. It is shown that failure to take into account the effects of gametic-phase disequilibrium can result in substantial overestimation of the response to selection as well as to changes in rank of the merits of alternative breeding schemes. For a given scheme, ignoring gametic-phase disequilibrium has only small effects on defining the optimum allocation of plots and the numbers of families tested.  相似文献   
8.
The house sparrow is an important model species for studying physiological, ecological and evolutionary processes in wild populations. Here, we present a medium density, genome wide linkage map for house sparrow (Passer domesticus) that has aided the assembly of the house sparrow reference genome, and that will provide an important resource for ongoing mapping of genes controlling important traits in the ecology and evolution of this species. Using a custom house sparrow 10 K iSelect Illumina SNP chip we have assigned 6,498 SNPs to 29 autosomal linkage groups, based on a mean of 430 informative meioses per SNP. The map was constructed by combining the information from linkage with that of the physical position of SNPs within scaffold sequences in an iterative process. Averaged between the sexes; the linkage map had a total length of 2,004 cM, with a longer map for females (2,240 cM) than males (1,801 cM). Additionally, recombination rates also varied along the chromosomes. Comparison of the linkage map to the reference genomes of zebra finch, collared flycatcher and chicken, showed a chromosome fusion of the two avian chromosomes 8 and 4A in house sparrow. Lastly, information from the linkage map was utilized to conduct analysis of linkage disequilibrium (LD) in eight populations with different effective population sizes (Ne) in order to quantify the background level LD. Together, these results aid the design of future association studies, facilitate the development of new genomic tools and support the body of research that describes the evolution of the avian genome.  相似文献   
9.
With the aim of understanding relationship between genetic and phenotypic variations in cultivated tomato, single nucleotide polymorphism (SNP) markers covering the whole genome of cultivated tomato were developed and genome-wide association studies (GWAS) were performed. The whole genomes of six tomato lines were sequenced with the ABI-5500xl SOLiD sequencer. Sequence reads covering ∼13.7× of the genome for each line were obtained, and mapped onto tomato reference genomes (SL2.40) to detect ∼1.5 million SNP candidates. Of the identified SNPs, 1.5% were considered to confer gene functions. In the subsequent Illumina GoldenGate assay for 1536 SNPs, 1293 SNPs were successfully genotyped, and 1248 showed polymorphisms among 663 tomato accessions. The whole-genome linkage disequilibrium (LD) analysis detected highly biased LD decays between euchromatic (58 kb) and heterochromatic regions (13.8 Mb). Subsequent GWAS identified SNPs that were significantly associated with agronomical traits, with SNP loci located near genes that were previously reported as candidates for these traits. This study demonstrates that attractive loci can be identified by performing GWAS with a large number of SNPs obtained from re-sequencing analysis.  相似文献   
10.

Objectives

Ischemic stroke is influenced by both environmental and genetic factors. The CD40/CD40L system is related to proinflammatory and prothrombogenic responses, which are involved in the pathophysiology of ischemic stroke. The aim of this study was to evaluate association between the CD40 -1C/T single nucleotide polymorphism (SNP) and ischemic stroke in a Chinese population.

Methods

We conducted a case–control study including 286 ischemic stroke patients and 336 controls. CD40 -1C/T SNP was genotyped using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and DNA sequencing methods, and evaluated its relevance to ischemic stroke susceptibility.

Results

Significantly increased ischemic stroke risk was found to be associated with the T allele of CD40 -1C/T (OR = 1.273, 95% CI = 1.016–1.594). The frequencies of CT and TT/CT genotypes of CD40 -1C/T in ischemic stroke patients were significantly higher than those of controls, respectively (for CT: OR = 2.350, 95% CI = 1.601–3.449; for TT/CT: OR = 2.148, 95% CI = 1.479–3.119). And, similar results were obtained after adjusting non-matched variables. We found that the frequency of carried T genotypes (TT and TT/CT) was significantly increased in patients with history of stroke compared with patients without (for TT: OR = 6.538, 95%CI = 1.655–25.833; for TT/CT: OR = 3.469, 95%CI = 1.031–11.670), respectively.

Conclusions

The findings suggested that the CD40 -1C/T polymorphism might contribute to the susceptibility to ischemic stroke in the Chinese population, and might be associated with history of previous stroke.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号