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排序方式: 共有1027条查询结果,搜索用时 140 毫秒
1.
The emerging limbs and twigs of the East Asian mtDNA tree   总被引:33,自引:0,他引:33  
We determine the phylogenetic backbone of the East Asian mtDNA tree by using published complete mtDNA sequences and assessing both coding and control region variation in 69 Han individuals from southern China. This approach assists in the interpretation of published mtDNA data on East Asians based on either control region sequencing or restriction fragment length polymorphism (RFLP) typing. Our results confirm that the East Asian mtDNA pool is locally region-specific and completely covered by the two superhaplogroups M and N. The phylogenetic partitioning based on complete mtDNA sequences corroborates existing RFLP-based classification of Asian mtDNA types and supports the distinction between northern and southern populations. We describe new haplogroups M7, M8, M9, N9, and R9 and demonstrate by way of example that hierarchically subdividing the major branches of the mtDNA tree aids in recognizing the settlement processes of any particular region in appropriate time scale. This is illustrated by the characteristically southern distribution of haplogroup M7 in East Asia, whereas its daughter-groups, M7a and M7b2, specific for Japanese and Korean populations, testify to a presumably (pre-)Jomon contribution to the modern mtDNA pool of Japan.  相似文献
2.
Kim Y  Stephan W 《Genetics》2002,160(2):765-777
The theory of genetic hitchhiking predicts that the level of genetic variation is greatly reduced at the site of strong directional selection and increases as the recombinational distance from the site of selection increases. This characteristic pattern can be used to detect recent directional selection on the basis of DNA polymorphism data. However, the large variance of nucleotide diversity in samples of moderate size imposes difficulties in detecting such patterns. We investigated the patterns of genetic variation along a recombining chromosome by constructing ancestral recombination graphs that are modified to incorporate the effect of genetic hitchhiking. A statistical method is proposed to test the significance of a local reduction of variation and a skew of the frequency spectrum caused by a hitchhiking event. This method also allows us to estimate the strength and the location of directional selection from DNA sequence data.  相似文献
3.
Deep common ancestry of indian and western-Eurasian mitochondrial DNA lineages   总被引:22,自引:0,他引:22  
About a fifth of the human gene pool belongs largely either to Indo-European or Dravidic speaking people inhabiting the Indian peninsula. The 'Caucasoid share' in their gene pool is thought to be related predominantly to the Indo-European speakers. A commonly held hypothesis, albeit not the only one, suggests a massive Indo-Aryan invasion to India some 4,000 years ago [1]. Recent limited analysis of maternally inherited mitochondrial DNA (mtDNA) of Indian populations has been interpreted as supporting this concept [2] [3]. Here, this interpretation is questioned. We found an extensive deep late Pleistocene genetic link between contemporary Europeans and Indians, provided by the mtDNA haplogroup U, which encompasses roughly a fifth of mtDNA lineages of both populations. Our estimate for this split is close to the suggested time for the peopling of Asia and the first expansion of anatomically modern humans in Eurasia [4] [5] [6] [7] [8] and likely pre-dates their spread to Europe. Only a small fraction of the 'Caucasoid-specific' mtDNA lineages found in Indian populations can be ascribed to a relatively recent admixture.  相似文献
4.
Microsatellites: simple sequences with complex evolution   总被引:15,自引:0,他引:15  
5.
To investigate which aspects of contemporary human Y-chromosome variation in Europe are characteristic of primary colonization, late-glacial expansions from refuge areas, Neolithic dispersals, or more recent events of gene flow, we have analyzed, in detail, haplogroup I (Hg I), the only major clade of the Y phylogeny that is widespread over Europe but virtually absent elsewhere. The analysis of 1,104 Hg I Y chromosomes, which were identified in the survey of 7,574 males from 60 population samples, revealed several subclades with distinct geographic distributions. Subclade I1a accounts for most of Hg I in Scandinavia, with a rapidly decreasing frequency toward both the East European Plain and the Atlantic fringe, but microsatellite diversity reveals that France could be the source region of the early spread of both I1a and the less common I1c. Also, I1b*, which extends from the eastern Adriatic to eastern Europe and declines noticeably toward the southern Balkans and abruptly toward the periphery of northern Italy, probably diffused after the Last Glacial Maximum from a homeland in eastern Europe or the Balkans. In contrast, I1b2 most likely arose in southern France/Iberia. Similarly to the other subclades, it underwent a postglacial expansion and marked the human colonization of Sardinia ~9,000 years ago.  相似文献
6.
Dosage compensation: do birds do it as well?   总被引:11,自引:0,他引:11  
7.
To date, few attempts have been made to benchmark the alignment algorithms upon nucleic acid sequences. Frequently, sophisticated PAM or BLOSUM like models are used to align proteins, yet equivalents are not considered for nucleic acids; instead, rather ad hoc models are generally favoured. Here, we systematically test the performance of existing alignment algorithms on structural RNAs. This work was aimed at achieving the following goals: (i) to determine conditions where it is appropriate to apply common sequence alignment methods to the structural RNA alignment problem. This indicates where and when researchers should consider augmenting the alignment process with auxiliary information, such as secondary structure and (ii) to determine which sequence alignment algorithms perform well under the broadest range of conditions. We find that sequence alignment alone, using the current algorithms, is generally inappropriate <50–60% sequence identity. Second, we note that the probabilistic method ProAlign and the aging Clustal algorithms generally outperform other sequence-based algorithms, under the broadest range of applications.  相似文献
8.
Hens, cocks and avian sex determination. A quest for genes on Z or W?   总被引:11,自引:0,他引:11       下载免费PDF全文
Ellegren H 《EMBO reports》2001,2(3):192-196
The sex of an individual is generally determined genetically by genes on one of the two sex chromosomes. In mammals, for instance, the presence of the male-specific Y chromosome confers maleness, whereas in Drosophila melanogaster and Caenorhabditis elegans it is the number of X chromosomes that matters. For birds (males ZZ, females ZW), however, the situation remains unclear. The recent discovery that the Z-linked DMRT1 gene, which is conserved across phyla as a gene involved in sexual differentiation, is expressed early in male development suggests that it might be the number of Z chromosomes that regulate sex in birds. On the other hand, the recent identification of the first protein unique to female birds, encoded by the W-linked PKCIW gene, and the observation that it is expressed early in female gonads, suggests that the W chromosome plays a role in avian sexual differentiation. Clearly defining the roles of the DMRT1 and PKC1W genes in gonadal development, and ultimately determining whether avian sex is dependent on Z or W, will require transgenic experiments.  相似文献
9.
Sex allocation theory for simultaneous hermaphrodites predicts an influence of the mating group size on sex allocation. Mating group size may depend on the size of the group in which an individual lives, or on the density, but studies to date have not distinguished between the two factors. We performed an experiment in which we raised a transparent simultaneous hermaphrodite, the flatworm Macrostomum sp., in different group sizes (pairs, triplets, quartets and octets) and in different enclosure sizes (small and large). This design allows us to differentiate between the effects of group size and density. After worms reached maturity we determined their reproductive allocation patterns from microscopic images taken in vivo. The results suggest that the mating group size is a function of the group size, and not of the density. They support the shift to higher male allocation in larger mating groups predicted by sex allocation theory. To our knowledge, this is the first study that unambiguously shows phenotypically plastic sex allocation in response to mating group size in a simultaneous hermaphrodite.  相似文献
10.
Compositional evolution of noncoding DNA in the human and chimpanzee genomes   总被引:11,自引:0,他引:11  
We have examined the compositional evolution of noncoding DNA in the primate genome by comparison of lineage-specific substitutions observed in 1.8 Mb of genomic alignments of human, chimpanzee, and baboon with 6542 human single-nucleotide polymorphisms (SNPs) rooted using chimpanzee sequence. The pattern of compositional evolution, measured in terms of the numbers of GC-->AT and AT-->GC changes, differs significantly between fixed and polymorphic sites, and indicates that there is a bias toward fixation of AT-->GC mutations, which could result from weak directional selection or biased gene conversion in favor of high GC content. Comparison of the frequency distributions of a subset of the SNPs revealed no significant difference between GC-->AT and AT-->GC polymorphisms, although AT-->GC polymorphisms in regions of high GC segregate at slightly higher frequencies on average than GC-->AT polymorphisms, which is consistent with a fixation bias favoring high GC in these regions. However, the substitution data suggest that this fixation bias is relatively weak, because the compositional structure of the human and chimpanzee genomes is becoming homogenized, with regions of high GC decreasing in GC content and regions of low GC increasing in GC content. The rate and pattern of nucleotide substitution in 333 Alu repeats within the human-chimpanzee-baboon alignments are not significantly affected by the GC content of the region in which they are inserted, providing further evidence that, since the time of the human-chimpanzee ancestor, there has been little or no regional variation in mutation bias.  相似文献
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