全文获取类型
收费全文 | 147篇 |
免费 | 9篇 |
专业分类
156篇 |
出版年
2021年 | 1篇 |
2019年 | 1篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 4篇 |
2015年 | 9篇 |
2014年 | 6篇 |
2013年 | 1篇 |
2012年 | 4篇 |
2011年 | 4篇 |
2010年 | 5篇 |
2009年 | 5篇 |
2008年 | 2篇 |
2007年 | 2篇 |
2006年 | 3篇 |
2005年 | 1篇 |
2004年 | 3篇 |
2003年 | 1篇 |
2002年 | 3篇 |
2001年 | 3篇 |
1999年 | 3篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 6篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1990年 | 5篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有156条查询结果,搜索用时 15 毫秒
131.
132.
133.
134.
The purpose of this paper is to assess the extent of gene identity and
differentiation at 33 dinucleotide repeat loci (377 total alleles) within
and among three European and three Native American populations. In order to
do this, we show that a maximum-likelihood method proposed for phylogenetic
trees (Cavalli-Sforza and Piazza 1975) can be used to estimate gene
identity (Nei 1987) with respect to any hierarchical structure. This method
allows gene differentiation to be evaluated with respect to any internal
node of a hierarchy. It also allows a generalization of F- and G-statistics
to situations with unequal expected levels of differentiation. Our
principal finding is that levels of genetic differentiation are unique to
specific populations and levels of nesting. The populations of European
origin show very little internal differentiation; moreover, their
continental average is close to the total population defined by the
aggregate of Europeans and Native Americans. By contrast, the Native
American populations show moderate levels of internal differentiation, and
a great distance between their continental average and the total. The
results of analyses of subsets of loci that were selected to have high gene
diversities in either Europeans or Native Americans closely parallel those
from the total set of loci. This suggests that the principal results are
unlikely to be caused by a European ascertainment bias in locus selection.
In summary, our findings demonstrate that partitions of gene diversity into
within- and between-populations components are heavily biased by the
populations analyzed and the models fitted. Optimistically, however, more
information is available to analyze population history and evolution by
quantifying, as we have done, the uniqueness of patterns of
differentiation.
相似文献
135.
Lopez JV; Culver M; Stephens JC; Johnson WE; O'Brien SJ 《Molecular biology and evolution》1997,14(3):277-286
Differential rates of nucleotide substitution among different gene segments
and between distinct evolutionary lineages is well documented among
mitochondrial genes and is likely a consequence of locus-specific selective
constraints that delimit mutational divergence over evolutionary time. We
compared sequence variation of 18 homologous loci (15 coding genes and 3
parts of the control region) among 10 mammalian mitochondrial DNA genomes
which allowed us to describe different mitochondrial evolutionary patterns
and to produce an estimation of the relative order of gene divergence. The
relative rates of divergence of mitochondrial DNA genes in the family
Felidae were estimated by comparing their divergence from homologous
counterpart genes included in nuclear mitochondrial DNA (Numt, pronounced
"new might"), a genomic fossil that represents an ancient transfer of 7.9
kb of mitochondrial DNA to the nuclear genome of an ancestral species of
the domestic cat (Felis catus). Phylogenetic analyses of mitochondrial
(mtDNA) sequences with multiple outgroup species were conducted to date the
ancestral node common to the Numt and the cytoplasmic (Cymt) mtDNA genes
and to calibrate the rate of sequence divergence of mitochondrial genes
relative to nuclear homologous counterparts. By setting the fastest
substitution rate as strictly mutational, an empirical "selective
retardation index" is computed to quantify the sum of all constraints,
selective and otherwise, that limit sequence divergence of mitochondrial
gene sequences over time.
相似文献
136.
Camarodont sea urchins possess a rapidly evolving actin gene family whose
members are expressed in distinct cell lineages in a developmentally
regulated fashion. Evolutionary changes in the actin gene family of
echinoids include alterations in number of family members, site of
expression, and gene linkage, and a dichotomy between rapidly and slowly
evolving isoform-specific 3' untranslated regions. We present sequence
comparisons and an analysis of the actin gene family in two congeneric sea
urchins that develop in radically different modes, Heliocidaris
erythrogramma and H. tuberculata. The sequences of several actin genes from
the related species Lytechinus variegatus are also presented. We compare
the features of the Heliocidaris and Lytechinus actin genes to those of the
the actin gene families of other closely related sea urchins and discuss
the nature of the evolutionary changes among sea urchin actins and their
relationship to developmental mode.
相似文献
137.
138.
Bacterioplankton Dynamics in the McMurdo Dry Valley Lakes, Antarctica: Production and Biomass Loss over Four Seasons 总被引:4,自引:0,他引:4
Abstract Research of the microbial ecology of McMurdo Dry Valley lakes has concentrated primarily on phototrophs; relatively little is known about the heterotrophic bacterioplankton. Bacteria represent a substantial proportion of water column biomass in these lakes, comprising 30 to 60% of total microplankton biomass. Bacterial production and cell numbers were measured 3 to 5 times, within four Antarctic seasons (October to January), in Lakes Fryxell, Hoare, and Bonney. The winter-spring transition (September to October) was included during one year. Lake Fryxell was the most productive, but variable, lake, followed by Lakes Bonney and Hoare. Bacterial production ranged from 0 to 0.009 μg C ml-1 d-1; bacterial populations ranged from 3.2 x 10(4) to 4.4 x 10(7) cells ml-1. Bacterial production was always greatest just below the ice cover at the beginning of the season. A second maximum developed just above the chemocline of all the lakes, as the season progressed. Total bacterioplankton biomass in the lakes decreased as much as 88% between successive sampling dates in the summer, as evidenced by areal integration of bacterial populations; the largest decreases in biomass typically occurred in mid-December. A forward difference model of bacterial loss in the trophogenic zone and the entire water column of these lakes showed that loss rates in the summer reached 6.3 x 10(14) cells m-2 d-1 and 4.16 x 10(12) cells m-2 d-1, respectively. These results imply that bacteria may be a source of carbon to higher trophic levels in these lakes, through grazing. 相似文献
139.
PCR-based assays of mendelian polymorphisms from anonymous single-copy nuclear DNA: techniques and applications for population genetics 总被引:5,自引:0,他引:5
This paper outlines a PCR-based approach for population genetics that
offers several advantages over conventional Southern blotting methods for
revealing restriction-fragment-length polymorphisms (RFLPs) in nuclear DNA.
Primers are constructed from clones isolated from a nuclear DNA library,
and these primers subsequently are employed in in vitro syntheses of
homologous regions. Amplified products are then screened directly for RFLPs
by using gel-staining procedures. Population applications for this
PCR-based approach, including potential strengths and weaknesses, are
exemplified by two RFLP data sets generated to estimate (a) male-mediated
gene flow in the green turtle (Chelonia mydas) and (b) geographic
population genetic structure in the American oyster (Crassostrea
virginica). Restriction assays of amplified products from 14 or 15
independent primer pairs in each species revealed polymorphisms at several
loci that proved highly informative in the population genetic analyses. In
general, the Mendelian polymorphisms produced by this PCR-based approach
will provide useful genetic markers for population studies, particularly in
situations where simpler and less expensive allozyme methods have failed,
for whatever reason, to provide adequate information.
相似文献
140.