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   297篇
  免费   7篇
  国内免费   5篇
  2023年   1篇
  2022年   2篇
  2021年   2篇
  2020年   1篇
  2019年   1篇
  2018年   2篇
  2017年   2篇
  2016年   2篇
  2015年   6篇
  2014年   4篇
  2013年   1篇
  2012年   10篇
  2011年   10篇
  2010年   6篇
  2009年   18篇
  2008年   23篇
  2007年   21篇
  2006年   11篇
  2005年   11篇
  2004年   12篇
  2003年   9篇
  2002年   3篇
  2001年   4篇
  2000年   5篇
  1999年   13篇
  1998年   5篇
  1997年   3篇
  1996年   12篇
  1995年   9篇
  1994年   17篇
  1993年   10篇
  1992年   4篇
  1991年   5篇
  1990年   12篇
  1989年   11篇
  1988年   8篇
  1987年   6篇
  1986年   2篇
  1985年   6篇
  1984年   4篇
  1983年   1篇
  1982年   6篇
  1980年   4篇
  1979年   1篇
  1978年   2篇
  1977年   1篇
排序方式: 共有309条查询结果,搜索用时 21 毫秒
1.
Summary Fertility parameters were estimated inVaccinium myrtillus andV. vitis-idaea after self- and cross-pollinations performed in growth chamber. We showed a drastic decrease in fertility after self-pollination as compared to cross-pollination. Number of plump seeds per berry was compared with previous field data. In both species, growth room conditions improved plump seed number after cross-pollination but not after self-pollination. In addition, in order to enhance resources supply to young developing fruits, cytokinin application was tested inV. myrtillus but no effect was detected. We hypothesize that the partial self-sterility is due to inbreeding depression based on the expression of recessive lethals.  相似文献   
2.
Inbreeding mating systems are uncommon because of inbreeding depression. Mating among close relatives can evolve, however, when outcrossing is constrained. Social spiders show obligatory mating among siblings. In combination with a female‐biased sex ratio, sib‐mating results in small effective populations. In such a system, high genetic homozygosity is expected, and drift may cause population divergence. We tested the effect of outcrossing in the social spider Stegodyphus dumicola. Females were mated to sib‐males, to a non‐nestmate within the population, or to a male from a distant population, and fitness traits of F1s were compared. We found reduced hatching success of broods from between‐population crosses, suggesting the presence of population divergence at a large geographical scale that may result in population incompatibility. However, a lack of a difference in offspring performance between inbred and outbred crosses indicates little genetic variation between populations, and could suggest recent colonization by a common ancestor. This is consistent with population dynamics of frequent colonizations by single sib‐mated females of common origin, and extinctions of populations after few generations. Although drift or single mutations can lead to population divergence at a relatively short time scale, it is possible that dynamic population processes homogenize these effects at longer time scales.  相似文献   
3.
A potential bias in estimation of inbreeding depression when using pedigree relationships to assess the degree of homozygosity for loci under selection is indicated. A comparison of inbreeding coefficients based on either pedigree or genotypic frequencies indicated that, as a result of selection, the inbreeding coefficient based on pedigree might not correspond with the random drift of allelic frequencies. Apparent differences in average levels of both inbreeding coefficients were obtained depending on the genetic model (additive versus dominance, initial allelic frequencies, heritability) and the selection system assumed (no versus mass selection). In the absence of selection, allelic frequencies within a small population change over generations due to random drift, and the pedigree-based inbreeding coefficient gives a proper assessment of the accompanying probability of increased homozygosity within a replicate by indicating the variance of allelic frequencies over replicates. With selection, in addition to random drift, directional change in allelic frequencies is not accounted for by the pedigree-based inbreeding coefficient. This result implies that estimation of inbreeding depression for traits under either direct or indirect selection, estimated by a regression of performance on pedigree-based coefficients, should be carefully interpreted.Deceased  相似文献   
4.
 Sugar beet hybrids are produced by crossing a cytoplasmic male-sterile (CMS) line with a pollinator. New CMS lines are produced by crossing a fertile plant to an existing CMS line. The fertile plant is also selfed. In the following generation, one of the selfed, fertile progeny is paired and isolated with one of the crossed, CMS progeny, to give a second generation of selfing and crossing. Over a series of such crosses and selfs, a new fertile inbred line and its corresponding, near-isogenic CMS partner are produced. Selection among lines takes place at one or more stages of the backcrossing programme. A method is presented here for calculating the genetic variances and covariances within and between lines and generations based on a derivation of additive genetic relationships modified from an approach widely employed in animal breeding. The genetic variances and covariances are used to predict response to selection from varying strategies, from which optimum schemes can be determined. Results suggest that selection should generally take place after three generations of backcrossing when the fertile plant used to initiate the backcrossing process is not inbred, but can take place after generation two when the fertile plant is inbred. Doubled haploid production is unlikely to provide an extra advantage that would be worthwhile in such a system. The method developed here can be used to explore a wide range of more complex breeding systems. Received: 27 July 1998 / Accepted: 19 October 1998  相似文献   
5.
Reintroduction programs aim at reinstalling a self-sustained population into the wild via a period of supplementation with captive-bred individuals. This procedure can rapidly generate inbreeding among offspring because of the mating scheme and this inbreeding might be further enhanced by the reintroduction scenario. First, we used simulations to assess the consequences of breeding designs on mean inbreeding index F among offspring when the genetic diversity of breeders, the number and sex ratios of breeders, and the proportion of successful crosses vary. A high number of breeders, a balanced sex ratio, a high proportion of effective crosses and a genetically diverse source population generally contribute to lower F values. However, moderately high (≥20) numbers of breeders combined with all but the most biased sex ratios produced mean F values near minimal values. The variability in F was negligible in all parameter combinations except for a very small number of breeders (5) and very biased sex ratios (≤ 1M : 19F). We also simulated the long-term inbreeding dynamics in the introduced population under various demographic scenarios. Our main finding was that the annual number of introduced offspring is a decisive factor in establishing long-term F values in the supplemented population. Low supplementation levels (102) quickly generated an almost completely inbred population whereas high levels (≥104) produced stable F values close to that of the introduced offspring. Simulations were run based on the life history and specific demographics of the bloater (Coregonus hoyi), whose reintroduction in Lake Ontario is being considered.  相似文献   
6.
Summary Genetic variability in the non-compound portion of the genomes of compound-chromosome (CC) strains intended for genetic control can be increased by the use of bridging strains which can be crossed to both CC and normal strains. Two bridging systems are described for chromosome-5 CC strains of Lucilia cuprina. The first system relies on the established viability and fertility of males trisomic for chromosome 5R. Males carrying the (5L.YL)23 half-translocation, a C(5R), and a normal chromosome 5 were crossed successfully to a CC strain and a normal strain. The second system uses a pair of reciprocal whole-arm 4;5 translocations to generate gametes disomic for 5R and nullosomic for 5L, which in combination with C(5L)-bearing gametes form viable near-euploid offspring with only small duplications and deficiencies. These offspring (C(5L); (4L.5R)357; (4R.5R)194; (4L.4R)) were crossed successfully with both CC and T(4;5)357/ + individuals. The latter were in turn crossed successfully with normal strains. The T(Y;5)23 system allows replacement of the non-CC genome with wild material more rapidly than the T(4;5)357/T(4;5)194 system, but unlike the latter does not allow replacement of the Y chromosome in the CC strain. The double translocation system is currently being used in L. cuprina.  相似文献   
7.
This paper describes the incidence of multiple births in the Yerkes Chimpanzee colony for a period of about 63 years. Pedigrees are presented for all multiple births. They demonstrate the recurrence of twins in relatively few family lines which can be traced back to a small number of ancestors. The findings suggest that twinning is a family trait in chimpanzees and they support the hypothesis that the incidence of multiple births is genetically influenced in this colony. In addition, a relatively high level of inbreeding may have enhanced the incidence of multiple births in these family lines. Similar findings are known from human populations.  相似文献   
8.
The adaptive significance of inversion polymorphism of Drosophila subobscura is well established. However, little is known about gene combinations within inversions which are coadapted because of population-specific effects. We studied this aspect of Dobzhansky's coadaptation hypothesis, using the systematic inbreeding method. Differences in magnitude and quality of inversion polymorphism in two ecologically and topologically distinct habitats were compared after several generations of continuous full-sib inbreeding. Populations from the two habitats differ in frequency of homokaryotypes after third and fifth generation of inbreeding and in the levels of homozygosity of different gene arrangements. The effect of homozygosity appears population and chromosome specific.  相似文献   
9.
In previous studies, we have investigated the strength of self-incompatibility (SI) in Solanum carolinense, a highly successful weed with a fully functional SI system that inhabits early successional and other disturbed habitats. We have found that the SI response in S. carolinense is a plastic trait—its strength being affected by the age of the flowers, and the presence of developing fruits and that there are genetic differences among families in their self-fertility. However, in species with a fully functional SI response, selfing would not be that common. As a result, deleterious recessives scattered though the genome of horsenettle are only occasionally exposed to selection. It has been suggested that deleterious recessives accumulate near S-alleles in strong SI species because the S-locus is located in a non-recombining region of the genome and because strong S-alleles are never in the homozygous state, thus sheltering some of the genetic load near the S-locus from selection. We performed a series of laboratory and greenhouse experiments to determine the extent to which sheltered load adds to the overall magnitude of inbreeding depression in horsenettle. Specifically, we amplified and sequenced the S-alleles from 16 genets collected from a large population in Pennsylvania and performed a series of controlled self-pollinations. We then grew the selfed progeny in the greenhouse; recorded various measures of growth and reproductive output; and amplified and sequenced their S-allele(s). We found that the heterozygous progeny of self-pollinations produce more flowers and have a greater ability to set both self and cross seed than S-homozygous progeny. We also found evidence of variation in the magnitude of load among S-alleles. These results suggest that sheltered load might slow the fixation of weak (partially compatible) S-alleles in this population, thus adding to the maintenance of a mixed mating system rather than leading to the fixation of the selfing alleles.  相似文献   
10.
1983年4月至1984年5月,对新疆伊犁哈萨克自治州五个民族的近亲结婚情况进行了调查。调查的婚姻数为维吾尔2553起,哈萨克1079起,回1235起,锡伯1222起,蒙古446起。近亲结婚率与平均近交系数分别为:维吾尔——8.23%与 46.74×10~(-4),回——8.10%与45.07×10~(-4),锡伯——4.66%与24.93×10~(-4),哈萨克——2.87%与11.31×10~(-4)。蒙古——0.45%与 2.80×10~(-4)。解放以来,维吾尔族与回族的近亲结婚率有上升趋势,而锡伯族的则有下降趋势。各族近亲结婚中大部分为亲表亲结婚,但堂亲结婚在维吾尔族的近亲结婚中竟占26.18%,在回族中也占16.00%。1303名近亲结婚子女中的七岁前死亡率(12.89%)及先天性缺陷与遗传性疾病发生率(2.99%)显著高于6370名非近亲结婚子女的相应值(7.65%及0.38%)。  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