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1.
Flocks participating in sire referencing schemes can achieve greater genetic gains than those achievable by within-flock selection. However, requirements for joining these schemes can be prohibitive to some producers. The objectives of this study were to determine whether less restrictive schemes or schemes of shorter duration could achieve rates of gain and reduce inbreeding as efficiently as continuous sire referencing schemes (SRS) and to investigate whether bias from different genetic means could be reduced by these alternative schemes. Pedigree and performance data for a single trait with a within-flock heritability of 0.25 were simulated (50 replications) for 15 flocks with 40 to 140 ewes per flock. Founder genetic means for each flock were sampled from a normal distribution with mean 0 and SD equal to the trait's genetic SD. After 10 yr of random mating, flocks had the opportunity to join an SRS and begin selection for the simulated trait. Yearling rams were chosen as reference sires randomly from the top one-sixth of the population ranked on BLUP EBV. Every year, in each flock, 3 reference sires were mated to 10 ewes. Six sire referencing scenarios were considered, in which all flocks participated in a SRS for 1) 15 yr; 2) 5 yr before discontinuing the scheme; 3) 10 yr before discontinuing the scheme; 4) 2 out of every 3 yr; 5) 15 yr with reference sire mating by natural service; and 6) no years (no use of SRS). Ewes not mated to reference sires were mated either to their own home-bred sires exclusively or to a mixture of homebred and unrelated purchased rams of unknown merit. Genetic gain was equivalent whether the SRS used AI or natural service matings, although inbreeding was lower with natural service. Across all scenarios, genetic gain and inbreeding were greater when excess ewes were mated exclusively to homebred sires. Genetic gains without SRS were 80 to 82% lower than when the scheme operated for 15 yr, whereas inbreeding was considerably greater. Other scenarios were intermediate in both gain and inbreeding levels. In all SRS scenarios, bias in EBV attributable to differing flock genetic means rapidly decreased in the first 5 yr of sire referencing. Levels of bias did not substantially increase when flocks discontinued SRS after 5 or 10 yr, suggesting that further participation in an SRS may not be necessary to manage risk. Natural service and noncontinuous SRS are viable options to continuous AI SRS in terms of genetic gain, inbreeding, and bias reduction. 相似文献
2.
基因组选配（genomic mating,GM）是利用基因组信息进行优化的选种选配,可以有效控制群体近交水平的同时实现最大化的遗传进展。但基因组选配是对群体中所有个体进行选配,这与实际的育种工作有点相悖。本研究模拟了遗传力为0.5的9 000头个体的基础群数据,每个世代根据GEBV选择30头公畜、900头母畜作为种用个体,而后使用基因组选配、同质选配、异质选配、随机交配4种不同的选配方案。其中基因组选配中分别选取遗传进展最大的解、家系间方差最大的解、近交最小的解所对应的交配方案进行选育。每种方案选育5个世代,比较其后代群体的平均GEBV、每世代的遗传进展、近交系数、遗传方差,并重复5次取平均值。结果表明,3种基因组选配方案的ΔG均显著高于随机交配和异质选配（P<0.01）,而且,选取遗传进展最大的基因组选配方案的ΔG比同质选配还高出4.3%。3种基因组选配的方案的ΔF比同质选配低22.2%~94.1%,而且选取近交最小的基因组选配方案ΔF比异质选配低11.8%。同质选配的遗传方差迅速降低,在第5世代显著低于除基因组选配中选择遗传进展最大的方案以外的所有方案（P<0.05）,3种基因组选配方案的遗传方差比同质选配高10.8%~32.2%。这表明基因组选配不仅可以获得比同质选配更高的遗传进展,同时有效的降低了近交水平,并且减缓了遗传方差降低速度,保证了一定的遗传变异。基因组选配作为一种有效的可持续育种方法,在畜禽育种中开展十分有必要。 相似文献
3.
T. Nomura 《Zeitschrift für Tierzüchtung und Züchtungsbiologie》1999,116(5):351-361
Increased rate of inbreeding in selection programmes may have an important effect on mid- and long-term selection response and reproductive performance through reduction in genetic variance and inbreeding depression. Selection on an inherited trait inflates the rate of inbreeding and reduces the effective population size (R obertson 1961; S antiago and C aballero 1995). This can be particularly important in selection based on index with information from relatives (L ush 1947) or best liner unbiased prediction (BLUP) with an animal model (H enderson 1984). In recent years, various methods have been proposed to reduce the rates of inbreeding in selection programmes while keeping genetic gains at the same level. These methods assume various selection and mating strategies. G rundy et al. (1994) showed that the use of biased heritability estimates for BLUP evaluation is one of the simplest and most efficient methods. A direct reduction in the weight on family mean in index selection (T oro and P erez -E nciso 1990), selection for weighted ancestral Mendelian sampling estimates (W oolliams and T hompson 1994; G rundy et al. 1998) and limited use of selected parents (T oro and N ieto 1984; W ei 1995) have also been shown to be efficient methods. Other methods include nonrandom matings of selected parents, such as factorial mating designs (W oolliams 1989), minimum coancestry mating (T oro et al. 1988) and compensatory mating (S antiago and C aballero 1995). Simultaneous optimization of the selection of candidates and their mating allocations has been also considered through mate selection with linear programming techniques (T oro and P erez -E nciso 1990). Among these methods, compensatory mating is a very simple and efficient method (G rundy et al. 1994; S antiago and C aballero 1995; C aballero et al. 1996). This mating system was derived from the theoretical consideration on effective population size under selection (S antiago and C aballero 1995). Although S antiago and C aballero (1995) considered that implementation of this mating could counteract the cumulative effect of selection on the effective population size, the theoretical basis has been little studied. In this paper, the author gives the theoretical basis of compensatory mating. A modification to enhance the effect of compensatory mating is also proposed and the efficiency is examined by stochastic simulation. 相似文献
4.
Inbreeding depression estimates obtained by regression of the individual performance on the inbreeding were studied by stochastic simulation under various genetic models (solely additive, partial dominance, overdominance and epistasis), and mating strategies (random mating versus selection). In all models, inbreeding depression estimates based on the individual pedigree inbreeding coefficients were compared with estimates based on the true level of autozygosity. For the model with partial dominance and selection, the estimates of inbreeding depression from pedigree information were more negative (lower) than those based on true inbreeding coefficients whereas, in contrast, they were less negative (higher) for the models with overdominance and selection. The difference in the variation of true and pedigree individual inbreeding coefficient indicated that biased estimates might occur even in random mating populations. The estimation of inbreeding depression was further complicated when epistatic effects were present. The sign and the magnitude of the inbreeding effect (depression) estimates might be rather heterogeneous if additive by dominance effects are present because they are strongly dependent on the gene frequency. It was also shown that inbreeding depression is possible in models with negative additive by dominance effects. In models with dominance by dominance inheritance it was difficult to assess the non-linear relationship between performance and inbreeding, while at the same time, non-linear estimates based on pedigree information were extremely biased. The results obtained indicate that new or additional methodologies are required if reliable conclusions about consequences of inbreeding depression are needed. 相似文献
5.
P. Korpiaho I. Strandén E. A. Mäntysaari 《Acta Agriculturae Scandinavica, Section A - Animal Sciences》2013,63(2):74-82
Different bull selection strategies and female reproductive rates were simulated under various selection intensities and mating strategies in a nucleus with testing capacity of 80 cows. In all schemes, donor cows were selected after first lactation and bulls were progeny tested. In some schemes, non-progeny tested nucleus bulls could be selected. The nucleus was open on the male side but closed on the female side after nucleus born cows were available. Pedigrees of animals born outside the nucleus were sampled from the Finnish Ayrshire population. Female reproduction rate was either constant or variable. Schemes were compared according to genetic response, and risks measured by rate of inbreeding and coefficient of variation in genetic response between simulation years 5 and 20. Inclusion of one- or three-year-old bulls increased genetic response by 9% and rate of inbreeding up to 81%. Variation in female reproductive rate reduced genetic response by at most 8%. Increase in risks was averted by limiting the number of female full-sibs. Schemes with low selection intensity and factorial mating provided the best results. 相似文献
6.
Despite challenges in the implementation of livestock genetic improvement programs in developing regions, including centralized nucleus breeding schemes, these programs can contribute to the improvement of the livelihood of smallholder farmers. In this paper, we present a community- or village-based breeding scheme in which breeding activities are carried out by communities of smallholder farmers. We evaluated genetic responses and the rate of inbreeding from alternative village sheep breeding schemes that were based on a survey of existing flock structure and breeding management in a sheep-barley system in Ethiopia. This survey showed that individual flock sizes were small, and that the majority of farmers practiced mixed grazing and uncontrolled mating of their flocks in communal grazing lands within villages. Here we evaluated within-village schemes (selection across flocks within a village) and across-village schemes (selection across villages) at different intensities of ram selection (i.e. proportions of rams selected, P). Our results showed that under within-village schemes, intensity of selection could not be increased (i.e. P could not be decreased below 0.149) when the rate of inbreeding was constrained to an acceptable level of 0.01, resulting in low genetic gain. The most optimal scheme was found to be across-village selection with at least three villages cooperating and P = 0.05. Our results also indicated that genetic gain from village breeding schemes with mass selection and BLUP selection is comparable. Village breeding schemes can make a significant contribution to the genetic improvement of livestock in Ethiopia and other developing countries. 相似文献
7.
Atsushi NAKAMURA Kenji TOGASHI Naoyuki YAMAMOTO Akiko NISHIURA 《Animal Science Journal》2002,73(3):175-184
In order to control rates of response and inbreeding, mate selection using fuzzy selective mating criteria (FMC) was investigated in adult multiple ovulation and embryo transfer nucleus schemes for dairy cattle. Stochastic simulation was used to model the closed nucleus scheme. This mate selection was examined in four alternative mating and male selection schemes: (i) a hierarchical scheme; (ii) a hierarchical sibship scheme (two males per sibship); (iii) a factorial scheme (two sires per dam); and (iv) a factorial sibship scheme (two males per sibship and two sires per dam). Genetic response and inbreeding rate tended to be reduced by increasing the trade-off parameter of FMC between the expected breeding value and inbreeding of progeny. Inbreeding rates in all schemes were reduced by reducing the variance of family size through selection and the average coancestry of mating pairs through mate allocation. 相似文献
8.
Selection on the best estimate of the breeding value of individuals should, in large populations, provide the maximal response in breeding value. However, many breeders deal with the selection of small numbers of animals from relatively small populations and therefore there is a trend for inbreeding to rise because of genetic drift. Moreover, as the evaluation of candidates is traditionally based on methodologies including information from relatives [selection indices, best linear unbiased predictor (BLUP)] more individuals are selected from the best families and so closely related individuals will generate most of the offspring. This effect is more important for traits with low heritability as phenotype gives little information on the breeding value of the individuals and more weight is given to relatives’ data. The need for controlling inbreeding refers not only to a better use of the genetic variability available and to a reduced inbreeding depression in the selected trait, but also to a reduced depression of fitness-related traits, which may be the most serious drawback at present due to the increase in inbreeding in domestic populations (M euwissen and W oolliams 1994). In recent years considerable work has been carried out on the design of strategies to maintain genetic diversity in selection programmes. These strategies are aimed at simultaneously optimizing genetic gain and inbreeding, either by reducing the rate of inbreeding (or variance of response) while keeping genetic gains at a predetermined level, or by increasing selection response under a restriction on inbreeding (or on variance of response). Following T oro and P& eacute ; rez -E nciso (1990) the different strategies can be classified according to the factor on which they act: (i) the selection criterion used; (ii) the mating system imposed; (iii) the number of selected individuals and their contribution to the next generation. The first group of strategies proposes the use of a suboptimal selection criterion that reduces the weight given to family information or the use of an upward-biased heritability in BLUP evaluation (T oro and P& eacute ; rez -E nciso 1990; see G rundy et al. 1998a for the latest development of this idea). The second group of strategies proposes action on the mating system including factorial mating designs, minimum co-ancestry mating (using linear programming) or compensatory mating (see review by C aballero et al. 1996). The third group of strategies includes the ones considered in the present work. The first possibility is to modify the contribution of the selected individuals of generation t to the selected individuals of generation t + 1, by practising some form of within-family selection with respect to BLUP values. Two strategies of this type were considered: modified within-family selection (MWFS) and restricted co-ancestry selection (RCS). The second possibility is to modify the contribution of the selected individuals of generation t to the evaluated individuals of generation t + 1 (instead of to the selected individuals) by a strategy called weighted selection (T oro and N ieto 1984). Three strategies were considered in this case: weighted selection (WS), restricted co-ancestry weighted selection (RCWS) and pair weighted selection (PWS). More specifically, the aim of the present paper is to show how these five strategies can be implemented using mathematical programming techniques. A small example comparing all of these strategies with standard truncation selection (TS) is also given for illustration. 相似文献
9.
Effective population size and inbreeding depression on litter size in rabbits. A case study 
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下载免费PDF全文 M. Ragab J.P. Sánchez M. Baselga 《Zeitschrift für Tierzüchtung und Züchtungsbiologie》2015,132(1):68-73
The purpose of this study is to use demographic and litter size data on four Spanish maternal lines of rabbits (A, V, H and LP), as a case study, in order to: (i) estimate the effective population size of the lines, as a measure of the rate of increase of inbreeding, and (ii) study whether the inbreeding effect on litter size traits depends on the pattern of its accumulation over time. The lines are being selected for litter size at weaning and are kept closed at the same selection nucleus under the same selection and management programme. The study considered 47 794 l and a pedigree of 14 622 animals. Some practices in mating and selection management allow an increase of the inbreeding coefficient lower than 0.01 per generation in these lines of around 25 males and 125 females. Their effective population size (Ne) was around 57.3, showing that the effect of selection, increasing the inbreeding, was counterbalanced by the management practices, intended to reduce the rate of inbreeding increase. The inbreeding of each individual was broken down into three components: old, intermediate and new inbreeding. The coefficients of regression of the old, intermediate and new inbreeding on total born (TB), number born alive (NBA) and number weaned (NW) per litter showed a decreasing trend from positive to negative values. Regression coefficients significantly different from zero were those for the old inbreeding on TB (6.79 ± 2.37) and NBA (5.92 ± 2.37). The contrast between the coefficients of regression between the old and new inbreeding were significant for the three litter size traits: 7.57 ± 1.72 for TB; 6.66 ± 1.73 for NBA and 5.13 ± 1.67 for NW. These results have been interpreted as the combined action of purging unfavourable genes and artificial selection favoured by the inbreeding throughout the generations of selection. 相似文献
10.
1. The aim of the study was to evaluate the genetic and economic breeding objectives for an indigenous chicken (IC) breeding programme in Kenya. 2. A closed three-tier nucleus breeding programme with three breeding objectives and two selection schemes was simulated. The breeding objectives included IC dual-purpose (ICD) for both eggs and meat, IC layer (ICL) for eggs and IC broiler (ICB) for meat production. 3. Pure line selection scheme (PLS) for development of IC pure breeds and crossbreeding scheme (CBS) for the production of hybrids were considered. Two-and three-way crossbreeding strategies were evaluated under CBS and the impact of nucleus size on genetic gains and profitability of the breeding programme were investigated. 4. Males were the main contributors to genetic gains. The highest genetic gains for egg number (2·71 eggs) and growth traits (1·74?g average daily gain and 57·96?g live weight at 16 weeks) were realised under PLS in ICL and ICB, respectively. 5. The genetic response for age at first egg was desirable in all the breeding objectives, while that for fertility and hatchability were only favourable under ICL and PLS in ICD. Faecal egg count and immune antibody response had low, but positive gains except under PLS where the later was unfavourable. ICB was the most profitable breeding objective, followed by ICD and ICL under all the selection schemes. 6. Although PLS was superior in genetic gains and profitability and recommended in breeding programmes targeting ICL and ICB, a three line CBS should be considered in development of a dual-purpose breed. 7. Increasing the nucleus size beyond 5% of the IC population was not attractive as it resulted in declining profitability of the breeding programme. 相似文献
11.
S. Ieiri T. Nomura H. Hirooka M. Satoh 《Zeitschrift für Tierzüchtung und Züchtungsbiologie》2004,121(2):90-100
Using Monte Carlo simulation, two schemes of restricted selection were compared under various combinations of genetic parameters and constraints on the genetic gains. The first selection scheme is the combination of best linear unbiased prediction (BLUP) evaluation and linear programming technique (BLUP + LP), and the second one is based on the restricted BLUP selection (RBLUP). Selection for two traits was supposed, in which animals were selected to maximize the genetic gain in trait 2 (Δg2) under a proportional restriction on the genetic gain in trait 1 (Δg1) to satisfy the intended ratio (Δg1:Δg2). Little differences were found between the two selection schemes with respect to the genetic gains averaged over replicates. However, in all the cases studied, the variance of genetic gains among replicates under BLUP + LP selection was smaller and less sensitive to the genetic parameters and the intended restriction than RBLUP selection. Under the situations of antagonistic selection, the difference tended to be larger. When the heritabilities of the two traits were different, RBLUP selection remarkably increased the variance of genetic gain in a trait with a higher heritability. These results suggest that BLUP + LP selection should always be preferable to RBLUP selection because of the smaller risk of selection. This choice is especially important for the situation where the genetic parameters act as limiting factors for the achievement of intended genetic gains. 相似文献
12.
Rates of inbreeding and genetic adaptation for populations managed as herds in zoos with a rotational mating system or with optimized contribution of parents 下载免费PDF全文
下载免费PDF全文 This study compares two genetic management scenarios for species kept in herds, such as deer. The simulations were designed so that their results can be extended to a wide range of zoo populations. In the first scenario, the simulated populations of size 3 × 20, 6 × 40 or 20 × 60 (herds × animals in herd) were managed with a rotational mating (RM) scheme in which 10%, 20% or 50% of males were selected for breeding and moved between herds in a circular fashion. The second scenario was based on optimal contribution theory (OC). OC requires an accurate pedigree to calculate kinship; males were selected and assigned numbers of offspring to minimize kinship in the next generation. RM was efficient in restriction of inbreeding and produced results comparable with OC. However, RM can result in genetic adaptation of the population to the zoo environment, in particular when 20% or less males are selected for rotation and selection of animals is not random. Lowest rates of inbreeding were obtained by combining OC with rotation of males as in the RM scheme. RM is easy to implement in practice and does not require pedigree data. When full pedigree is available, OC management is preferable. 相似文献
13.
不同来源大白猪总产仔数近交衰退评估 总被引：2，自引：2，他引：0
旨在评估两个不同来源大白猪群体经过近8个世代的选育后总产仔数（total number of piglets born，TNB）近交衰退的程度。本研究对1 937头大白猪使用GeneSeek GGP Porcine HD芯片进行分型，其中1 039头来自加系大白猪和898头来自法系大白猪，且两品系均有表型记录和系谱记录，系谱共由3 086头大白猪组成。分别使用系谱、SNP和ROH进行个体近交系数估计，并将近交系数作为协变量利用动物模型对总产仔数进行近交衰退评估。为了精准定位导致总产仔数衰退的基因组片段，又进一步对每条染色体以及显著染色体分段计算近交系数并估计其效应，检测是否能引起总产仔数发生近交衰退现象。对于加系群体，FROH、FGRM和FPED估计的近交系数均值分别为0.124、0.042和0.013，其中FROH和FPED相关最高，相关系数为0.358；对于法系群体，FROH、FGRM和FPED均值分别为0.123、0.052和0.007，其中FROH和FGRM相关最高，相关系数为0.371。利用3种不同计算方法所得近交系数用于估计近交衰退时，加系群体的总产仔数均检测到显著的近交衰退，而且当FROH、FGRM和FPED每增加10%时，总产仔数分别减少0.571、0.341和0.823头；但法系群体仅有FROH估计的总产仔数检测到显著近交衰退，FROH每增加10%时，总产仔数减少0.690头。为了锁定相关的染色体和基因组区段，首先利用ROH估计每条染色体近交系数并进行近交衰退分析发现，加系群体中检测到第6、7、8和13号染色体产生了显著近的总产仔数交衰退，而法系群体未检测到与近交衰退相关的染色体。然后，又将与加系总产仔数近交衰退显著相关的4条染色体平均分为2、4、6、8个片段进行近交衰退检测，其中平均分成8段后的染色片段的长度范围为15.1~25.8 Mb。在第6、7和8号染色体分别检测到1、2和3个与总产仔数相关的近交衰退染色体片段。这些区域注释到了CUL7、MAPK14和PPARD基因与胎盘发育相关，AREG和EREG基因与卵母细胞成熟有关。本研究利用3种近交系数计算方法对两个不同来源的大白猪总产仔数进行近交衰退评估，在加系大白猪中3种估计方法都能检测到近交衰退的现象，而法系群体中只有FROH才能检测到。而且通过ROH方法进一步确定了能引起加系大白猪总产仔数衰退的4条染色体和6个特定的染色体区段，还注释到了与繁殖相关的候选基因。这为揭示近交衰退的遗传机制提供了新的研究手段，也为基因组选种选配提供了参考依据。 相似文献
14.
This article presents a deterministic method to predict rates of inbreeding (deltaF) for typical livestock improvement schemes. The method is based on a recently developed general theory to predict rates of inbreeding, which uses the concept of long-term genetic contributions. A typical livestock breeding population was modeled, with overlapping generations, BLUP selection, and progeny testing of male selection candidates. Two types of selection were practiced: animals were either selected by truncation on estimated breeding values (EBV) across age classes, or the number of parents selected from each age class was set to a fixed value and truncation selection was practiced within age classes. Bulmer's equilibrium genetic parameters were obtained by iterating on a pseudo-BLUP selection index and deltaF was predicted for the equilibrium situation. Predictions were substantially more accurate than predictions from other available methods, which ignore the effect of selection on deltaF. Predictions were accurate for schemes with up to 20 sires. Predicted deltaF was somewhat too low for schemes with more than 20 sires, which was due to the use of simple linear models to predict genetic contributions. The present method provides a computationally feasible (i.e., deterministic) tool to consider both the rate of inbreeding and the rate of genetic gain when optimizing livestock improvement schemes. 相似文献
15.
I. Strandén P. Korpiaho M. Pakula E. A. Mäntysaari 《Acta Agriculturae Scandinavica, Section A - Animal Sciences》2013,63(4):235-245
Centralized multiple ovulation and embryo transfer (MOET) nucleus breeding schemes with limited testing capacity were compared with a conventional artificial insemination (AI) scheme using a simulation. The donor cows were selected after their first lactation. In the hybrid MOET schemes, bulls were progeny tested in the AI population. In comparison, two other schemes used nucleus-born adult bulls only. The best hybrid scheme gave 77% or 40% better genetic response in milk protein yield than that estimated in the national AI scheme when the assumed genetic progress in the AI scheme was low or high, respectively. The best scheme had acceptable coefficients of variation of genetic response, although the rate of inbreeding was 2.5 times higher than in the national AI scheme. The schemes using adult bulls had lower responses and higher rates of inbreeding than the hybrid schemes. The outstanding performance of the hybrid schemes was due to more successful cow selection path than in a conventional AI scheme. A high accuracy of selection of bulls was also important. 相似文献
16.
Stochastic computer simulation was used to investigate the potential extra genetic gains obtained from gene-assisted selection (GAS) by combining 1) optimization of genetic contributions for maximizing gain, while restricting the rate of inbreeding with 2) optimization of the relative emphasis given to the QTL over generations. The genetic model assumed implied a mixed inheritance model in which a single quantitative trait locus (i.e., QTL) is segregating together with polygenes. When compared with standard GAS (i.e., fixed contributions and equal emphasis on the QTL and polygenic EBV), combined optimization of contributions of selection candidates and weights on the QTL across generations allowed substantial increases in gain at a fixed rate of inbreeding and avoided the conflict between short- and long-term responses in GAS schemes. Most of the increase of gain was produced by optimization of selection candidates' contributions. Optimization of the relative emphasis given to the QTL over generations had, however, a greater effect on avoiding the long-term loss usually observed in GAS schemes. Optimized contribution schemes led to lower gametic phase disequilibrium between the QTL and polygenes and to higher selection intensities both on the QTL and polygenes than with standard truncation selection with fixed contributions of selection candidates. 相似文献
17.
INO CURIK JOHANN SÖLKNER & NIKOLA STIPIC 《Zeitschrift für Tierzüchtung und Züchtungsbiologie》2002,119(2):101-115
Considering the effects of selection, dominance, epistasis and linkage, a stochastic computer simulation was performed to study how well inbreeding coefficients calculated from pedigree (fped ) and genotypic frequencies (fhet ) correspond to the inbreeding coefficient that is defined as the proportion of autozygous loci in the modelled genome (i.e. the level of autozygosity, faut ). Although in random mating populations all three inbreeding coefficients show almost (with slight deviations in models with two loci) the same expectation, they represent three separate variables. First, faut , fped and fhet responded differently to selection, dominance, epistasis and linkage. Second, they did not have the same standard deviations, which means that the effects of random drift, especially in models under selection, were not affecting all three coefficients in the same way. Finally, they were not always defined in the same domain. With selection as the most important factor responsible for the observed discrepancies, the bias (discrepancy) was present in both directions, thus leading to overestimation or underestimation of the observed level of autozygosity depending on the genetic model, linkage and initial gene frequency. Variation of the autozygosity level (between replicates) was increased notably in models with additive inheritance under selection and was an additional potential source of bias. Thus, when the trait is, to a large extent, controlled by a finite number of loci and when selection is present, the bias in the estimation of the autozygosity is likely to occur and caution is necessary whenever conclusions are based on inbreeding coefficients estimated from the pedigree or decrease in heterozygosity. 相似文献
18.
A fundamental strategy in selection programs is to combine maximum rate of response and minimum rate of inbreeding, these goals being in conflict with each other. Maximum selection response can be achieved at a cost of erosion in the effective number of breeding animals (a measure of the inbreeding level); reciprocally, the maximum effective number under selection can be preserved with a low response. The simultaneous consideration of both factors makes it difficult to decide on the use of individual (more effective in conserving effective number) or combined selection (maximizes response but yields low effective size). Q uinton et al. (1992) showed that comparing selection methods at the same level of inbreeding, rather than at the same selection intensity, changes the perspectives of current selection theory. If low to moderate inbreeding levels are considered, then phenotypic selection can yield higher response than selection on more accurate methods. Different methods have been proposed for maximizing selection response at the same level of inbreeding, i.e. to restrict the number of close relatives selected (N icholas and S mith 1983), to use false high heritability estimates in the genetic evaluation (G rundy and H ill 1993), to use assortative (S mith and H ammond 1986) or compensatory (G rundy et al. 1994) matings, to adjust estimated breeding values for the relationship with the already selected ones (G oddard and S mith 1990), to avoid matings of related individuals (T oro and P erez -E nciso 1990), or to use factorial rather than hierarchical matings (W oolliams 1989; L eitch et al. 1994). Q uinton and S mith (1995) compared the merits of these methods using stochastic simulation; they concluded that none of the methods was best over all conditions, and that the use of false high heritabilities, or adjusted estimated breeding values with the relationships, does not seem to be recommended; besides, mating together those individuals with the lowest relationship has little effect on the accumulated inbreeding. W ray and G oddard (1994), and B risbane and G ibson (1995) indicated that if Gn is the genetic mean after n generations of selection and Fn is the mean inbreeding coefficient, a reasonable selection objective is Gn ? DFn, where D is the value of a unit of inbreeding relative to a unit of genetic gain. M euwissen (1997) showed that these methods do not guarantee maximum genetic gains at some level of inbreeding and presented a rule for maximizing the genetic response with a predefined rate of inbreeding. His algorithm can be used to put a constraint on the variance of the selection response by replacing the additive relationship matrix by the prediction error variance (W oolliams and M euwissen 1993). W ei (1995a) developed a restricted phenotypic selection by considering limits on the number of individuals that will be selected from a family or on the family number selected. This less sophisticated method balances response and inbreeding. A restriction on the family number may lead to an increased response (but a decreased effective size), whereas restricting the proportion of selected individuals from a family is an efficient way to control the inbreeding (decreased response). W ei (1995b) generalized the method by introducing both restrictions. In this study, rates of response were compared under between-family, within-family, or both restrictions for a two-trait selection index in a short-term experiment with Tribolium. 相似文献
19.
Klieve HM Kinghorn BP Barwick SA 《Zeitschrift für Tierzüchtung und Züchtungsbiologie》1994,111(1-6):81-88
SUMMARY: Stochastic simulation was used to evaluate a range of selection strategies with respect to both additive genetic response and inbreeding. Strategies involving selection on BLUP ebvs or individual phenotype, followed by random mating, were compared with mate selection strategies which used portfolio analysis to give joint consideration to genetic merit and inbreeding. An adapted Mean Of Total Absolute Deviations (MOTAD) method was used in a mate selection model to define optimal matings with regard to aggregate genetic merit and inbreeding for a base population h(2) of 0.2. Compared with random mating following selection on BLUP ebvs, inbreeding levels after 10 years of selection were able to be reduced under BLUP plus mate selection from ～.23 to as little as .11. Additive genetic gain was either little compromised or increased. The results suggest that information linking expected levels of genetic merit and inbreeding can be used to find the preferred selection strategy. ZUSAMMENFASSUNG: Gemeinsame Kontrolle von Zuchtfortschritt und Inzucht bei Partnerselektion Es wurde stochastische Simulation zur Auswertung einer Reihe von Selektionsstrategien hinsichtlich Zuchtwertzuwachs und Inzucht verwendet. Strategien mit Selektion auf der Basis von BLUP ebvs oder individuellem Ph?notyp mit nachfolgender Zufallspaarung wurden mit Partnerselektionsstrategien verglichen, die Portfolioanalyse zur gemeinsamen Beachtung von Zuchtwert und Inzucht verwendeten. Eine Methode adaptierter MITTELWERTE TOTALER ABSOLUTER ABWEICHUNGEN (MOTAD) Methode wurde beim Partnerselektionsmodell zur Definition optimaler Paarungen in Hinblick auf Gesamtzuchtwert und Inzucht bei einer Populationsheritabilit?t von 0,2 verwendet. Verglichen mit Zufallspaarung nach Selektion auf BLUP ebvs waren die Inzuchtgrade nach 10 Selektionsjahren von 0,23 auf 0,11 reduziert und additiver Zuchtfortschritt war dabei wenig beeintr?chtigt oder nahm sogar zu. Die Ergebnisse weisen darauf hin, da? Information, die Zuchtwert und Inzucht verbindet, zur Identifikation erwünschter Selektionsstrategien führen kann. 相似文献
20.
Theoretical rates of annual genetic responses to selection in beef cattle were compared for conventional and multiple ovulation and embryo transfer (MOET) breeding schemes. Several combinations of replacement policy, mating ratio and type of selection were considered for both schemes with low, medium and high heritabilities. For MOET, four rates of embryo transfers per donor were used to represent low to moderate MOET levels. The results indicated that annual genetic responses to selection could be up to 1.3, 1.6 and 1.8 times as great for MOET compared with conventional breeding for traits of low, medium and high heritability, respectively; however, the annual inbreeding rates also were high for the MOET schemes considered. Embryo splitting, or cloning, was shown to increase accuracy of selection by 8 to 35% through the production of identical genotypes. The use of MOET in conjunction with embryo splitting in elite nucleus units could substantially increase genetic improvement for traits with low, medium and high heritabilities in beef cattle populations. 相似文献