Estimates Of Additive Genetic Variance Research Articles

Estimation of additive genetic variance when base populations are selected.

A population of size 40 was simulated 1,000 times for 10 generations. Five out of twenty males were selected each generation, and each male was mated to four females to have two progeny. The additive genetic variance (sigma 2a) before selection was 10, and the initial heritability was .5. Due to covariances among animals, inbreeding and gametic disequilibrium, the genetic variance was reduced to 6.72 after 10 generations of selection. Reduction of variance was lower in another population simulated with size 400 and 10% of the males selected. Restricted Maximum Likelihood was used to estimate sigma 2a using an animal model. The estimate of sigma 2a was empirically unbiased when all data and all relationships were used. Omitting data from selected ancestors caused biased estimates of sigma 2a due to not accounting for all gametic disequilibrium. Including additional relationships between assumed base animals adjusted for inbreeding and for covariances. Bias from gametic disequilibrium decreased slightly with the use of more relationship information, and it was smaller in the small population and(or) when selection had been practiced for just a few generations.

Laboratory estimates of heritabilities and genetic correlations in nature.

A lower bound on heritability in a natural environment can be determined from the regression of offspring raised in the laboratory on parents raised in nature. An estimate of additive genetic variance in the laboratory is also required. The estimated lower bounds on heritabilities can sometimes be used to demonstrate a significant genetic correlation between two traits in nature, if their genetic and phenotypic correlations in nature have the same sign, and if sample sizes are large, and heritabilities and phenotypic and genetic correlations are high.

Observed and Predicted Genetic Gains from Single and Multi‐Trait Selections Using Three Mating Designs in Pearl Millet*

AbstractThe objectives of the study were to estimate the genetic components of variance, predict the response to selection and to evaluate observed response to selection using two populations namely, PSB 7 and PSB 3, of pearl millet (Pennisetum americanum [L.] Leeks). The observed response was studied using four methods of selection based on single and multitrait criteria in three types of families, namely S1, half‐sib (HS) and full‐sib (FS) families. One hundred SI, HS and FS families each were developed in PSB 7 and PSB 3. Plants within a population were pollinated with bulk pollen of other population to develop HS families. Crosses were made between a pair of plants using one plant from each of the two populations to develop PS families. Data were recorded on days to flower, plant height, tiller number, spike length, spike girth, seed yield per plant and spike weight. Additive genetic variance estimates were significant for all traits. The estimates of dominance and additive × location and dominance × location interaction components were not significant, except additive × location component in PSB 7 for seed yield and spike weight. The genotypic component of variance among S1 families and S1 × location interactions were significant except S1 × location interactions in PSB 7 for days to flower, spike length and spike girth.Twenty‐four strains were developed by intermating superior families identified on the basis of four selection methods in each of the six types of families.The selection methods were high seed yield (YS) per se; index selection (IS) for high tiller number and long and thick spike; truncation selection (TS) for high yield among the families having days to flower and plant height lower than the population mean: and visual selection (VS) after completion of flowering for good vigour, uniformity in plant height and good inflorescence attributes. YS and IS were found to be superior to TS and VS with respect to observed genetic gains for seed yield. That advance was higher in selection among SI families as compared with FS and HS families. In general, a good agreement was noted between observed and expected responses. Expected genetic gain from FS reciprocal (R) recurrent selection was higher than the average gain from HS‐R selection.

Effects of inbreeding and estimates of additive genetic variance within seven summer oilseed rape cultivars

Seven groups of inbred lines derived from the summer oilseed rape (Brassica napus ssp. oleifera) cultivars 'Westar', 'Regent', 'Lergo', 'Marnoo', 'Ariel', 'Karat', and 'R83-11' were evaluated at three sites over 2 years, for agronomic characters and seed oil and protein concentrations. Estimates of additive genetic variance and heritability were calculated. Comparison of the mean of inbred lines with the mean of their respective source cultivars indicated little, if any, inbreeding depression in all cultivars except 'Ariel'. It was concluded that dominance is not a major factor in the genetic determination of the characters studied. For every trait in every cultivar, individual inbred lines were identified that exceeded or equaled their respective source cultivar means, indicating that neither heterozygosity per se nor genetic heterogeneity is required to maintain maximum performance in summer oilseed rape. Therefore, the oilseed rape cultivars used in this study were considered to be genetically heterogeneous populations, consisting of some number of generally homozygous lines. Significant additive genetic variance was detected for most traits in most cultivars, indicating that superior inbred lines could be extracted from within cultivars.Key words: inbreeding depression, heritability, Brassica napus L.

Genetics of Seed Yield and its Components in Common Bean (Phaseolus vulgaris L.) of Middle‐American Origin

AbstractGenetic variance, heritability, and expected response from selection arc useful in devising alternative methods and criteria of: selection. The objectives of this study were to estimate these for seed yield and its components from 200 F2: populations involving 80 cultivars and lines of mostly small‐seeded dry bush bean (Phaseolus vulgaris L.) of habits growth I, II, and III of Middle‐American origin. All cultivars and lines were crossed in eight sets of ten parents each in a Design II mating system. The F2 populations, without parents, were evaluated in the field in a replicates‐in‐sets design at two locations in Colombia in 1983.Estimates of additive genetic variance were significant for yield, pods/m2, seeds/pod, and seed weight. Interaction with environments was also significant. Values for nonadditive genetic variance were not significant for either yield or yield components. The estimates of narrow sense heritability, based on the F2 population mean and unbiased by genotype x environment interaction, were 0.21 ± 0.13 for yield. 20 ± 0.13 for pods/m2, 0.57 ± 0.13 for seeds/pod, and 0.74 ± 0.15 for seed weight.The expected direct response from selection of the top 20 % of F2 populations for yield per se would result in a 4.30 % increase in yield with a correlated response of 0.21 % in seed weight. In contrast, the expected gain from direct selection for seed weight would result in a 11.76 % increase in seed weight with a, correlated gain of 0.28 % for yield. Direct selection for pods/m2 would decrease yield, seeds/pod and seed weight, while direct selection for seeds/pod would reduce pods/m2 and seed weight but increase seed yield by 0.37 %.Data on yield from replicated trials in the early segregating generations could be utilized for identification and selection of promising crosses and families or lines with crosses for dry bean yield improvement.

Effect of Natural Inbreeding on Variance Structure in Tests of Wind-Pollination Douglas-Fir Progenies

Abstract Studies of the mating habits of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) have shown that wind-pollination families contain a small proportion of very slow-growing natural inbreds. The effect of these very small trees on means, variances, and variance ratios was evaluated for height and diameter in a 16-year-old plantation by systematically deleting from analyses the shortest individuals in wind-pollination (W) and diallel (D) progenies of six parent trees. The progenies were growing together in a common field test. The D population served as a control (no inbreds). The low-vigor trees in the W population type gave a downward bias to means and an upward bias to within-plot and family x block variance estimates. Amount of bias differed among W families. Before deletion, estimates of additive genetic variance and heritability for height in the W test were about one-third and one-sixth the estimates from the D test. Deleting the shortest 5 to 8% of the trees from the W population did not remove all the discrepancy, but it did result in much closer agreement between W and D tests. For the D test alone, deletion of the smallest trees had a negligible effect on estimates of additive genetic variance and heritability. For. Sci 34(1):102-118.

Components of Genetic Variance and Covariance for Linear Type Traits in Jersey Cattle

First available appraisals for 67,644 Jersey cows were used to estimate relative magnitudes of additive and selected sources of nonadditive genetic variance and covariance for 13 type traits scored linearly from 50 to 99 points.Covariances among four types of relatives were used to estimate components of genetic variance and covariance for each of the 13 traits. Direct additive, direct dominance, and additive maternal genetic variances and the direct-maternal additive genetic covariance were estimated using covariances among paternal half-sibs, full-sibs, daughters and dams, and maternal half-sibs.Estimates of additive genetic variance were 11 to 36% of variance within herd-appraiser subclasses with largest values for stature, strength, rump angle, rump width, and udder depth. Dominance components were two to four times the magnitude of additive components for dairy character, rump width, fore udder, rear udder height, rear udder width, and suspensory ligament but much smaller for other traits. Maternal effects and the direct-maternal additive genetic covariance appeared small for all linear type traits.

PREDICTIONS OF THE EFFECTIVENESS OF INTRAPOPULATION SELECTION VERSUS SELECTION FOR SPECIFIC COMBINING ABILITY IN SWINE

Additive genetic variance estimates for purebred (Lacombe) and crossbred (Lacombe × Yorkshire) populations and the additive genetic covariance between purebred and crossbred progeny were calculated for postweaning average daily gain, total probe fat, total carcass fat, and litter size in swine. These estimates were used to predict the effectiveness of four methods of intrapopulation selection (IP) relative to selection for specific combining ability (SCA) to determine the most effective means of improving crossbred performance. The intrapopulation methods were mass selection based on information from both sexes (BS), mass selection based on information from one sex (OS), full-sib selection (FS), and half-sib selection (HS). The Lacombe population was selected over 12 generations for increased postweaning average daily gain and the Lacombe × Yorkshire population was generated by breeding Lacombe boars with randomly selected gilts from a Yorkshire control population. Selection for combining ability was the most effective means of improving average daily gain except when information was available on both sexes and the relative selection intensity (SCA/IP) was less than 0.60. Mass selection was superior to SCA for improving total probe fat except when information was restricted to one sex and the relative selection intensity was less than 0.47. For total probe fat and total carcass fat, SCA was superior to FS and HS for relative selection intensities less than 0.65 and 0.74, respectively. Selection for combining ability was superior to OS for litter size regardless of the generation interval length. Key words: Intrapopulation selection, selection for combining ability, additive genetic variance, additive genetic covariance, swine

Indirect negative evidence for frequency-dependent selection with respect to viability in Drosophila melanogaster.

Using the estimates of additive genetic variance and dominance variance as well as the regression coefficient of heterozygotes on the sums of the component homozygotes (βY•X) and the inverse of the regression coefficient of the sum of the two homozygotes on their heterozygotes (1/βX•Y) with respect to viability estimated by the Cy method, it was tested whether or not frequency- dependent selection is a general rule with respect to viability polygenes. Two types of frequency-dependent selection models were considered: (1) gene frequency-dependent model and (2) zygotic frequency-dependent model. The βY•X and 1/βY•X values predicted on the basis of the frequency-dependent selection models and the ratio of the dominance variance to the additive variance are inconsistent with the actual estimates of these parameters for the Florida, Ishigaki-jima (Japan), Raleigh (N.C.) and Aomori (Japan) populations. The same conclusion was obtained for newly arisen polygenic viability mutations. Thus, it is speculated that frequency-dependent selection is not a general rule for viability polygenes as well as polymorphic isozyme genes.

Estimating the Precision of Estimates of Genetic Parameters Realized from Multiple-Trait Selection Experiments

The genetic change from multiple-trait selection experiments can be equated to the regression of genotype on phenotype. This gives rise to a method of obtaining estimates of additive genetic variances and covariances. The method requires the use of selection weights, derived by means of the index-in-retrospect, to provide invariant solutions. Solution variance estimates obtained from Monte Carlo simulation do not agree with variance estimates from ordinary least squares methods. This indicates that the errors are distributed with some structure V. A form of V is proposed which utilizes knowledge of the errors. Monte Carlo variance estimates from generalized least squares (GLS) methods agree closely with the average variance estimates from GLS when the proposed V is used. Use of an estimated V, derived after the initial estimation procedure, is shown to provide adequate information on the variance of the estimates.

High Intensity Selection for Percent Oil in Corn1

Seven cycles of high‐intensity within half‐sib family selection for percent oil in the ‘Reid Yellow Dent’ corn (Zea mays L.) cultivar resulted in an increase in oil from 4.0 to 9.1%, while kernel weight and ear height were reduced slightly and grain moisture at harvest increased. No significant changes in yield, silking date, or plant height were found. Realized heritability for percent oil was 43%.Estimates of genetic variance for percent oil and 100‐kernel‐weight were obtained in 1977 and 1978, for Cycles 0 and 5. All estimates of additive genetic variance (ó2A) were greater than zero; however, change in ó2A occurred between cycles. Estimates of dominance genetic variance did not differ from zero.A comparison of the predicted gains from within half‐sib family selection, mass selection, and combined selection among and within half‐sib families revealed that maximum gain is expected when a higher selection intensity is used within half‐sib families than among half‐sib families.

Estimating Genetic Variance in Sugarcane Using a Factorial Cross Design1

Progeny from seven factorials of crosses were studied to examine the quantitative genetics of a breeding population of clones used in the Hawaiian sugarcane (Saccharum spp. L.) breeding program and the applicability of quantitative genetic theory to sugarcane. Nonadditive genetic effects were equally important to additive genetic effects, which suggests that increased yields could result from utilizing both polycrosses and crosses where both parents are known. There was relatively little genetic variability for sugar content as measured by refractometer solids compared to the genetic variability for yield of cane as measured by volume. Thus, the greatest progress in breeding and selection is more likely to come from increasing the yield of cane rather than from increasing sugar content.The estimates of additive genetic variance, σ2A1 and σ2A2, were obtained, respectively, from the regression of progeny means on mid‐parent means and the progeny analysis; the estimates of total genetic variance, σ2G! and σ2G2 were obtained, respectively, from the analysis of parental clones and the progenies. The ratios of σ2A1/σ2A2 and σ2G1/σ2G2, however, were generally less than one, which suggests that the assumptions underlying quantitative inheritance analysis were violated. This is probably because the clones in the breeding population were highly selected for yield potential.

Genetics of tassel branch number in maize and its implications for a selection program for small tassel size

Tassel branch numbers of six crosses of maize (Zea mays L.) were analyzed to determine inheritance of this trait. Generation mean analyses were used to estimate genetic effects, and additive and nonadditive components of variance were calculated and evaluated for bias due to linkage. Both narrow-sense and broad-sense heritabilities were estimated. Additive genetic variance estimates were significant in five of the six crosses, whereas estimates of variance due to nonadditive components were significant in only three crosses. Additionally, estimates of additive variance components usually were larger than corresponding nonadditive components. There was no evidence for linkage bias in these estimates. Estimates of additive genetic effects were significant in four of six crosses, but significant dominance, additive × additive and additive × dominance effects also were detected. Additive, dominance, and epistatic gene action, therefore, all influenced the inheritance of tassel branch number, but additive gene action was most important. Both narrow-sense and broadsense heritability estimates were larger than those reported for other physiological traits of maize and corroborated conclusions concerning the importance of additive gene action inferred from analyses of genetic effects and variances. We concluded that selection for smalltasseled inbreds could be accomplished most easily through a mass-selection and/or pedigree-selection system. Production of a small-tasseled hybrid would require crossing of two small-tasseled inbreds. We proposed two genetic models to explain unexpected results obtained for two crosses. One model involved five interacting loci and the other employed two loci displaying only additive and additive × additive gene action.

Genetic parameters in aKy 14 xKy Ex 42 burley population ofNicotiana tabacum L.

In the F2 generation of the cross between burley varieties,Ky 14 andKy Ex 42, estimates of additive genetic variance were significant for yield, days to flower, and number of leaves. All estimates of dominance variance, additive X additive variance, and interactions between genetic variances and years were smaller than twice their standard errors. The gain expected from selecting and recombining the best 10% of theF 2 plants ranged from 4% for number of leaves to 8.6% for yield. Additive genetic correlations between pairs of agronomic characters were positive. Yield and number of leaves were negatively correlated with total alkaloids.

Genetic studies in an exotic population of corn (Zea mays L.) grown under two plant densities

Progenies of a Design II [Comstock and Robinson (1948)] using random S 1 lines from an exotic population of corn (Zea mays L.) were evaluated in a randomized incomplete block design with two replications at two plant-population densities (1 7,222 plants/ha and 68,888 plants/ha) in 1970 and 1971, at Lincoln, Nebraska. Five traits were studied i.e. grain weight, number of ears, days to flower, plant height and ear height.Under both densities the estimates of additive genetic variance were much larger than those of dominance genetic variance for all traits. The ratio of dominance to additive genetic variance estimates was less than 0.5 suggesting that for the majority of loci controlling the traits, partial to complete dominance is likely.The estimates of additive genetic x year interaction variance were high and significantly different from zero under both densities, indicating that estimates of additive genetic variance in this population obtained from experiments conducted in only one year may be seriously biased. The estimates of dominance genetic x year interaction variance were not significant and most of them were negative.Under both densities high genetic inter-relationships were indicated between grain weight and number of ears, days to flower and plant height, days to flower and ear height, and plant height and ear height.Even though there was a large difference between the two densities used in the study, the differences between the estimates of genetic parameters were not significant in all cases.The sample size of S 1 plants representing each S0 parent in the crossing nursery used in the present study (11.75) caused a small upward bias in the estimates of additive genetic variance, but it caused an upward bias in the estimates of dominance genetic variance of 6-7% of the total genetic variance.It is suggested that a trait such as grain weight should be expressed on a unit area basis when genetic parameters (except for correlation and the ratio between two values) obtained from experiments with different plant-population densities are to be compared.

Genetic Variance in Autotetraploid Maize1

This study represents the first attempt to characterize genetic variability in an autotetraploid maize population. Estimates of additive and digenic genetic variance were obtained from a sib analysis in autotetraploid maize (Zea mays L.), Synthetic B. Estimates of additive genetic variance larger than one standard error were obtained for yield, ear length, ears per plant, plant height, and percent oil. Positive estimates of digenic variance larger than one standard error were found only for plant height. The estimate of additive genetic variance for yield is smaller in the autotetraploid, Synthetic B, than is norreally encountered in diploid populations.

MASS SELECTION FOR POST-WEANING GROWTH IN SWINE: II. RESPONSE TO SELECTION

Recurrent mass selection for post-weaning average daily gain was practiced for seven generations in a population of Lacombe swine. Response to selection was positive, but the observed response was estimated to be only 0.33 of the predicted. The pooled estimate of additive genetic variance was 0.0013 ± 0.0002. Realized heritability was 0.126 ± 0.029. The pooled estimate of heritability obtained from components of variance and parent-offspring regression was 0.377 ± 0.058. Litter size was positively correlated with average daily gain (0.14 piglet per generation). No significant effects of selection in reducing additive genetic variance were observed.

MASS SELECTION FOR POST-WEANING GROWTH IN SWINE: I. THE VALUE OF A PEDIGREED CONTROL POPULATION

A pedigreed control population of Yorkshire swine has been described. The average size of the breeding population was 20 males and 40 females in each of the seven generations. The effective number of parents per generation was estimated at 87.5. There has been little or no evidence of any significant changes in the genetic value of the population over the seven generations as determined by estimates of additive genetic variance, selection differentials, and other related parameters. The performance of the control population was compared with that of a selected population and with crossbreds produced by mating select population males to control population females. Comparisons were made in the same environments. The comparisons of the effects of the selection program on several traits measured in the two populations have been made, based on the absolute trends in these populations over the seven generations and as a deviation from the performance of the control population.

Genetic Variability for Yield after Four Cycles of Reciprocal Recurrent Selections in Maize1

The primary objective of my study was to determine the relative change in genetic variation after four cycles of reciprocal recurrent selection for yield in maize (Zea mays L.). Estimates of additive genetic variance (σ2A) and variance due to dominance deviations (σ2D) were obtained for the original (C0) Stiff Stalk Synthetic and Corn Borer Synthetic #1 populations; for the syn. 3 generation of the reconstituted populations (C4) after four cycles of reciprocal recurrent selection for yield; and for the syn. 3 generation of the hybrid populations formed by crossing the original synthetic varieties (CO ✕ CO) and the CA reconstituted populations (C4 ✕ C4). cross‐classification mating design (Design II) was imposed on each population to form half‐sib and full.sib progenies. Twenty sets were produced, each containing 16 full‐sib progenies formed from crossing four randomly selected S0 males and Sl females, for each population, and grown at three locations.There was no difference in the estimates of σ2A for the CO and C4 Corn Borer Synthetic No. 1 populations. The estimates for Stiff Stalk Synthetic showed a decrease from the CO to the C4 , which approached the 10% probability level. There was a significant reduction in the estimates of σ2A from the CO ✕ CO to the C4 ✕ C4 hybrid populations. All the estimates of σ2A exceeded twice their standard errors. None of the changes in the estimates of σ2DD was significant between the populationsFour cycles of reciprocal recurrent selection showed an increase in yield of the Stiff Stalk Synthetic and hybrid populations and a decrease in Corn Borer Synthetic No. I populations. It appeared that only modest gains have been made for yield with a reduction in genetic variation.

Variation and Covariation in a High Oil Population of Corn (Zea mays L.) and Their Implications in Selection1

Sixty‐four half‐sib families, each containing four full‐sib families, were grown in replicated trials in five environments. Data were collected for silking date, plant and ear heights, kernel size, lodging percentage, shelling percentage, oil percentage, yield of grain, and yield of oil. Estimates of additive and dominance variances were calculated for each trait, and these variance estimates were used to predict the genetic gain from different selection methods. Phenotypic and additive genetic correlations among traits were calculated.All estimates of additive genetic variance were significantly greater than zero. None of the estimates of dominance variance was significant except for yield of grain in one environment. Mass selection appeared to be the most effective method of selection for higher oil percentage in this population, but recurrent selection for general combining ability would be the most effective method for yield of grain and yield of oil. The only traits significantly correlated with oil percentage were silking date and yield of oil. A selection index for improving yield of oil appeared to have little advantage over selection for yield of oil alone.