Estimates Of Additive Genetic Effects Research Articles

Tuning the genomic evaluation system of Holstein-Friesian cattle

The aim of this study was to compare the CPU time of two most widely used Fortran compilers – the Absoft and the Intel, in the context of two programs used in the genomic evaluation system of dairy cattle - a program for re-coding of the raw genotyping data of single nucleotide polymorphisms and a program for the estimation of additive genetic effects of single nucleotide polymorphisms. The data set used for the analysis corresponded to the Polish national routine genomic evaluation for non-return rate of heifers and comprised genotypes of 46,267 single nucleotide polymorphisms for each of 2904 evaluated bulls, which resulted in 130,393,459 non-missing genotype records. Programs were compiled under several different compilation options using the Absoft (release 2015) and the Intel (release 2019) compilers. The CPU times of both programmes significantly differed between compilers. In the program for genotype re-coding the Intel compiler always resulted in shorter running times than the Absoft compiler. More variation was observed in CPU times of the program for effect estimation, for which the Absoft compiler resulted in both, the shortest and the longest computing times. A more detailed run-time memory consumption visualisation of single runs of the fastest Absoft and Intel compiled program executions demonstrated that the former compiler allows for the utilisation of more of the available memory, which in the case of a memory intensive program for effect estimation resulted in a faster execution time. In conclusion, an overall performance of the Intel compiler was better than this of the Absoft compiler. Even though significant differences in CPU time were observed, the Intel compiled execution times were less dependent on the pre-imposed compilation parameters. However, it is important to note that for programs performing a large number of arithmetic evaluations it is possible to tune the Absoft compiler to outperform the Intel compiler. The key compilation parameters are those, which allow for lower floating point accuracy and dedicated optimization of mathematical operations. Although blind use of such options is not recommended, in the case of genomic evaluation the math and floating point based optimisations did not affect the convergence of the solver nor the estimates of single nucleotide polymorphisms’ effects.

Genotype-environment interaction for age at first calving in Limousine and Charolais cattle raised in Italy, employing reaction norm model

The genotypes express differentiated responses depending on the environmental conditions and this can cause a re-ordering of the genotype in the environmental gradient. Therefore, our objective was to evaluate the genotype-environment interaction (GxE) for age at first calving (AFC) in Limousine and Charolais cattle raised in Italy, using reaction norm model (RNM). The data comprises information from 35,255 Limousine and 7,339 Charolais animals, born between 1999 and 2017. The standard animal model (AM), which ignores the G x E interaction, and the hierarchical models of reaction norms with homogeneous and heterogeneous residual variances with one and two steps (MHNRHO1P, MHNRHO2P, MHNRHE1P, and MHNRHE2P) were used for the analysis. In both breeds, the MHNRHOP1 model presented the best fit. In favorable environments (lower AFC), there was less phenotypic variation; the AFC expression of the animals was close and this may have caused lower estimates of additive genetic effects (σA2) and consequently lower heritabilities (h²). In Charolais, the h² for the animal model (AM) was 0.13, whereas for MHNRHOP1 the value varied throughout the environmental gradient from 0.06 to 0.13, for low and high AFC environments, respectively. In Limousine breed, a lower h² (0.06) was observed in the animal model (AM), while in the MHNRHOP1 model it varied from 0.00 to 0.15. In both breeds, bulls with higher genetic merits for one environment were not for another, demonstrating the existence of the GxE. In Charolais and Limousine breeds, more than 93.75% and 87.72% of the bulls presented extremely robust or robust genotypes, respectively. Although most bulls are robust, there are bulls reactive to the environment, which can lead to incorrect selection of breeders. The results suggest that the genotype-environment interaction should be considered in the selection for AFC in both breeds raised in Italy.

Evaluation of non-additive genetic variation in feed-related traits of broiler chickens

Genome-wide association mapping and genomic predictions of phenotype of individuals in livestock are predominately based on the detection and estimation of additive genetic effects. Non-additive genetic effects are largely ignored. Studies in animals, plants, and humans to assess the impact of non-additive genetic effects in genetic analyses have led to differing conclusions. In this paper, we examined the consequences of including non-additive genetic effects in genome-wide association mapping and genomic prediction of total genetic values in a commercial population of 5,658 broiler chickens genotyped for 45,176 single nucleotide polymorphism (SNP) markers. We employed mixed-model equations and restricted maximum likelihood to analyze 7 feed related traits (TRT1 – TRT7). Dominance variance accounted for a significant proportion of the total genetic variance in all 7 traits, ranging from 29.5% for TRT1 to 58.4% for TRT7. Using a 5-fold cross-validation schema, we found that in spite of the large dominance component, including the estimated dominance effects in the prediction of total genetic values did not improve the accuracy of the predictions for any of the phenotypes. We offer some possible explanations for this counter-intuitive result including the possible confounding of dominance deviations with common environmental effects such as hatch, different directional effects of SNP additive and dominance variations, and the gene-gene interactions’ failure to contribute to the level of variance.

A novel sibling-based design to quantify genetic and shared environmental effects: application to drug abuse, alcohol use disorder and criminal behavior.

Twin studies have been criticized for upwardly biased estimates that might contribute to the missing heritability problem. We identified, from the general Swedish population born 1960-1990, informative sibships containing a proband, one reared-together full- or half-sibling and a full-, step- or half-sibling with varying degrees of childhood cohabitation with the proband. Estimates of genetic, shared and individual specific environment for drug abuse (DA), alcohol use disorder (AUD) and criminal behavior (CB), assessed from medical, legal or pharmacy registries, were obtained using Mplus. Aggregate estimates of additive genetic effects for DA, AUD and CB obtained separately in males and females varied from 0.46 to 0.73 and agreed with those obtained from monozygotic and dizygotic twins from the same population. Of 54 heritability estimates from individual classes of informative sibling trios (3 syndromes × 9 classes of trios × 2 sexes), heritability estimates from the siblings were lower, tied and higher than those from obtained from twins in 26, one and 27 comparisons, respectively. By contrast, of 54 shared environmental estimates, 33 were lower than those found in twins, one tied and 20 were higher. With adequate information, human populations can provide many methods for estimating genetic and shared environmental effects. For the three externalizing syndromes examined, concerns that heritability estimates from twin studies are upwardly biased or were not generalizable to more typical kinds of siblings were not supported. Overestimation of heritability from twin studies is not a likely explanation for the missing heritability problem.

Accounting for heterogeneity of phenotypic variance in Iranian Holstein test-day milk yield records

First lactation milk yield data consisting of 1,576,102 test-day records for 221,862 Iranian Holstein cows having lactations between 1983 and 2008 were used to study the existence and effect of heterogeneity of variance (HOV) on estimated breeding values and the performance of random regression models (RRMs) with different orders of fit. A pre-correction method based on phenotypic variance, assuming equal heritability for different levels of herd-test date classes and a genetic correlation of one between them, was used to correct for HOV. RRMs with Legendre polynomial functions were used to analyze adjusted and unadjusted records. Some re-ranking of animals occurred from the adjustment, but the correction method only had slight effects on the overall ranking and rank correlations of animals. Data correction had considerable effects on top animals, such that 10% and 17% of top sires and dams, respectively, were replaced from the list of top 1% animals when compared to the homogeneous variance scenario. Application of the adjustment method resulted in slightly higher heritabilities, which may be due to the more accurate estimation of additive genetic effects when HOV is considered. An index consisting of different comparison criteria was used to investigate the effect of HOV on fitting orders of Legendre polynomials and to compare RRMs. In general, the rank of models was improved by increasing the order of fit, but models with smaller orders of fit and correction for HOV performed better than models with higher orders of fit without correction for variance heterogeneity. The results of this research indicate that the accuracy of estimated breeding values may be increased and the genetic progress of the herds could be affected by accounting for HOV as part of genetic evaluations in Iran.

Including dominance effects in the genomic BLUP method for genomic evaluation.

We evaluated the performance of GBLUP including dominance genetic effect (GBLUP-D) by estimating variances and predicting genetic merits in a computer simulation and 2 actual traits (T4 and T5) in pigs. In simulation data, GBLUP-D explained more than 50% of dominance genetic variance. Moreover, GBLUP-D yielded estimated total genetic effects over 1.2% more accurate than those yielded by GBLUP. In particular, when the dominance genetic variance was large, the accuracy could be substantially improved by increasing the number of markers. The dominance genetic variances in T4 and T5 accounted for 9.6% and 6.3% of the phenotypic variances, respectively. Estimates of such small dominance genetic variances contributed little to the improvement of the accuracies of estimated total genetic effects. In both simulation and pig data, there were nearly no differences in the estimates of additive genetic effects or their variance between GBLUP-D and GBLUP. Therefore, we conclude GBLUP-D is a feasible approach to improve genetic performance in crossbred populations with large dominance genetic variation and identify mating systems with good combining ability.

Genetic evaluation of a complete diallel cross involving three populations of freshwater prawn (Macrobrachium rosenbergii) from different geographical regions of India

Estimates of additive genetic, reciprocal and average heterosis effects were obtained from a three by three complete diallel cross experiment for giant freshwater prawn Macrobrachium rosenbergii. The data were generated in two batches. Three populations were sampled from the states of Gujarat (West), Kerala (South West) and Orissa (East) in India. The two mating batches of diallel crosses were combined in the analysis. A total 4773 animals (2233 in Batch 1 and 2540 in Batch 2) were tagged with VIA (Visible Implant Alpha numeric tags). A total of 2545 animals (979 and 1566 from Batch 1 and 2, respectively) were harvested with readable VIA tags and were; therefore available for analysis. The fixed effects included in the models were test environment (ponds 1 or 2 in batch 1, ponds 3 or 4 in batch 2), sex (male or female) and batch (1 or 2). Age at harvest was fitted as linear covariate. The genotype effect was partitioned into additive, non additive and reciprocal cross effects. Body weight, standard length and carapace length were recorded during tagging, sampling and harvesting. Estimates of additive genetic effect in the three stocks showed that the Kerala stock had greater body weight, carapace length and standard length, followed by Orissa then Gujarat. Average heterosis for body traits was low (3.1 to 6.3%) and not significantly different from zero. The results are discussed with respect to the choice of the most appropriate breeding strategy to improve the performance of M. rosenbergii in India.

Association Test for X-Linked QTL in Family-Based Designs

Family-based association methods for detecting quantitative trait loci (QTL) have been developed primarily for autosomes, and comparable methods for X-linked QTL have received less attention. We have developed a family-based association test for quantitative traits, named XQTL, which uses X-linked markers in a nuclear family design. XQTL adopts the framework of the orthogonal model implemented in the QTDT program, modifying the sex-specific score for X-linked genotypes. XQTL also takes into account the dosage effect due to female X chromosome inactivation. Restricted maximum likelihood (REML) and Fisher's scoring method are used to estimate variance components of random effects. Fixed effects, derived from the phenotypic differences among and within families, are estimated by the least-squares method. Our proposed XQTL can perform allelic and two-locus haplotypic association tests and can provide estimates of additive genetic effects and variance components. Simulation studies show correct type I error rates under the null hypothesis and robust statistical power under alternative scenarios. The loss of power observed when parental genotypes are missing can be compensated by an increase of offspring number. By treating age at onset of Parkinson disease as a quantitative trait, we illustrate our method, using MAO polymorphisms in 780 families.

Estimation of Genetic Parameters for Cumulative Egg Numbers in a Broiler Dam Line by Using a Random Regression Model

The random regression model (RRM) methodology was applied to the estimation of genetic parameters for cumulative egg numbers and monthly egg production in a broiler dam line. The data were extracted from records of a commercial dam line in 2001 to 2003. A total of 99,193 records from 6,475 hens and 9,111 pedigreed animals were used in the current study. The variance components were estimated using Gibbs sampling procedure. According to the Bayesian information criterion and Bayes factor, an RRM with Legendre polynomial of 2 orders for hatching groups and additive genetic effects and of 4 orders for permanent environmental effects was chosen as the optimal model for cumulative egg numbers in the broiler dam line. The heritability estimates of the cumulative egg numbers between wk 1 and 40 of production ranged from 0.16 to 0.54, whereas heritability estimates from wk 12 to 20 of production were moderate. The ratios of permanent environmental variance to phenotypic variance were large, indicating that the RRM could produce better estimates of additive genetic effects. The genetic and phenotypic correlations between cumulative egg numbers at different production weeks estimated with the optimal RRM were generally higher when the overlapping weeks were greater. In addition, genetic parameters for monthly egg production could also be obtained by the optimal RRM, and the heritability estimates ranged from 0.03 to 0.18. It was suggested that early selection based on cumulative egg numbers in the first 19 wk of production could effectively improve annual egg production in the broiler dam line.

Reading Skills in Early Readers

The present study combined parallel data from the Northeast-Northwest Collaborative Adoption Projects (N2CAP) and the Western Reserve Reading Project (WRRP) to examine sibling similarity and quantitative genetic model estimates for measures of reading skills in 272 school-age sibling pairs from three family types (monozygotic twins, dizygotic twins, and unrelated adoptive siblings). The study included measures of letter and word identification, phonological awareness, phonological decoding, rapid automatized naming, and general cognitive ability. Estimates of additive genetic effects and shared environmental effects were moderate and significant. Furthermore, shared environmental effects estimated in twins were generally similar in magnitude to adoptive sibling correlations, suggesting highly replicable estimates across different study designs.

Twin analyses of chronic fatigue in a Swedish national sample

Chronic fatigue has infrequently been studied in twins. Data from twin studies can inform clinical and research approaches to the management and etiology of human complex traits. The authors obtained telephone interview data on current chronic fatigue from 31406 individuals twins in the Swedish Twin Registry (aged 42-64 years, 75.68% response rate), from both members of 12407 pairs and from one member of 6592 pairs. Of the complete pairs, 3269 pairs were monozygotic, 9010 pairs dizygotic, and 128 pairs of unknown zygosity. Structural equation twin modeling was used to estimate the latent genetic architecture of varying definitions of fatiguing illness. Estimates of additive genetic effects, shared environmental effects, and individual-specific environmental effects were similar in males and females. No definition of current fatiguing illness (ranging from any fatigue to CFS-like illness) was strikingly distinctive. Individual-specific effects were the predominant source of variation, followed by modest genetic influences. We could not exclude a small but conceptually important contribution of shared environmental effects. Current fatiguing illness appears to be a complex trait resulting from both environmental and genetic sources of variation without pronounced differences by gender.

ADHD: sibling interaction or dominance: an evaluation of statistical power.

Sibling interaction effects are suggested by a difference in phenotypic variance between monozygotic (MZ) twins and dizygotic (DZ) twins, and a pattern of twin correlations that is inconsistent with additive genetic influences. Notably, negative sibling interaction will result in MZ correlations which are more than twice as high as DZ correlations, a pattern also seen in the presence of genetic dominance. Negative sibling interaction effects have been reported in most genetic studies on Attention Deficit Hyperactivity Disorder (ADHD) and related phenotypes, while the presence of genetic dominance is not always considered in these studies. In the present paper the statistical power to detect both negative sibling interaction effects and genetic dominance is explored. Power calculations are presented for univariate models including sources of variation due to additive genetic influences, unique environmental influences, dominant genetic influences and a negative sibling interaction (i.e., contrast effect) between phenotypes of twins. Parameter values for heritability and contrast effects are chosen in accordance with published behavior genetic studies on ADHD and associated phenotypes. Results show that when both genetic dominance and contrast effects are truly present and using a classical twin design, genetic dominance is more likely to go undetected than the contrast effect. Failure to detect the presence of genetic dominance consequently gives rise to slightly biased estimates of additive genetic effects, unique environmental effects, and the contrast effect. Contrast effects are more easily detected in the absence of genetic dominance. If the significance of the contrast effect is evaluated while also including genetic dominance, small contrast effects are likely to go undetected, resulting in a relatively large bias in estimates of the other parameters. Alternative genetic designs, such as adding pairs of unrelated siblings reared together to a classical twin design, or adding non-twin siblings to twin pairs, greatly enhances the statistical power to detect contrast effects as well as the power to distinguish between genetic dominance and contrast effects.

QUANTITATIVE GENETICS OF GROWTH AND DEVELOPMENT TIME IN THE BURYING BEETLE NICROPHORUS PUSTULATUS IN THE PRESENCE AND ABSENCE OF POST-HATCHING PARENTAL CARE

Despite a growing interest in the evolutionary aspects of maternal effects, few studies have examined the genetic consequences of maternal effects associated with parental care. To begin to provide data on nonlaboratory or nondomestic animals, we compared the effect of presence and absence of parental care on phenotype expression of larval mass and development time at different life-history stages in the burying beetle Nicrophorus pustulatus. This beetle has facultative care; parents can feed their larvae through regurgitation of digested carrion or offspring can feed by themselves from previously prepared carrion. To investigate larval responses to these two levels of care, including estimates of additive genetic effects, maternal effects, and genotype-by-environment interactions, we used a half-sibling split-family breeding experiment—raising half of the offspring of a family in the presence of their mother and the other half without their mother present. Larvae reared with their mother present were on average heavier and developed faster, although some of the differences in development decreased or were eliminated by the adult stage. These results suggest that presence or absence of post-hatching maternal care plays an important role in phenotype expression early in life, whereas later the phenotype of the offspring is determined mainly by the genotype and/or unshared environmental effects. Our study also permitted us to examine the differences in genetic effects between the two care environments. Heritabilities, maternal/common environment effect, and most genetic correlations did not differ between the care treatments. Genetic analyses revealed substantial additive genetic effects for development time but small effects for measures of body mass. Maternal plus common environment effects were high for measures of mass but low for development time, suggesting that indirect genetic effects of maternal and/or common environment are less important for the evolution of development time than for mass. Estimates of genetic correlations revealed a trade-off between the duration of the two development stages after the offspring left the carrion. There was also a negative genetic correlation between the time spent on carrion and the mass at 72 h, when mothers usually stop feeding. The analysis of genotype-by-environment interactions indicates substantial variation among maternal families in response to care. Presence or absence of parental care may therefore contribute to the additive genetic variance through its interaction with the maternal component of the additive genetic variance. The presence of this interaction further suggests that parents may vary in care strategies, with some parents dispersing after preparation of the carrion and some parents staying with the larvae. This interaction may help maintain genetic variation in growth, development time, and parental care behavior. Additional work is needed, however, to quantify indirect genetic effects and genetic variation in parental care behavior itself.Corresponding Editor: T. Mousseau

Quantitative genetics of growth and development time in the burying beetle Nicrophorus pustulatus in the presence and absence of post-hatching parental care.

Despite a growing interest in the evolutionary aspects of maternal effects, few studies have examined the genetic consequences of maternal effects associated with parental care. To begin to provide data on nonlaboratory or nondomestic animals, we compared the effect of presence and absence of parental care on phenotype expression of larval mass and development time at different life-history stages in the burying beetle Nicrophorus pustulatus. This beetle has facultative care; parents can feed their larvae through regurgitation of digested carrion or offspring can feed by themselves from previously prepared carrion. To investigate larval responses to these two levels of care, including estimates of additive genetic effects, maternal effects, and genotype-by-environment interactions, we used a half-sibling split-family breeding experiment-raising half of the offspring of a family in the presence of their mother and the other half without their mother present. Larvae reared with their mother present were on average heavier and developed faster, although some of the differences in development decreased or were eliminated by the adult stage. These results suggest that presence or absence of post-hatching maternal care plays an important role in phenotype expression early in life, whereas later the phenotype of the offspring is determined mainly by the genotype and/or unshared environmental effects. Our study also permitted us to examine the differences in genetic effects between the two care environments. Heritabilities, maternal/common environment effect, and most genetic correlations did not differ between the care treatments. Genetic analyses revealed substantial additive genetic effects for development time but small effects for measures of body mass. Maternal plus common environment effects were high for measures of mass but low for development time, suggesting that indirect genetic effects of maternal and/or common environment are less important for the evolution of development time than for mass. Estimates of genetic correlations revealed a trade-off between the duration of the two development stages after the offspring left the carrion. There was also a negative genetic correlation between the time spent on carrion and the mass at 72 h, when mothers usually stop feeding. The analysis of genotype-by-environment interactions indicates substantial variation among maternal families in response to care. Presence or absence of parental care may therefore contribute to the additive genetic variance through its interaction with the maternal component of the additive genetic variance. The presence of this interaction further suggests that parents may vary in care strategies, with some parents dispersing after preparation of the carrion and some parents staying with the larvae. This interaction may help maintain genetic variation in growth, development time, and parental care behavior. Additional work is needed, however, to quantify indirect genetic effects and genetic variation in parental care behavior itself.

Genetic influences on blood pressure with the cold-pressor test: a twin study.

To determine the genetic and environmental contributions to resting blood pressure, the level of blood pressure during the cold-pressor test and the increase in blood pressure with the cold-pressor test in an adult cohort of normotensive twins. Ninety-one monozygotic and 41 dizygotic normal twin pairs were recruited by advertisement. The mean age was 34 +/- 14 years (mean +/- SD). Systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate were measured continuously at the finger (using a Finapres device) and verified at the upper arm oscillometrically (using a Dinamap device) every minute. The cold-pressor test was conducted by immersing the non-dominant hand into cold (< 4 degrees C) water for 2 min. Statistical analysis was performed by using the SPSS program; parameters of the quantitative genetic models were estimated by path-analysis techniques using the LISREL 8 program. Heritability estimates of additive genetic effects were statistically significant for SBP and DBP but not for heart rate during rest and during the cold-pressor test. Furthermore, the path analysis indicated shared as well as specific genetic components both for the blood pressure level at rest and for that during the cold-pressor test. However, the genetic influences on the blood pressure level at rest and on the increase in blood pressure during the cold-pressor test (the blood pressure level during the cold-pressor test minus that during rest) were entirely independent of one another. A significant genetic covariation exists for SBP and DBP during rest and during the cold-pressor test, as well as a significant genetic variation that is specific to the cold-pressor stress condition. These findings suggest that different genes or sets of genes contribute to blood pressure regulation during rest and to blood pressure reactivity to cold-pressor stress.

Generation means analysis for productivity in two diverse peanut crosses

Utilization of exotic germplasm resources for population improvement in peanut (Arachis hypogaea L.) has increased as the need to increase genetic diversity among peanut cultivars was recognized. Progeny of crosses of two unadapted germplasm lines (GP-NC 343 and FESR-11-P11-32) with an adapted cultivar ('NCV11') of peanut were evaluated for the genetic factors influencing the inheritance of yield and fruit characters in crosses among diverse lines. Objectives were to (1) estimate the relative importance of additive and nonadditive genetic effects in the inheritance of yield and fruit characters in two diverse peanut crosses; (2) determine the proportion of exotic germplasm that gave the optimum combination of mean productivity and genetic variability for each of the crosses; (3) relate the results to theories regarding the transfer of desirable alleles from exotic germplasm into adapted breeding populations. Crosses and backcrosses were made to generate germplasm lines (ten generations) ranging from 0 to 100% exotic germplasm for each cross. The populations were evaluated in replicated field trials. Yield and six fruit characters were measured, and a weighted analysis of variance was conducted to determine if significant differences existed among generations. Generation means analyses were performed for each trait measured in each of the crosses using both three- and six-parameter models, which were tested for goodness-of-fit with a joint-scaling test. Significant differences were detected among generations for most traits measured in both crosses. Estimates of additive genetic effects were significant for pod weight and seed weight in cross 1 ('NC-V11' x GP-NC 343) and for all traits in cross 2 ('NC-V11' x FESR-11-P11-32) except seed∶pod ratio. Significant estimates of dominance effects were found for pod length, pod width, and pod weight in cross 1 and for pod length in cross 2. No significant estimates of digenic effects were observed in cross 1, whereas in cross 2 estimates of additive x dominance epistatic effects were significant for yield and pod length, while estimates of additive x additive effects were significant for seed number. Regression of trait means on generations showed a curvilinear response for all traits in cross 1 except seed weight, which gave a linear response. For all traits in cross 2, the relationship between productivity and proportion of unadapted germplasm was effectively linear. Based on generation means and variances, progeny from the first or second backcross generation to the recurrent parent should be expected to give an optimum combination of mean productivity and relative variability in the population.

Genetic differences in learning behavior in honeybees (Apis mellifera capensis)

Workers from colonies of Cape honeybees show marked phenotypic differences in performance in proboscis extension reflex (PER) conditioning. Analysis of these differences using parthenogenetic offspring groups permitted the estimation of genotypic values and revealed a high degree of genetic variability that is evident among related as well as unrelated bees. The results obtained from related groups are of particular importance, since they demonstrated the existence of strong genetic variability among individuals of the same colony. Quantitative analysis yielded high estimates of additive genetic effects and low estimates of dominance effects. Selection of individual workers resulted in an explicit increase in genetic variance of the next generation (G1). However, selection of bees from the parthenogenetic G1 generation, which was done to obtain parthenogenetic G2 offspring, did not lead to further improvement in selection. This observation suggests that recombination of linked genes underlying proboscis extension reflex was negligible during selection in parthenogenetic groups. Taken together with further behavioral analysis (Brandes and Menzel, 1990; Brandes et al., 1988), results from these quantitative genetic experiments suggest that additive genetic factors contribute significantly to variability among individuals for associative learning.

Genetics of nitrate content of lettuce, 1: Analysis of generation means

The genetics of nitrate content in butterhead lettuce (Lactuca sativa L.) was studied using the mean values of five parental genotypes and several generations obtained from crosses between them. One high nitrate parental genotype was chosen and four low nitrate ones. A diallel analysis showed additive genetic effects to be the major source of variation in generation means. Estimates of additive genetic effects differed significantly between experiments, indicating genotype x experiment interactions. Effects of dominance were relatively small. The size and direction of dominance varied between experiments. Reciprocal differences were of very limited size and also varied between experiments. The inheritance of nitrate content in lettuce fitted the additive-dominance genetic model.

Estimates of Additive Genetic Effects, Maternal Genetic Effects, Individual Heterosis, Maternal Heterosis, and Egg Cytoplasmic Effects for Growth in Tilapia nilotica

AbstractLengths and weights of Auburn University‐Egypt and Auburn University‐Ivory Coast strains of Tilapia nilotica and their F1 F2, and backcross hybrids were analyzed by multiple regression to estimate strain additive genetic effects, strain maternal genetic effects, individual heterosis (specific combining ability), maternal heterosis, and strain egg cytoplasmic effects for growth to determine why heterosis of the F2 and backcross hybrids was greater than that of the FI hybrids. Individual heterosis and maternal heterosis made the greatest contributions to growth. The F2 and backcross hybrids were larger than the F1 hybrids because of maternal heterosis. Regression coefficients were used to predict growth for any possible hybrid combination. Egypt ♀× Ivory Coast ♂ FI hybrid females backcrossed to Egypt males and Egypt ♀× Ivory Coast ♂ F2 hybrids were the largest groups evaluated in this study and were also predicted to have faster growth rates than any other possible Egypt‐Ivory Coast hybrid.

Estimates of additive genetic effects, maternal effects, specific combining ability, maternal heterosis, and egg cytoplasm effects for cold tolerance in Oreochromis niloticus (L.)

. Cold tolerances of Auburn University-Egypt and Auburn University-Ivory Coast strains of Oreochromis niloticus (L.) and their F1. F2, and backcross hybrids were analysed by multiple regression to estimate strain additive genetic effects, strain maternal effects, specific combining ability (individual heterosis), maternal heterosis, and strain egg cytoplasm effects for cold tolerance. The Egypt strain was more cold tolerant than the Ivory Coast strain. Cold tolerance of F1 hybrids was intermediate between those of the parental strains. Heterosis for cold tolerances of F1, F2, and backcross hybrids was -1.39%, -0.83%, and -2.08% respectively. Additive genetic effects had the greatest effect on cold tolerance; Egypt genes for cold tolerance were superior to Ivory Coast genes for cold tolerance. Regressions for specific combining ability and maternal heterosis were small and were not different from zero, which explains why F1, F2, and backcross hybrids exhibited no hybrid vigour for cold tolerance. Cold tolerance in O. niloticus is controlled by additive rather than by dominance genetic variance; consequently, selection is the breeding technique that should be used to maximize cold tolerance. Regressions were used to predict cold tolerances of all possible Egypt-Ivory Coast hybrids. Predicted cold tolerances suggested that no possible Egypt-Ivory Coast hybrid would be more cold tolerant than the Egypt strain.