The effect of birthweight on tooth-size variability in twins.

Studies indicate that low birthweight (LBW) children display reduced deciduous tooth size but there is little information about permanent tooth size. It has also been shown that dental fluctuating asymmetry (FA) increases in response to various environmental influences, but the relationship between birthweight and FA remains unclear. The aim of this study was to compare tooth size and asymmetry, according to birthweight, in the deciduous and permanent dentitions of a sample of Australian twins. The study sample comprised 436 twins, classified into 2 groups: normal birthweight (NBW > 2500 g) and low birthweight (LBW < or = 2500 g). For each individual it was generally possible to measure maximum mesiodistal crown diameters of both deciduous and permanent central incisors from serial dental models. Correlations were calculated between tooth-size variables and birthweight; subsequently comparisons of tooth size and FA were made between the LBW and NBW samples using Student's t tests. Small positive correlations (around .1) were noted between birthweight and tooth-size variables. There was no evidence of tooth-size reduction in the LBW male sample, but the LBW females displayed tooth-size reduction of approximately 2-3% for both deciduous and permanent incisors, compared to the NBW females. There was no evidence of increased FA in the LBW individuals of either sex. These findings indicate that developing teeth are generally well-protected from developmental disturbances during prenatal and perinatal periods. Further research is needed to clarify the biological basis of an apparently true but weak association between tooth size and birthweight.     

GENETIC AND ENVIRONMENTAL VARIATION IN LIFE-HISTORY TRAITS OF A MONOCARPIC PERENNIAL: A DECADE-LONG FIELD EXPERIMENT

Directional and stabilizing selection tend to deplete additive genetic variance. On the other hand, genetic variance in traits related to fitness could be retained through polygenic mutation, spatially varying selection, genotype-environment interaction, or antagonistic pleiotropy. Most estimates of genetic variance in fitness-related traits have come from laboratory studies, with few estimates of heritability made under natural conditions, particularly for longer lived organisms. Here I estimated additive genetic variance in life-history characters of a monocarpic herb, Ipomopsis aggregata, that lives for up to a decade. Experimental crosses yielded 229 full-sibships nested within 32 paternal half-sibships. More than 5000 offspring were planted as seeds into natural field sites and were followed in most cases through their entire life cycle. Survival showed substantial additive genetic variance (genetic coefficient of variation ≈ 54%). Small differences at seedling emergence were magnified over time, such that the genetic variability in survival was only detectable by tracking the success of offspring for several years starting from seed. In contrast to survival, reproductive traits such as flower number, seeds per flower, and age at flowering showed little or no genetic variability. Despite relatively high levels of additive genetic variation for some life-history characters, high environmental variance in survival resulted in very low heritabilities (0–9%) for all of these characters. Maternal effects were evident in seed mass and remained strong throughout the lengthy vegetative period. No negative genetic correlations between major components of female fitness were detected. Mean corolla width for a paternal family was, however, negatively correlated with the finite rate of increase based on female fitness. That negative correlation could help to maintain additive genetic variance in the face of strong selection through male function for wide corollas.     

Genetics of Life History in DROSOPHILA MELANOGASTER. I. Sib Analysis of Adult Females

A sib analysis of adult life-history characters was performed on about twelve hundred females from a laboratory Drosophila melanogaster population that had been sampled from nature and cultured so as to preserve its genetic variability. The following results were found. There was no detectable trend with age in additive or dominance genetic variances for age-specific fecundity. Environmental variance for age-specific fecundity increased with age. The genetic variance for fecundity characters was primarily additive. The genetic variance for longevity was primarily dominance variance. There were negative genetic correlations between early fecundity and lifespan, as well as between mean egg-laying rate and longevity.     

Genetic parameters related to environmental variability of weight traits in a selection experiment for weight gain in mice; signs of correlated canalised response

Data from an experimental mice population selected from 18 generations to increase weight gain were used to estimate the genetic parameters associated with environmental variability. The analysis involved three traits: weight at 21 days, weight at 42 days and weight gain between 21 and 42 days. A dataset of 5273 records for males was studied. Data were analysed using Bayesian procedures by comparing the Deviance Information Criterion (DIC) value of two different models: one assuming homogeneous environmental variances and another assuming them as heterogeneous. The model assuming heterogeneity was better in all cases and also showed higher additive genetic variances and lower common environmental variances. The heterogeneity of residual variance was associated with systematic and additive genetic effects thus making reduction by selection possible. Genetic correlations between the additive genetic effects on mean and environmental variance of the traits analysed were always negative, ranging from -0.19 to -0.38. An increase in the heritability of the traits was found when considering the genetic determination of the environmental variability. A suggested correlated canalised response was found in terms of coefficient of variation but it could be insufficient to compensate for the scale effect associated with an increase of the mean.     

Genetic study of the height and weight process during infancy.

Longitudinal height and weight data from 4649 Dutch twin pairs between birth and 2.5 years of age were analyzed. The data were first summarized into parameters of a polynomial of degree 4 by a mixed-effects procedure. Next, the variation and covariation in the parameters of the growth curve (size at one year of age, growth velocity, deceleration of growth, rate of change in deceleration [i.e., jerk] and rate of change in jerk [i.e., snap]) were decomposed into genetic and nongenetic sources. Additionally, the variation in the estimated size at birth and at 2 years of age interpolated from the polynomial was decomposed into genetic and nongenetic components. Variation in growth was best characterized by a genetic model which included additive genetic, common environmental and specific environmental influences, plus effects of gestational age. The effect of gestational age was largest for size at birth, explaining 39% of the variance. The differences between monozygotic and dizygotic twin correlations were largest for size at 1 and 2 years of age and growth velocity of weight, which suggests that these parameters are more influenced by heritability than size at birth, deceleration and jerk. The percentage of variance explained by additive genetic influences for height at 2 years of age was 52% for females and 58% for males. For weight at 2 years of age, heritability was approximately 58% for both sexes. Variation in snap height for males was also mainly influenced by additive genetic factors, while snap for females was influenced by both additive genetic and common environmental factors. The correlations for the additive genetic and common environmental factors for deceleration and snap are large, indicating that these parameters are almost entirely under control of the same additive genetic and common environmental factors. Female jerk and snap, and also female height at birth and height at 2 years of age, are mostly under control of the same additive genetic factor.     

Within-generation mutation variance for litter size in inbred mice

The mutational input of genetic variance per generation (sigma(m)(2)) is the lower limit of the genetic variability in inbred strains of mice, although greater values could be expected due to the accumulation of new mutations in successive generations. A mixed-model analysis using Bayesian methods was applied to estimate sigma(m)(2) and the across-generation accumulated genetic variability on litter size in 46 generations of a C57BL/6J inbred strain. This allowed for a separate inference on sigma(m)(2) and on the additive genetic variance in the base population (sigma(a)(2)). The additive genetic variance in the base generation was 0.151 and quickly decreased to almost null estimates in generation 10. On the other hand, sigma(m)(2) was moderate (0.035) and the within-generation mutational variance increased up to generation 14, then oscillating between 0.102 and 0.234 in remaining generations. This pattern suggested the existence of a continuous uploading of genetic variability for litter size (h(2)=0.045). Relevant genetic drift was not detected in this population. In conclusion, our approach allowed for separate estimation of sigma(a)(2) and sigma(m)(2) within the mixed-model framework, and the heritability obtained highlighted the significant and continuous influence of new genetic variability affecting the genetic stability of inbred strains.     

Effects on phenotypic variability of directional selection arising through genetic differences in residual variability

In standard models of quantitative traits, genotypes are assumed to differ in mean but not variance of the trait. Here we consider directional selection for a quantitative trait for which genotypes also confer differences in variability, viewed either as differences in residual phenotypic variance when individual loci are concerned or as differences in environmental variability when the whole genome is considered. At an individual locus with additive effects, the selective value of the increasing allele is given by ia/sigma + 1/2 ixb/sigma2, where i is the selection intensity, x is the standardized truncation point, sigma2 is the phenotypic variance, and a/sigma and b/sigma2 are the standardized differences in mean and variance respectively between genotypes at the locus. Assuming additive effects on mean and variance across loci, the response to selection on phenotype in mean is isigma2(Am)/sigma + 1/2 ixcov(Amv)/sigma2 and in variance is icov(Amv)/sigma + 1/2 ixsigma2(Av)/sigma2, where sigma2(Am) is the (usual) additive genetic variance of effects of genes on the mean, sigma2(Av) is the corresponding additive genetic variance of their effects on the variance, and cov(Amv) is the additive genetic covariance of their effects. Changes in variance also have to be corrected for any changes due to gene frequency change and for the Bulmer effect, and relevant formulae are given. It is shown that effects on variance are likely to be greatest when selection is intense and when selection is on individual phenotype or within family deviation rather than on family mean performance. The evidence for and implications of such variability in variance are discussed.     

Genetic variability in the diapause response of the burnet moth Zygaena trifolii (Lepidoptera: Zygaenidae)

A high degree of phenotypic variability was observed in the diapause response of the burnet moth Zygaena trifolii. In this study, we show that the observed variability is partly based on genetic differences between individuals. In a selection experiment, the larval instar at which diapause occurs was changed within six generations. Diapause instars were dependent on the time of pre-diapause development of larvae, which varied considerably between larvae. A heritability analysis indicates that a part of the variability in development time is based on additive genetic variance. The maintenance of genetic variability in the development time and the diapause response of Z. trifolii is discussed in the context of spatially and temporally changing selection pressures.     

Dominance Genetic Variance for Traits Under Directional Selection in Drosophila serrata

In contrast to our growing understanding of patterns of additive genetic variance in single- and multi-trait combinations, the relative contribution of nonadditive genetic variance, particularly dominance variance, to multivariate phenotypes is largely unknown. While mechanisms for the evolution of dominance genetic variance have been, and to some degree remain, subject to debate, the pervasiveness of dominance is widely recognized and may play a key role in several evolutionary processes. Theoretical and empirical evidence suggests that the contribution of dominance variance to phenotypic variance may increase with the correlation between a trait and fitness; however, direct tests of this hypothesis are few. Using a multigenerational breeding design in an unmanipulated population of Drosophila serrata, we estimated additive and dominance genetic covariance matrices for multivariate wing-shape phenotypes, together with a comprehensive measure of fitness, to determine whether there is an association between directional selection and dominance variance. Fitness, a trait unequivocally under directional selection, had no detectable additive genetic variance, but significant dominance genetic variance contributing 32% of the phenotypic variance. For single and multivariate morphological traits, however, no relationship was observed between trait–fitness correlations and dominance variance. A similar proportion of additive and dominance variance was found to contribute to phenotypic variance for single traits, and double the amount of additive compared to dominance variance was found for the multivariate trait combination under directional selection. These data suggest that for many fitness components a positive association between directional selection and dominance genetic variance may not be expected.     

Dissecting total genetic variance into additive and dominance components of purebred and crossbred pig traits

The partition of the total genetic variance into its additive and non-additive components can differ from trait to trait, and between purebred and crossbred populations. A quantification of these genetic variance components will determine the extent to which it would be of interest to account for dominance in genomic evaluations or to establish mate allocation strategies along different populations and traits. This study aims at assessing the contribution of the additive and dominance genomic variances to the phenotype expression of several purebred Piétrain and crossbred (Piétrain × Large White) pig performances. A total of 636 purebred and 720 crossbred male piglets were phenotyped for 22 traits that can be classified into six groups of traits: growth rate and feed efficiency, carcass composition, meat quality, behaviour, boar taint and puberty. Additive and dominance variances estimated in univariate genotypic models, including additive and dominance genotypic effects, and a genomic inbreeding covariate allowed to retrieve the additive and dominance single nucleotide polymorphism variances for purebred and crossbred performances. These estimated variances were used, together with the allelic frequencies of the parental populations, to obtain additive and dominance variances in terms of genetic breeding values and dominance deviations. Estimates of the Piétrain and Large White allelic contributions to the crossbred variance were of about the same magnitude in all the traits. Estimates of additive genetic variances were similar regardless of the inclusion of dominance. Some traits showed relevant amount of dominance genetic variance with respect to phenotypic variance in both populations (i.e. growth rate 8%, feed conversion ratio 9% to 12%, backfat thickness 14% to 12%, purebreds-crossbreds). Other traits showed higher amount in crossbreds (i.e. ham cut 8% to 13%, loin 7% to 16%, pH semimembranosus 13% to 18%, pH longissimus dorsi 9% to 14%, androstenone 5% to 13% and estradiol 6% to 11%, purebreds-crossbreds). It was not encountered a clear common pattern of dominance expression between groups of analysed traits and between populations. These estimates give initial hints regarding which traits could benefit from accounting for dominance for example to improve genomic estimated breeding value accuracy in genetic evaluations or to boost the total genetic value of progeny by means of assortative mating.     

Genetics of Mandible Form in the Mouse

The underlying determination of phenotypic variability and covariability is described for 14 traits that define the morphological size and shape of the mature mouse mandible. Variability is partitioned into components due to direct additive and dominance genetic effects, indirect maternal additive genetic effects, genetic covariance between direct additive and indirect maternal additive effects and common and residual environmental effects. Multivariate analyses of the dimensionality of genetic variability indicate several complex and independent genetic components underlie the morphological form of the mandible. The multidimensional nature of the genetic components suggests a complex picture with regard to the consequences of selection on mandibular form.     

Additive and non-additive genetic parameters from clonally replicated and seedling progenies of<Emphasis Type="Italic"> Eucalyptus globulus</Emphasis>

The first estimates of the importance of epistatic effects within Eucalyptus globulus were obtained from analysis of clonally replicated full-sib progeny tests grown in Portugal. Parents comprised diverse selections from the Portuguese landrace. Variance components were estimated for 4-year-old diameter growth and pilodyn penetration, an indirect measure of wood density, both key traits in the pulpwood breeding objective. The experimental components of variance were used to estimate heritabilities and proportions of the phenotypic variance due to dominance and epistasis. The additive variance was the only significant genetic component affecting either diameter or pilodyn. Estimates of the additive, dominance and epistatic effects accounted for 8–10%, 0–4% and 0.4% of the phenotypic variance in diameter, and for 11–17%, 0% and 5% of the phenotypic variance in pilodyn, respectively. A comparison of residual coefficients of variation within seedling and cloned progenies indicated that C effects within clones were not a serious source of random variability. Despite the test sites encompassing a diverse range of locations, no important genotype by environment interaction was detected. The results suggested that an improvement strategy combining both recurrent selection for additive genetic merit and clonal testing may be adequate for optimizing genetic gains from this genetic base.Communicated by O. Savolainen     

Age-specific genetic and maternal effects in fecundity of preindustrial Finnish women

A population's potential for evolutionary change depends on the amount of genetic variability expressed in traits under selection. Studies attempting to measure this variability typically do so over the life span of individuals, but theory suggests that the amount of additive genetic variance can change during the course of individuals' lives. Here we use pedigree data from historical Finns and a quantitative genetic framework to investigate how female fecundity, throughout an individual's reproductive life, is influenced by "maternal" versus additive genetic effects. We show that although maternal effects explain variation in female fecundity early in life, these effects wane with female age. Moreover, this decline in maternal effects is associated with a concomitant increase in additive genetic variance with age. Our results thus highlight that single over-lifetime estimates of trait heritability may give a misleading view of a trait's potential to respond to changing selection pressures.     

Ontogeny of additive and maternal genetic effects: lessons from domestic mammals

Evolution of size and growth depends on heritable variation arising from additive and maternal genetic effects. Levels of heritable (and nonheritable) variation might change over ontogeny, increasing through "variance compounding" or decreasing through "compensatory growth." We test for these processes using a meta-analysis of age-specific weight traits in domestic ungulates. Generally, mean standardized variance components decrease with age, consistent with compensatory growth. Phenotypic convergence among adult sheep occurs through decreasing environmental and maternal genetic variation. Maternal variation similarly declines in cattle. Maternal genetic effects are thus reduced with age (both in absolute and relative terms). Significant trends in heritability (decreasing in cattle, increasing in sheep) result from declining maternal and environmental components rather than from changing additive variation. There was no evidence for increasing standardized variance components. Any compounding must therefore be masked by more important compensatory processes. While extrapolation of these patterns to processes in natural population is difficult, our results highlight the inadequacy of assuming constancy in genetic parameters over ontogeny. Negative covariance between direct and maternal genetic effects was common. Negative correlations with additive and maternal genetic variances indicate that antagonistic pleiotropy (between additive and maternal genetic effects) may maintain genetic variance and limit responses to selection.     

Reduced tooth size in 45,X (Turner syndrome) females

Mean values and variances of deciduous and permanent tooth dimensions were compared between 121 45,X (Turner syndrome) females and 171 control subjects to clarify the role of the X chromosome on dental development. Although deciduous molars tended to be smaller than normal in 45,X females, there was no evidence of a reduction in tooth size for deciduous anterior teeth. In the permanent dentition, all mesiodistal dimensions were significantly smaller in 45,X females but only some of the buccolingual dimensions were smaller. The findings for deciduous tooth-size may reflect a sampling effect related to the extremely high frequency of spontaneous abortion in 45,X individuals. Results for permanent teeth are consistent with the concept of a decrease in enamel thickness in 45,X females.     

Common genetic and environmental factors among craniofacial traits in Belgian nuclear families: Comparing skeletal and soft-tissue related phenotypes

The major objective of this study was to determine the possible effects of common genetic and environmental factors among 18 craniofacial anthropometric traits, with special attention to the differences between skeletal and soft-tissue related phenotypes. The studied sample consisted of 122 nuclear families living in Brussels and included 251 males and 258 females aged from 13 to 72 years. Univariate and bivariate quantitative genetic analyses were performed using a variance components procedure implemented in SOLAR software.All phenotypes were significantly influenced by additive genetic factors with heritability estimates ranging from 0.46 (nose height) to 0.72 (external biocular breadth). Sex, age and their interactions explained 7-46% of the total phenotypic variance of the traits. Bivariate analysis revealed that several traits share a common genetic and/or environmental basis while other traits show genetic and environmental independence from one another. More and greater genetic and environmental correlations were observed among skeletal phenotypes, than among soft-tissue traits and between both categories. Apart from the tissue composition, other characteristics of the craniofacial morphology such as the orientation (e.g. heights, breadths) have shown to be important factors in determining pleiotropy and common environmental effects between some pairs of traits. In conclusion, the results confirm that overall head configuration is largely determined by additive genetic effects, and that common genetic and environmental factors affecting craniofacial size and shape are stronger for the skeletal traits than for the soft-tissue traits.     

Quantitative Genetics of DROSOPHILA MELANOGASTER. I. Sexual Dimorphism in Genetic Parameters for Wing Traits

Sexual dimorphism in genetic parameters is examined for wing dimensions of Drosophila melanogaster. Data are fit to a quantitative genetic model where phenotypic variance is a linear function of additive genetic autosomal variance (common to both sexes), additive genetic X-linked variances distinct for each sex, variance due to common rearing environment of families, residual environmental variance, random error variance due to replication, and variance due to measurement error and developmental asymmetry (left vs. right sides). Polygenic dosage compensation and its effect on genetic variances and covariances between sexes is discussed. Variance estimates for wing length and other wing dimensions highly correlated with length support the hypothesis that the Drosophila system of dosage compensation will cause male X-linked genetic variance to be substantially larger than female X-linked variance. Results for various wing dimensions differ, suggesting that the level of dosage compensation may differ for different traits. Genetic correlations between sexes for the same trait are presented. Total additive genetic correlations are near unity for most wing traits; this indicates that selection in the same direction in both sexes would have a minor effect on changing the magnitude of difference between sexes. Additive X-linked correlations suggest some genotype x sex interactions for X-linked effects.     

The role of genetic constraints on the diversification of Iberian taxa of the genus Aquilegia (Ranunculaceae)

The microevolutionary process of adaptive phenotypic differentiation of quantitative traits between populations or closely‐related taxa depends on the response of populations to the action of natural selection. However, this response can be constrained by the structure of the matrix of additive genetic variance and covariance between traits in each population ( G matrix). In the present study, we obtained additive genetic variance and narrow sense heritability for 25 floral and vegetative traits of three subspecies of Aquilegia vulgaris, and one subspecies of Aquilegia pyrenaica through a common garden crossing experiment. For two vegetative and one floral trait, we also obtained the G matrix and genetic correlations between traits in each subspecies. The amount of genetic variation available in wild populations is not responsible for the larger differentiation of vegetative than floral traits found in this group of columbines. However, the low heritability of some traits constrained their evolution because phenotypic variability among taxa was larger for traits with larger heritability. We confirmed that the process of diversification of the studied taxa involved shifts in the G matrix, mainly determined by changes in the genetic covariance between floral and vegetative traits, probably caused by linkage disequilibrium in narrow endemic taxa. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 252–261.     

Partitioning additive genetic variance into genomic and remaining polygenic components for complex traits in dairy cattle

ABSTRACT: BACKGROUND: Low cost genotyping of individuals using high density genomic markers were recently introduced as genomic selection in genetic improvement programs in dairy cattle. Most implementations of genomic selection only use marker information, in the models used for prediction of genetic merit. However, in other species it has been shown that only a fraction of the total genetic variance can be explained by markers. Using 5217 bulls in the Nordic Holstein population that were genotyped and had genetic evaluations based on progeny, we partitioned the total additive genetic variance into a genomic component explained by markers and a remaining component explained by familial relationships. The traits analyzed were production and fitness related traits in dairy cattle. Furthermore, we estimated the genomic variance that can be attributed to individual chromosomes and we illustrate methods that can predict the amount of additive genetic variance that can be explained by sets of markers with different density. RESULTS: The amount of additive genetic variance that can be explained by markers was estimated by an analysis of the matrix of genomic relationships. For the traits in the analysis, most of the additive genetic variance can be explained by 44 K informative SNP markers. The same amount of variance can be attributed to individual chromosomes but surprisingly the relation between chromosomal variance and chromosome length was weak. In models including both genomic (marker) and familial (pedigree) effects most (on average 77.2%) of total additive genetic variance was explained by genomic effects while the remaining was explained by familial relationships. CONCLUSIONS: Most of the additive genetic variance for the traits in the Nordic Holstein population can be explained using 44 K informative SNP markers. By analyzing the genomic relationship matrix it is possible to predict the amount of additive genetic variance that can be explained by a reduced (or increased) set of markers. For the population analyzed the improvement of genomic prediction by increasing marker density beyond 44 K is limited.     

Inheritance of 18 quantitative dermatoglyphic traits based on factors in MZ and DZ twins

18 quantitative finger and palmar dermatoglyphic traits were analyzed with the aim of determining genetic effects and common familial environmental influences on a large (358 nuclear pedigrees) number of twins (MZ and DZ). Genetic analysis based on principal factors includes variance and bivariate variance decomposition analysis. Especially, Factor 1 (digital pattern size) is remarkable, due to its degree of universality. The results of genetic analysis revealed all three extracted factors have significant proportion of additive genetic variance (93.5% to 72.9%). The main results of bivariate variance decomposition analysis appears significant correlation in residual variance between digital pattern size factor (Factor 1) versus finger pattern intensity factor (Factor 4), and palmar main lines factor (Factor 2) verses a-b ridge count (Factor 3), but there was no significant correlation in the genetic variance of factors.