Genetic correlation and response to selection in simulated populations

Summary Effects of truncation selection of a primary trait upon genetic correlation with a secondary trait were examined over 30 generations in genetic populations simulated by computer. Populations were 24 males and 24 females mated randomly with replacement; number of offspring was determined by intensity of selection. Each trait was controlled by 48 loci segregating independently, effects were equal at every locus, and gene frequency was arbitrarily set at 0.5 at each locus in the initial generation. All combinations of three genetic correlations, three intensities of selection, and three environmental variances were simulated. Gene action was additive. Genetic correlation was set by number of loci which affected both traits and was measured each generation as the product-moment correlation of genotypic values and estimated by two methods of combining phenotypic covariances between parent and offspring.Genetic correlations in each offspring generation remained consistently near initial correlations for all environmental variances when fraction of offspring saved as parents was as large as one-half. When the fraction of offspring saved was as small as one-fifth, genetic correlations decreased but most rapidly with heritability high and after the 15th generation of selection. Truncation selection caused genetic correlation to decrease in those offspring selected to become parents of the next generation. Amount of reduction depended on heritability of the selected trait rather than on degree of truncation selection. Estimates of genetic correlation from phenotypic covariances between parent and offspring fluctuated markedly from real correlations in the small populations simulated.Michigan Agricultural Experiment Station Journal Article 4836. Part of North Central Regional Project NC-2.     

Genetic correlation and response to selection in simulated populations

Summary One of two quantitative traits was selected and correlated response in the other trait was measured in each of 30 generations for models of additive genes and of complete dominance. Each trait was controlled by 48 loci with equal effects, segregating independently from frequencies of 0.5 in the initial generation. Intensity of selection regulated the number of offspring from randomly mating 24 males and 24 females each generation. Three each of genetic correlations between traits, intensities of selection, and amounts of environmental variation were simulated.In the additive model correlated responses of the unselected trait to selection of the primary trait agreed closely with responses expected from theoretical considerations. In the model of complete dominance, responses of genotypic means of the unselected trait to selection of the primary trait in opposite directions were quite symmetrical for the first few generations but became distinctly asymmetrical in later generations. With little selection, response was fairly linear but became distinctly curvilinear as intensity of selection increased and environmental variance decreased. Between 15th and 30th generations some gains in the correlated trait to the 15th generation were lost.Michigan Agricultural Experiment Station Journal Article 4847. Part of North Central Regional Project NC-2.     

Optimised parent selection and minimum inbreeding mating in small aquaculture breeding schemes: a simulation study

The effectiveness of low cost breeding scheme designs for small aquaculture breeding programmes were assessed for their ability to achieve genetic gain while managing inbreeding using stochastic simulation. Individuals with trait data were simulated over 15 generations with selection on a single trait. Combinations of selection methods, mating strategies and genetic evaluation options were evaluated with and without the presence of common environmental effects. An Optimal Parent Selection (OPS) method using semi-definite programming was compared with a truncation selection (TS) method. OPS constrains the rate of inbreeding while maximising genetic gain. For either selection method, mating pairs were assigned from the selected parents by either random mating (RM) or Minimum Inbreeding Mating (MIM), which used integer programming to determine mating pairs. Offspring were simulated for each mating pair with equal numbers of offspring per pair and these offspring were the candidates for selection of parents of the next generation. Inbreeding and genetic gain for each generation were averaged over 25 replicates. Combined OPS and MIM led to a similar level of genetic gain to TS and RM, but inbreeding levels were around 75% lower than TS and RM after 15 generations. Results demonstrate that it would be possible to manage inbreeding over 15 generations within small breeding programmes comprised of 30 to 40 males and 30 to 40 females with the use of OPS and MIM. Selection on breeding values computed using Best Linear Unbiased Prediction (BLUP) with all individuals genotyped to obtain pedigree information resulted in an 11% increase in genetic merit and a 90% increase in the average inbreeding coefficient of progeny after 15 generations compared with selection on raw phenotype. Genetic evaluation strategies using BLUP wherein elite individuals by raw phenotype are genotyped to obtain parentage along with a range of different samples of remaining individuals did not increase genetic progress in comparison to selection on raw phenotype. When common environmental effects on full-sib families were simulated, performance of small breeding scheme designs was little affected. This was because the majority of selection must anyway be applied within family due to inbreeding constraints.     

Genetic effects on flight capacity in the beet armyworm,Spodoptera exigua (Lep., Noctuidae)

The beet armyworm, Spodoptera exigua, is an important migratory insect pest in tropical and subtropical regions worldwide. The current study investigated genetic variation in the flight capacity of both female and male moths, using a quantitative genetics approach. The offspring–parent regression showed that parents had a significant influence on the flight duration of offspring, and the heritability estimated as 0.302. The upward selection increased mean flight duration from 123.7 to 284.6 min in females and from 113.9 to 254.0 min in males during 8 h of flight test; by contrast, downward selection decreased it from 123.7 to 65.6 min in females and from 113.9 to 29.8 min in males, while it did not change significantly in either females or males of the control line over eight generations. The mean realized heritability was estimated as 0.432 based on upward selection but 0.130 on downward selection, suggesting the asymmetry of response to selection on flight capacity. Reciprocal crosses between the two selected lines confirmed the dominance of ‘long‐flying genes’ in the inheritance of flight capacity. A positive genetic correlation was found between increased flight duration and pupal weight. The presence of such additive genetic variance and covariance for flight capacity and the fitness trait, pupal weight, in the population of S. exigua may have underpinned the evolution of its migratory behaviour.     

On the assignment of fitness to parents and offspring: whose fitness is it and when does it matter?

There has been a long‐standing conceptual debate over the legitimacy of assigning components of offspring fitness to parents for purposes of evolutionary analysis. The benefits and risks inherent in assigning fitness of offspring to parents have been given primarily as verbal arguments and no explicit theoretical analyses have examined quantitatively how the assignment of fitness can affect evolutionary inferences. Using a simple quantitative genetic model, we contrast the conclusions drawn about how selection acts on a maternal character when components of offspring fitness (such as early survival) are assigned to parents vs. when they are assigned directly to the individual offspring. We find that there are potential shortcomings of both possible assignments of fitness. In general, whenever there is a genetic correlation between the parental and direct effects on offspring fitness, assigning components of offspring fitness to parents yields incorrect dynamical equations and may even lead to incorrect conclusions about the direction of evolution. Assignment of offspring fitness to parents may also produce incorrect estimates of selection whenever environmental variation contributes to variance of the maternal trait. Whereas assignment of offspring fitness to the offspring avoids these potential problems, it introduces the possible problem of missing components of kin selection provided by the mother, which may not be detected in selection analyses. There are also certain conditions where either model can be appropriate because assignment of offspring fitness to parents may yield the same dynamical equations as assigning offspring fitness directly to offspring. We discuss these implications of the alternative assignments of fitness for modelling, selection analysis and experimentation in evolutionary biology.     

Diversity of parental environments increases phenotypic variation in Arabidopsis populations more than genetic diversity but similarly affects productivity

Background and AimsThe observed positive diversity effect on ecosystem functioning has rarely been assessed in terms of intraspecific trait variability within populations. Intraspecific phenotypic variability could stem both from underlying genetic diversity and from plasticity in response to environmental cues. The latter might derive from modifications to a plant’s epigenome and potentially last multiple generations in response to previous environmental conditions. We experimentally disentangled the role of genetic diversity and diversity of parental environments on population productivity, resistance against environmental fluctuations and intraspecific phenotypic variation.MethodsA glasshouse experiment was conducted in which different types of Arabidopsis thaliana populations were established: one population type with differing levels of genetic diversity and another type, genetically identical, but with varying diversity levels of the parental environments (parents grown in the same or different environments). The latter population type was further combined, or not, with experimental demethylation to reduce the potential epigenetic diversity produced by the diversity of parental environments. Furthermore, all populations were each grown under different environmental conditions (control, fertilization and waterlogging). Mortality, productivity and trait variability were measured in each population.Key ResultsParental environments triggered phenotypic modifications in the offspring, which translated into more functionally diverse populations when offspring from parents grown under different conditions were brought together in mixtures. In general, neither the increase in genetic diversity nor the increase in diversity of parental environments had a remarkable effect on productivity or resistance to environmental fluctuations. However, when the epigenetic variation was reduced via demethylation, mixtures were less productive than monocultures (i.e. negative net diversity effect), caused by the reduction of phenotypic differences between different parental origins.ConclusionsA diversity of environmental parental origins within a population could ameliorate the negative effect of competition between coexisting individuals by increasing intraspecific phenotypic variation. A diversity of parental environments could thus have comparable effects to genetic diversity. Disentangling the effect of genetic diversity and that of parental environments appears to be an important step in understanding the effect of intraspecific trait variability on coexistence and ecosystem functioning.     

Effect of selection on genetic parameters of correlated traits

Summary Changes in genetic parameters of correlated traits due to the buildup of linkage (gametic phase) disequilibrium from repeated truncation selection on a single trait are studied. After several generations of selection, an equilibrium is approached where there are no further changes in genetic parameters and limiting values are reached. Formulae are derived under an infinitesimal model for these limiting values of genetic variances and covariances, heritabilities, and genetic correlations between traits directly and indirectly selected. Changes from generation zero to the limit in all these parameters become greater as heritability of the trait under direct selection increases and, to a lesser extent, as intensity of selection increases. Change in heritability of a trait under indirect selection also increases as the absolute value of the correlation between the trait under indirect and the trait under direct selection increases. The change is maximum when the initial value of heritability is close to 0.5 and insignificant when the initital value is close to zero or one. Change in the genetic correlation between the trait under direct selection and the trait under indirect selection is maximum when its initial value is close to ±0.6 and insignificant when its initial value is close to zero or ±1. Heritability of the trait indirectly selected and genetic correlation between that trait and the trait directly selected always decrease in absolute value, whereas genetic correlation between two traits indirectly selected can either decrease or increase in absolute value. It is suggested that use be made of formulae at selection equilibrium in the prediction of correlated responses after several generations of selection.     

The effect of simultaneous selection on the genetic correlation

The theoretical effect of simultaneous selection on the genetic correlations between two traits over 20 generations was examined using simulation. For each generation, a population of 50 male and 50 female diploid gen otypes with 15 loci, each with two alleles, was synthesized. None of the loci exhibited dominance. Five loci affected only trait 1, 5 loci only trait 2 and 5 were pleiotropic (affected both traits). Initial allelic frequencies were equal at each locus. Phenotypes were created by adding a random normal deviation for each trait to the genotype. The size of this deviation for each trait determined its heritability (h²). Index selection with h² combinations of (0.15, 0.15), (0.15,0.45) and (0.45,0.45) and relative economic weights of (1, 1) and (1, 3) for each h² combination was employed. In each generation, the highest ranking 25 genotypes of each sex were used to generate the next generation with single-pair matings, each producing two male and two female offspring. One hundred replicates were run for both negative and positive correlations. With a positive initial value, the genetic correlation tended to decline (toward zero). The rates of change were moderately affected by index weights and h². With a negative initial value, the genetic correlation tended to decrease (towards -1). However, unequal heritabilities and unequal relative economic weights slowed the rate of change with the greatest imbalances tending to hold the correlation constant or move it toward zero. These simulations illustrate that changes in parameters over time can affect the selection practiced. Under some of the conditions simulated, the use of initial genetic parameter values without change could have potentially negative effects on overall genetic gain.     

The role of maternal and paternal effects in the evolution of parental quality by sexual selection

Genetic models of maternal effects and models of mate choice have focused on the evolutionary effects of variation in parental quality. There have been, however, few attempts to combine these into a single model for the evolution of sexually selected traits. We present a quantitative genetic model that considers how male and female parental quality (together or separately) affect the expression of a sexually selected offspring trait. We allow female choice of males based on this parentally affected trait and examine the evolution of mate choice, parental quality and the indicator trait. Our model reveals a number of consequences of maternal and paternal effects. (1) The force of sexual selection owing to adaptive mate choice can displace parental quality from its natural selection optimum. (2) The force of sexual selection can displace female parental quality from its natural selection optimum even when nonadaptive mate choice occurs (e.g. runaway sexual selection), because females of higher parental quality produce more attractive sons and these sons counterbalance the loss in fitness owing to over-investment in each offspring. (3) Maternal and paternal effects can provide a source of genetic variation for offspring traits, allowing evolution by sexual selection even when those traits do not show direct genetic variation (i.e. are not heritable). (4) The correlation between paternal investment and the offspring trait influenced by the parental effects can result in adaptive mate choice and lead to the elaboration of both female preference and the male sexually selected trait. When parental effects exist, sexual selection can drive the evolution of parental quality when investment increases the attractiveness of offspring, leading to the elaboration of indicator traits and higher than expected levels of parental investment.     

Genetic and Statistical Analyses of Strong Selection on Polygenic Traits: What,Me Normal?

We develop a general population genetic framework for analyzing selection on many loci, and apply it to strong truncation and disruptive selection on an additive polygenic trait. We first present statistical methods for analyzing the infinitesimal model, in which offspring breeding values are normally distributed around the mean of the parents, with fixed variance. These show that the usual assumption of a Gaussian distribution of breeding values in the population gives remarkably accurate predictions for the mean and the variance, even when disruptive selection generates substantial deviations from normality. We then set out a general genetic analysis of selection and recombination. The population is represented by multilocus cumulants describing the distribution of haploid genotypes, and selection is described by the relation between mean fitness and these cumulants. We provide exact recursions in terms of generating functions for the effects of selection on non-central moments. The effects of recombination are simply calculated as a weighted sum over all the permutations produced by meiosis. Finally, the new cumulants that describe the next generation are computed from the non-central moments. Although this scheme is applied here in detail only to selection on an additive trait, it is quite general. For arbitrary epistasis and linkage, we describe a consistent infinitesimal limit in which the short-term selection response is dominated by infinitesimal allele frequency changes and linkage disequilibria. Numerical multilocus results show that the standard Gaussian approximation gives accurate predictions for the dynamics of the mean and genetic variance in this limit. Even with intense truncation selection, linkage disequilibria of order three and higher never cause much deviation from normality. Thus, the empirical deviations frequently found between predicted and observed responses to artificial selection are not caused by linkage-disequilibrium-induced departures from normality. Disruptive selection can generate substantial four-way disequilibria, and hence kurtosis; but even then, the Gaussian assumption predicts the variance accurately. In contrast to the apparent simplicity of the infinitesimal limit, data suggest that changes in genetic variance after 10 or more generations of selection are likely to be dominated by allele frequency dynamics that depend on genetic details.     

The effect of index selection on allele frequencies and future genetic gains when traits are correlated

The application of the selection index in the case of an additive two-trait model in which the genetic effect on each trait is determined by a finite number of loci is examined. Simulation results indicate that the direction of change in the frequency of favourable alleles is not necessarily in the positive direction at all loci when index selection is used as the basis for truncation selection. When the genetic correlation was positive (or favourable with respect to the economic weights), there was little difference (<5%) in genetic gain over 20 generations and no difference in the direction of change in allele frequencies or genetic correlation whether or not updated values for the genetic (co)variances were used in constructing the selection index. However, when the genetic correlation was negative or unfavourable, the effect of using genetic parameters which were not updated had unexpected effects on the allele frequencies and genetic correlation and reduced the genetic gain by a greater amount (< 12%).     

Genetic analyses of a seasonal interval timer

Seasonal clocks (e.g., circannual clocks, seasonal interval timers) permit anticipation of regularly occurring environmental events by timing the onset of seasonal transitions in reproduction, metabolism, and behavior. Implicit in the concept that seasonal clocks reflect adaptations to the local environment is the unexamined assumption that heritable genetic variance exists in the critical features of such clocks, namely, their temporal properties. These experiments quantified the intraspecific variance in, and heritability of, the photorefractoriness interval timer in Siberian hamsters (Phodopus sungorus), a seasonal clock that provides temporal information to mechanisms that regulate seasonal transitions in body weight. Twenty-seven families consisting of 54 parents and 109 offspring were raised in a long-day photoperiod and transferred as adults to an inhibitory photoperiod (continuous darkness; DD). Weekly body weight measurements permitted specification of the interval of responsiveness to DD, a reflection of the duration of the interval timer, in each individual. Body weights of males and females decreased after exposure to DD, but 3 to 5 months later, somatic recrudescence occurred, indicative of photorefractoriness to DD. The interval timer was approximately 5 weeks longer and twice as variable in females relative to males. Analyses of variance of full siblings revealed an overall intraclass correlation of 0.71 +/- 0.04 (0.51 +/- 0.10 for male offspring and 0.80 +/- 0.06 for female offspring), suggesting a significant family resemblance in the duration of interval timers. Parent-offspring regression analyses yielded an overall heritability estimate of 0.61 +/- 0.2; h(2) estimates from parent-offspring regression analyses were significant for female offspring (0.91 +/- 0.4) but not for male offspring (0.35 +/- 0.2), indicating strong additive genetic components for this trait, primarily in females. In nature, individual differences, both within and between sexes, in the timekeeping properties of seasonal interval timers, and a strong heritable basis thereof, would provide ample substrate for selection to rapidly influence seasonal clocks. Balancing selection in environments where the onset of spring conditions varies from year to year could maintain genetic variance in interval timers and yield interval timers tuned to the local environment.     

Are offspring begging levels exaggerated beyond the parental optimum? Evidence from a bidirectional selection experiment

Parental care involves elaborate behavioural interactions between parents and their offspring, with offspring stimulating their parents via begging to provision resources. Thus, begging has direct fitness benefits as it enhances offspring growth and survival. It is nevertheless subject to a complex evolutionary trajectory, because begging may serve as a means for the offspring to manipulate parents in the context of evolutionary conflicts of interest. Furthermore, it has been hypothesized that begging is coadapted and potentially genetically correlated with parental care traits as a result of social selection. Further experiments on the causal processes that shape the evolution of begging are therefore essential. We applied bidirectional artificial selection on begging behaviour, using canaries (Serinus canaria) as a model species. We measured the response to selection, the consequences for offspring development, changes in parental care traits, here the rate of parental provisioning, as well as the effects on reproductive success. After three generations of selection, offspring differed in begging behaviour according to our artificial selection regime: nestlings of the high begging line begged significantly more than nestlings of the low begging line. Intriguingly, begging less benefitted the nestlings, as reflected by on average significantly higher growth rates, and increased reproductive success in terms of a higher number of fledglings in the low selected line. Begging could thus represent an exaggerated trait, possibly because parent–offspring conflict enhanced the selection on begging. We did not find evidence that we co‐selected on parental provisioning, which may be due to the lack of power, but may also suggest that the evolution of begging is probably not constrained by a genetic correlation between parental provisioning and offspring begging.     

Secondary theorem of natural selection in biocultural populations.

The "Secondary Theorem of Natural Selection," an extension of Fisher's fundamental theorem, states that the rate of change in the mean of an arbitrary character in response to selection is proportional to the additive genetic covariance between the character and fitness. Here I derive an expression for the change in the mean value of a trait subject to both genetic and cultural transmission. I start with the one-locus case under generalized mating and cultural transmission from parents to offspring, then proceed to the two-locus case. My results support previous work on the effects of nongenetic inheritance by showing that (i) cultural transmission introduces a timelag in the population response to selection; (ii) with cultural transmission the effects of selection persist even after selection is relaxed; and (iii) cultural transmission can either enhance or retard phenotypic evolution relative to that obtained under purely genetic transmission.     

The evolution of indicator traits for parental quality: the role of maternal and paternal effects

In systems where individuals provide material resources to their mates or offspring, mate choice based on traits that are phenotypically correlated with the quality of resources provided is expected to be adaptive. Several models have explored the evolution of mating preference where there are direct benefits to choice, but few have addressed how a phenotypic correlation can be established between a male indicator trait and the degree of parental investment. We present a model with three quantitative traits: male and female parental investment and a potential male indicator trait. In our model, the expression of the "indicator" trait in offspring is affected by parental investment. These effects are referred to as maternal or paternal effects, or as "indirect genetic effects" when parental investment is heritable. With genetic variation for degree of parental investment, offspring harbor genes for parental investment that are unexpressed before mating but will affect the investment that they provide when expressed. Because the investment received from the parents affects the expression of the indicator trait, there will be a correlation between the genes for parental investment inherited and the degree of expression of the indicator trait in the offspring. The indicator trait is thus an "honest" signal for the degree of paternal investment.     

The risk of cancer in the offspring and parental length of life

BackgroundWe investigated if cancer onset in offspring is related to having short-lived parents for different cancer types and to see if there was a difference in smoking- and non-smoking related cancers.MethodsOur study included 524,391 individuals born in Norway 1940–1950. All children were followed up for cancer from the age of 20 until they were between 59 and 69 years. Parental longevity was examined by grouping parental age of death into parents dying before 75 years of age and parents dying at 75 years of age or older.ResultsAn increased risk of 1.14 (95%CI = 1.10–1.19) among male offspring and 1.08 (95%CI = 1.04–1.12) among female offspring was observed for total cancer when both parents died before the age of 75 compared to offspring with two long-lived parents. The highest increase was found for cancer in the lungs and trachea for both male (HR = 1.67, 95%CI = 1.50–1.86) and female offspring (HR = 1.53, 95%CI = 1.33–1.76). For other smoking-related cancers, the risk was lower. No increased risk was observed for non-smoking-related cancers.ConclusionOffspring of long-lived parents have lower risk of developing cancer compared with offspring of short-lived parents. Intergenerational transmission of risk factors from parents to offspring may play an important role, especially for tobacco-related cancers. However, genetic factors cannot be ruled out, since consistent evidence has implicated genetic factors in smoking behaviour.     

Estimation of heritability, evolvability and genetic correlations of two pollen and pistil traits involved in a sexual conflict over timing of stigma receptivity in Collinsia heterophylla (Plantaginaceae)

Background and Aims

Heritable genetic variation is crucial for selection to operate, yet there is a paucity of studies quantifying such variation in interactive male/female sexual traits, especially those of plants. Previous work on the annual plant Collinsia heterophylla, a mixed-mating species, suggests that delayed stigma receptivity is involved in a sexual conflict: pollen from certain donors fertilize ovules earlier than others at the expense of reduced maternal seed set and lower levels of pollen competition.

Methods

Parent–offspring regressions and sib analyses were performed to test for heritable genetic variation and co-variation in male and female interactive traits related to the sexual conflict.

Key Results

Some heritable variation and evolvability were found for the female trait (delayed stigma receptivity in presence of pollen), but no evidence was found for genetic variation in the male trait (ability to fertilize ovules early). The results further indicated a marginally significant correlation between a male''s ability to fertilize early and early stigma receptivity in offspring. However, despite potential indirect selection of these traits, antagonistic co-evolution may not occur given the lack of heritability of the male trait.

Conclusions

To our knowledge, this is the first study of a plant or any hermaphrodite that examines patterns of genetic correlation between two interactive sexual traits, and also the first to assess heritabilities of plant traits putatively involved in a sexual conflict. It is concluded that the ability to delay fertilization in presence of pollen can respond to selection, while the pollen trait has lower evolutionary potential.     

EXPRESSION OF GENETIC VARIATION IN BODY SIZE OF THE COLLARED FLYCATCHER UNDER DIFFERENT ENVIRONMENTAL CONDITIONS

Heritability of body size in two experimentally created environments, representing good and poor feeding conditions, respectively, was estimated using cross-fostered collared flycatcher Ficedula albicollis nestlings. Young raised under poor feeding conditions attained smaller body size (tarsus length) than their full-sibs raised under good feeding conditions. Parent-offspring regressions revealed lower heritability (h²) of body size under poor than under good feeding conditions. Hence, as the same set of parents were used in the estimation of h² in both environments, this suggests environment-dependent change in additive genetic component of variance (V_A), or that the genetic correlation between parental and poor offspring environment was less than that between parental and good offspring environment. However, full-sib analyses failed to find evidence for genotype-environment interactions, although the power of these tests might have been low. Full-sib heritabilities in both environments tended to be higher than estimates from parent-offspring regressions, indicating that prehatching or early posthatching common environment/maternal effects might have inflated full-sib estimates of V_A. The effect of sibling competition on estimates of V_A was probably small as the nestling size-hierarchy at day 2 posthatch was not generally correlated with size-hierarchy at fledging. Furthermore, there was no correlation between maternal body condition during the incubation and final size of offspring, indicating that direct maternal effects related to nutritional status were small. A review of earlier quantitative genetic studies of body size variation in birds revealed that in eight of nine cases, heritability of body size was lower in poor than in good environmental conditions. The main implication of this relationship will be a decreased evolutionary response to selection under poor environmental conditions. On the other hand, this will retard the loss of genetic variation by reducing the accuracy of selection and might help explain the moderate to high heritabilities of body-size traits under good environmental conditions.     

Familial similarity in dental arch form and tooth position.

This study was done to quantify and compare similarities in dental arch form and tooth position among individuals of various degrees of biological kinship. The sample consisted of 360 dental casts selected from 102 Japanese families, each including both parents and one of their offspring. Most of the subjects had well-aligned permanent dentitions. Coefficients of dissimilarity were obtained by orienting two standard-sized dental arch diagrams plotted from occlusal photographs to the position of best fit. Dissimilarity between adults and children from different families was computed and served as the control data. With few exceptions, the mean dissimilarity between parents and offspring was smaller than that of the nonfamilial pairings; however, statistical differences were not always significant. Among all the teeth observed, the position of both upper and lower central incisors was found to exhibit the least familial similarity. Analysis of variance among pairings of different sources of variation suggested the existence of significant additive genetic effect, both autosomal and sex-linked. Relative contribution of the additive genetic effect in the total variability of this sample was estimated for dental arch form and tooth position to be in the range of 55-60% and 39-77%, respectively. Autosomal additive genetic effects appear to strongly account for the variation in position of the second premolar and of the first molar.     

Replicated selection for insulin-like growth factor-1 and body weight in mice

Summary Five generations of divergent selection for plasma concentration of insulin-like growth factor-1 (IGF-1) and for 12-week body weight were carried out in mice, including randomly selected control lines for each trait. All lines were replicated once (12 lines in total). Each replicate line consisted of eight male and eight female parents per generation. Litter size was standardized to eight pups at birth. Mass selection was applied in the selected lines and within-family random selection in the control lines. Blood was taken from the orbital sinus of individual mice at 12 weeks of age for IGF-1 assay. Realized heritabilities were 0.10±0.01 for IGF-1 and 0.41 ± 0.02 for 12-week weight. The realized genetic correlation between IGF-1 and 12-week weight was 0.58 ± 0.01, with a phenotypic correlation of 0.38. Although the genetic correlation between IGF-1 and body weight in mice is moderately positive, 12-week weight responded 3.5 times as fast to weight selection as to selection for IGF-1.