Midparent

Abstract: Measures of familial resemblance (spouse correlations, regressions of offspring on midparent, single-parent/single-child correlations, and sibling correlations) are presented for members of the two largest ethnic groups tested in the Hawaii Family Study of Cognition. Median spouse correlations (corrected for differences in test reliability) for 15 individual tests of specific cognitive abilities are 0.15 and 0.12 for Americans of European and of Japanese ancestry, respectively. With regard to the regressions of offspring on midparent value, corresponding median values are 0.50 and 0.35. Results of hierarchical multiple regression analyses, as well as the ordering of single-parent/single-child and sibling correlations, provide no evidence to support the hypothesis that spatial ability is influenced by a sex-linked, recessive gene.

Abstract: Heterosis is widely used in breeding, but the genetic basis of this biological phenomenon has not been elucidated. We postulate that additive and dominance genetic effects as well as two-locus interactions estimated in classical QTL analyses are not sufficient for quantifying the contributions of QTL to heterosis. A general theoretical framework for determining the contributions of different types of genetic effects to heterosis was developed. Additive × additive epistatic interactions of individual loci with the entire genetic background were identified as a major component of midparent heterosis. On the basis of these findings we defined a new type of heterotic effect denoted as augmented dominance effect di* that comprises the dominance effect at each QTL minus half the sum of additive × additive interactions with all other QTL. We demonstrate that genotypic expectations of QTL effects obtained from analyses with the design III using testcrosses of recombinant inbred lines and composite-interval mapping precisely equal genotypic expectations of midparent heterosis, thus identifying genomic regions relevant for expression of heterosis. The theory for QTL mapping of multiple traits is extended to the simultaneous mapping of newly defined genetic effects to improve the power of QTL detection and distinguish between dominance and overdominance.

Abstract: In bilateral characters, two kinds of asymmetries are common: fluctuating asymmetry (FA), or nondirectional variation between left and right sides, and directional asymmetry (DA), in which one side is consistently larger than the other. FA has been extensively used as a measure of developmental stability because of its presumed environmental basis whereas DA has not typically been recommended because it has been presumed to have at least some genetic basis. To test these two hypotheses, heritabilities were calculated via parent–offspring regression for both DA and FA in 10 triply measured mandible characters in random-bred mice. Midparent estimates of heritabilities of DA in the 10 characters were quite low (mean = 0.06), but significant for one character as well as the sum of the DA values over all characters (0.21). Midparent estimates of heritability of FA in the 10 characters also were low (mean = 0.03), but not significant for any individual character or the sum of the FA values over all characters. Heritabilities of developmental stability calculated from heritabilities and repeatabilities of FA in the mandible characters were higher in magnitude (mean of midparent estimates = 0.45), but all still were not statistically significant. It was concluded that both hypotheses were supported, but that genetic variation in DA was so small that the potential for DA as an indicator of developmental stability should be explored.

Abstract: Most traits of interest in hazelnut are quantitative, yet there is little information available on heritability, variance components, or correlations. In this study, 13 morphological and four phenological traits were observed for 3 years on 41 parental genotypes, and for 2 years on seed-lings representing 35 progenies. Narrow-sense heritability estimates obtained by regression of offspring means on midparent values were all high, ranging from 0.56 for amount of kernel fibre to 0.89 for nut depth. Data on the parent genotypes was used to analyse variance components and showed that genotypic variance accounted for most of the phenotypic variance for all traits, and there was very little variation among trees within genotypes. Genotype-year interaction accounted for only a very small part of the phenotypic variance for most traits, and changes in genotype rank were few and minor. Thus, selection based on genotype means is appropriate. The largest genotype-year interaction components were observed for time of nut maturity, time of stigma exsertion, time of catkin elongation, number of nuts per cluster and amount of kernel fibre. For these five traits, evaluation over a period of years is advised. Correlation coefficients were large and positive for four nut size traits, indicating some redundancy in these measurements. Three of the phenological traits (time of stigma exsertion, time of catkin elongation, and time of leaf budbreak) were also highly and positively correlated. A large genetic correlation (0.44 < r < 0.73) was noted between good blanching and early phenology (catkin elongation, stigma exsertion and leaf budbreak), although plots of individual seedling values showed a large degree of scattering. Genetic correlations also indicated that seedlings with small nuts also tend to have compressed nuts, short husks and catkins that elongate later, but plots of individual seedling values showed large scattering. Correlations among the six traits directly related to the objectives of the Oregon State University hazelnut breeding programme (kernel weight, per cent kernel, nut shape index, blanching ability, relative husk length and nut maturity) were very low, indicating that they can be handled as independent traits.