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The recent discovery of a major gene for rapid postweaning growth has reinforced the hypothesis that other quantitative continuous traits may be influenced by single genes of large effect. However, most methods for the detection of such genes rely on the discovery of multimodality in the population frequency distribution. The complicating effects of environment and artificial selection make the identification of such genes with field-collected data a formidable problem. An index is proposed that may serve as an indicator that a major gene is segregating within a population. The index is based on the assumption that under polygenic inheritance, an offspring's deviation from the midparent average is smaller than the deviation from either parent. Whereas, for the Mendelian segregation of a major gene, the opposite would be expected. A proposed class of indices is then based on the ratio |O − .5(S + D) |k/(|O − S |k/2 ?O − D |k/2 ) where O, S and D are the additive genetic values of an offspring and its sire and dam estimated via best linear unbiased prediction. Values of the index greater than 1. would be indicative of major gene inheritance. Simulation of small populations indicates that the index is quite sensitive to the existance of segregating major genes even in the absence of multimodality of the phenotypic distribution. However, the index remains dependent on the accuracy of genetic value estimation.

Conception rate, prenatal survival and litter size were recorded for 444 ewes of two age groups from five lines of grade Targhee sheep: two unselected control lines, HC1 and DC(C); two lines selected for 20 yr for increased 120-d weight, HW and DH(W); and a line selected for 18 yr for increased multiple births, T. Line T was equal or superior to the control lines in conception rate, prenatal survival and litter size in both age groups, although most of the differences were not significant. The W selected lines were inferior to the C and T lines in fertility and tended to be lower in prenatal survival, among mature ewes, resulting in a significantly lower number of lambs born per corpus luteum in the W lines than in the other two groups. Among yearlings, C ewes were non-significantly lower in fertility than T and W ewes, while W ewes were significantly lower than C and T ewes in prenatal survival. The T line ewes had higher overall reproductive performance than either of the other two groups. Ewes with two ovulations had a significantly higher conception rate than ewes with single ovulations. Gestation period was exceptionally uniform with a coefficient of variation of 1.3% and little difference due either to line or litter size. It was concluded that selection for multiple births improved overall reproductive performance, whereas selection for increased growth rate had an adverse effect on several components of reproduction, leading to a net decline in fitness.

Ovulation rate was measured by laparoscopy at two consecutive cycles on 366 ewes 2 yr old and over and 85 yearling ewes of five lines of Targhees from the base base population; 53 yearling linecross ewes were also included. The lines were two unselected controls (HC1 and DC), two selected for 21 yr for increased 120-d weight (HW and DH) and one selected for 19 yr for multiple births (T). Ewes were synchronized in late July or early August at the start of the normal breeding season with intravaginal pessaries impregnated with 60 mg methylacetoxyprogesterone and examined at first and second estrus. Ovulation had occurred in both cycles in 327 (89%) and 177 (85%) of the mature and yearling ewes, respectively. Overall mean numbers of corpora lutea at first and second estrus were 1.42 and 1.63, respectively for ewes 2 yr and over and 1.20 and 1.44 for yearlings, indicating an effect of synchronizing treatment, season, flushing, or a combination of these. Among mature ewes, ovulation rate was higher (P<.05) in DH (+.20), HW (+.19) and T (+.16) than in controls at first estrus, and in HW (+.29) and T (+.21) but not DH (−.04) at second estrus. Among yearlings, differences were significant only at second estrus (HW, +.40; T, +.35) and again not for DH (+.08). The failure of line DH to increase in ovulation rate from first to second estrus as did other lines was transmitted to linecross progeny. Body weight within line affected ovulation rate significantly, with a greater effect at second estrus, in both age groups. Adjustment for body weight removed the difference between HW and controls but not between T and controls. Repeatability of corpora lutea count was .27 and .25 for mature and yearling ewes, respectively.

Previous research from this laboratory identified the existence of a major gene for rapid growth in mice. This trait is expressed as rapid postweaning growth and is inherited as a single recessive gene, designated hg. The hg/hgmales and females had greater (P<.05) body weights at 49-d of age as compared with their Hg/- counterparts; 45.6 ± .64 and 34.0 ± .64 vs 33.2 ± .23 and 27.3 ± .15 g, respectively. Twenty-four male littermate pairs (hg/hgand Hg/-) were euthanized at 49 d of age for determination of carcass composition. Percentage body water was not affected by genotype (hg/hg, 66.7 ± .5; Hg/-, 67.8 ± .3). However, there was a decrease (P<.05) in percentage body protein (20.2 ± .1 vs 20.8 ± .1) and body ash (3.4 ± .01 vs 3.7 ± .01) in hg/hgmales as compared with Hg/- males, while percentage lipid was significantly increased in hg/hgmales (8.7 ± .5 vs 6.7 ± .3). When carcass weights were adjusted for differences in dry weight, only body protein was significantly affected by genotype, being greater for hg/hgmales (8.54 ± .19 vs 7.70 ± .19 g). All other components of body composition were not affected by genotype. The regression coefficient for the covariate of dry weight was significantly different from zero for all carcass components. However, there was no significant difference in the regression coefficient between Hg/- and hg/hgmales for each component. The hg/hgmice were not obese suggesting that these mice may be a valuable model for the study of genetic influences upon growth rate and mature body weight.

Frequently, exotic, hybrid and native sires are evaluated based on progeny records, using additive models for direct effects and assuming common genetic variances across breed groups. In some cases, however, additive maternal and nonadditive direct and maternal fixed and random effects may be important. Also, heterogeneous variances across genetic groups may exist. Hence, sire proofs from additive models will be biased and not of minimum variance. Sire-maternal grandsire models that account for direct and maternal, additive and nonadditive fixed and random effects as well as different additive and nonadditive variances across genetic groups are presented. Direct and maternal effects are treated as separate characters. Sire proofs for direct and maternal traits contain group (fixed) and sire (random) additive and nonadditive effects. Problems of confounding and multicollinearity may arise due to empty cells in the design. Ridge regression may be used to help remove problems of multicollinearity. Extension to multitrait evaluation is straightforward.

Previous research in this laboratory demonstrated the existence of a major gene (hg), inherited as a homozygous recessive, which increases postweaning growth by 40 to 50% in C57B1/6 mice (Ch) compared to the same genetic stock without the major gene (CH). Although its effect has not been previously evaluated, this single recessive allele is also in a line of mice selected for rapid postweaning gain for over 70 generations. Gh represents that line of mice with the major gene for growth (hg) in the growth-selected background and GH the growth-selected background without the major gene. Total body weight, daily weight gain, feed consumption and gain/feed, measured daily from 21 to 42 d of age, were all significantly greater (P<.01) in the two lines with the hg/hg genotype (Ch and Gh) compared with their respective control lines (CH and GH). Differences in body composition at 42 d of age between CH compared with Ch and GH compared with Gh were accounted for by difference in body weight. Gross and net energetic efficiency, calculated assuming a similar maintenance energy requirement, were improved (P<.01) in Ch and Gh compared to CH and GH, respectively. The results demonstrated that hg influences growth in growth-neutral and growth-selected backgrounds. The gene also alters energy metabolism by increasing energetic efficiency of growth and(or) decreasing maintenance energy requirement.