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The effectiveness of selection for high and low body weight at six weeks of age was studied in descendants of X-irradiated (R) and nonirradiated (C) inbred rats. There were two replicates of each of the direction of selection irradiation treatments. In C lines, there were no consistent responses to selection, probably due to a low level of genetic variability. In R rats, selection was effective only for decreased body weight. The results of this experiment do not suggest the use of irradiation combined with selection as a means of enhancing responses to selection in animals.

The genetic effects of one generation of spermatogonial X-irradiation in rats, by a single dose of 600r in one experiment and by a fractionated dose of 450r in another, were measured in three generations of their descendants. Estimates of dominant lethal mutation rates—(2 to 3) × 10 -4/gamete/r—from litter size differences between irradiated and nonirradiated stock were consistent with previous estimates from rats and mice. Similar consistency was found for estimates of sex-linked recessive mutation rates—(1 to 2) × 10-4 chromosome/r—from male proportions within strains; however, when measured in crossbreds the proportion of males was higher in the irradiated than in the nonirradiated lines. This inconsistency in results is in keeping with the contradictory results reported for recessive sex-linked lethal mutation rates in mice. The effects used to estimate recessive lethal mutation rates which were unusually high—(2 to 14) × 10-4/gamete/r—were not significant. Other factors that could have contributed to the observed effects are postulated.

Effects of nine generations of 450r per generation of ancestral spermatogonial X irradiation of inbred rats on body weight were examined. After six generations of random mating (avoiding inbreeding) following the termination of irradiation, descendants of irradiated males (R) were significantly lighter than their controls (C) at 3 and 6 weeks, but not at 10 weeks of age. However, differences in growth between R and C populations were small. Among-litter and within-litter variance estimates were generally larger in the R lines than in the C lines, suggesting that selection responses would be greater in R than in C lines. In conjunction with previous evidence—obtained during the irradiation phase of the experiment—this suggested that more rapid response to selection for 6-week body weight, in particular, might accrue in the R lines.

Effects of nine generations of 450r per generation of ancestral spermatogonial X irradiation of inbred rats on genetic parameters of body weight at 3, 6 and 10 weeks of age and of weight gains between these periods were studied. Covariances among relatives were estimated by mixed model and regression techniques in randomly selected lines with (R) and without (C) radiation history. Analyses of the data were based on five linear genetic models combining additive direct, additive indirect (maternal), dominance and environmental effects. Parameters in these models were estimated by generalized least-squares. A model including direct and indirect genetic effects fit more closely to the data in both R and C lines. Overdominance of induced mutations did not seem to be present. Ancestral irradiation increased maternal additive genetic variances of body weights and gains but not direct genetic variances. Theoretically, due to a negative direct-maternal genetic correlation, within full-sib family selection would be ineffective in increasing body weight at six weeks in both R and C lines. However, progress from mass selection would be expected to be faster in the R lines.

SUMMARY One hundred and .tWO puberal gilts from nine genetic groups were used to determine the association of two morphological types of Ovarian follicular loss (hemorrhagic and milky) with genetic background of ~gilts, level of feed intake and stage of estrous cycle. At estrus (day 0 of estrous cycle) a lesser prqportion of gilts (P

The effectiveness of selection for 3–9-week gain was examined in a population of rats with a history of past selection for high 3–9-week gain. Lines were selected for high (U line) and low (D line) 3–9-week gain with two replicates of each line. Two randomly selected lines were also kept, one originating from the same base population as the two selected lines (R line) and the other originating from a population that had been randomly mated for the previous 27 generations (C line). Two replicates of each of these lines were kept. After seven generations of selection, a randomly selected line (relaxed line) was formed from each of the two upward- and each of the two downward-selected lines. Results have been presented for 13 generations of selection. The environmental trend for 3–9-week gain, as indicated by the randomly selected R and C lines, was consistently negative in all four lines. Realized heritabilities calculated by deviating the response to selection from the trend in the R or C lines resulted in non-significantly higher values in the D lines than the U lines. Six generations of relaxation of selection indicated no effect of natural selection in the U lines or the D lines. The relative magnitude of the drift, error and common environmental variances were estimated by the methods given by Hill (1971). The estimates of these parameters then led to calculation of the degree of bias in the sampling variances of the realized heritability estimates. As was predicted by Hill (1971), estimates of the variance of realized heritabilities obtained by using standard regression techniques were less than those obtained using Hill's formulae. The results are discussed in relation to other similar studies with rats and mice.

Estimates of the effects of inbreeding and outbreeding in dairy cattle were made in an experiment designed specifically to study these effects in a population in which no artificial selection was practiced. The basic groups in this study were 6 unrelated lines of Holstein cattle; each line was based on a foundation stock of 1 sire, 1 or 2 inbred sons and 20 noninbred daughters. Animals were mated within each line and between lines to produce inbred and “outbred” progeny, respectively. Further outbreeding was conducted by crossing the inbred animals from different lines to produce 2-, 3-, and 4-line crosses. Only a fraction of the total posssible crosses was made. Significant (P

Summary Growth and production data of inbred and outbred progeny of Holstein-Friesian sires in three Wisconsin State Department of Public Welfare herds were collected and analyzed to determine the effect of inbreeding on body dimensions (height at withers, circumference of shin bone, heart girth and width of hips) at 6 and 18 months of age and at maturity and on milk and butterfat production and butterfat test. The average intra-sire partial regression (holding mature size of dam constant) of dimension on inbreeding was used to measure this effect. The partial regression of dimensions on inbreeding was essentially zero, except for heart girth at 18 months and maturity, which was significant only at 18 months. Intra-sire partial regressions (holding dam's record constant) of milk and butterfat production of 42 inbred and 47 outbred cows on inbreeding were significant and amounted to an average decrease of 74lb. of milk and 2.3lb. of butterfat for each 1 per cent increase in inbreeding. No evidence of an effect of inbreeding on butterfat percentage was indicated. Considerable variation was found in the partial regression coefficients of sires, indicating that offspring of some sires could be inbred without any apparent decrease in body size or production, possibly in part because the beneficial effect of increasing the relationship to a sire transmitting a superior level of production tends to offset detrimental effects of inbreeding to him.

Summary Factors affecting the incidence of cystic ovaries have been examined in a herd of Holstein cattle. The records covered 341 cows and 1,280 cow-service periods over a 10-yr. period. Cystic ovaries occurred in 18.8 per cent of the cows and 7 per cent of the service periods. The proportion of the cows becoming cystic for the first time averaged 0.058 per service period and no evidence was obtained for significant variation in this rate between service periods. Heritability of the occurrence of cystic ovaries sometime during life was estimated for this herd as 0.43. No evidence was obtained for an association between inherent level of butterfat production or test and the occurrence of cystic ovaries. Cows when not milked (mainly in the heifer service period) showed an incidence of cystic ovaries of 3.4 per cent; those in the milking herd, 6.8 per cent; those started on test and then removed to the milking herd, 8.5 per cent; and those on official test, 10.6 percent.

Summary Butterfat production per, day of life was evaluated as a possible selection criterion for genetic improvement of dairy cattle. The data provided little evidence of heterogeneity of the correlations of production to 36 mo. of age (and 305-day first lactation production) with production to 48, 60, 72 and 84 mo. of age. The predictive values of 36-mo. production and 305-day first lactation production to production at these same ages did not differ significantly. Age at first calving had significantly less influence on total production to 84 mo. than on total production to 36 mo. of age. Age at first calving and first 305-day lactation production accounted for 89 percent of the variance in 36-mo. production and 41 percent of the variance in 84-mo. production. There was no evidence of heritability of production per day of life at any of the ages studied from 36 to 84 mo., nor was age at first calving indicated to be heritable. The estimate of heritability of first lactation butterfat production from these data was 0.30.

An experiment designed to study the effect of various mating systems on sire-lines of dairy cattle is described. Inbreeding depressions were found in various production traits by comparing outbreds with inbreds (F, ≅ 0.25). The differences were 1,313 lb, 35 lb, 1,761 lb, and 48 lb in favor of the former, in actual milk, actual fat, M.E. milk, and M.E. fat. of the first lactation, respectively. For the same traits, differences of 1,832 lb, 74 lb, 2,455 lb, and 97 lb, respectively, were found in favor of the two-line crosses compared to inbreds of the same line. However, inbreeding effects varied significantly among sire-lines. There was no evidence that increase of production with age was more rapid in inbreds than in outbreds.