Section 9 — Population Genetics 



Beginning in the 5th generation, two or more 

 all-white dark-eyed males have been obtained 

 in each generation for a total of eleven. One 

 all-white female was obtained in the 8th genera- 

 tion from an all-white sire. Tests show the ab- 

 sence of albino and the absence of genes domi- 

 nant to wild type. 



The tremendous changes in the mean and 

 variance in this closed population from an initial 

 level of essentially zero are of primary impor- 

 tance. Although the mean of this sample does 

 not approach the absolute limit of selection, a 

 dozen individuals have reached the phenotypic 

 limit. 



9.36. Effects of Mating System on Ovulation, 

 Implantation and Litter Size in Mice. T. G. 



Martin, F. E. Harrington and D. L. Hill 

 (Lafayette, U.S.A.). 



Four inbred lines of mice were mated in a 

 diallel mating system producing sixteen genetical- 

 ly different groups of offspring. Samples of four 

 females for each trait in each genetic group were 

 drawn to estimate number of ova shed, number of 

 implantation sites and litter size. Embryonic 

 mortality was estimated by number of ova 

 shed minus litter size. Pureline versus crossline 

 comparisons made in the parental generation 

 yielded only one statistically significant differ- 

 ence with crossline and pureline matings having 

 9.17 and 8.73 implantations, respectively. Ave- 

 rages for the other traits were: litter size 8.33 

 and 8.38, ova shed 9.88 and 10.25, and embry- 

 onic mortality 25.9 and 26.9 per cent for cross- 

 line and pureline matings, respectively. In the 

 Fi generation, crossbred and pureline matings 

 were compared. Crossbreds performed at signi- 

 ficantly higher levels than pureline mice in all 

 traits except ovulation rate. Crossbred averages 

 were 11.24 ova, 11.39 implantations, 9.02 off- 

 spring and 19.8 per cent embryonic mortality. 

 Pureline averages were 10.29 ova, 8.63 implanta- 

 tions, 7.53 offspring and 29.0 per cent embryonic 

 mortality. Analysis of the Fi data indicated 

 strongly that specific combining ability was 

 a more important source of variation than gene- 

 ral combining ability thus indicating that a 

 large portion of genetic variation is due to 

 dominance and epistasis and a smaller portion 

 due to additive genetic effects. Mating system 

 influenced time of embryonic death with 36, 

 58 and 100 per cent of embryonic deaths occurr- 

 ing after implantation in pureline, crossline, 

 and crossbred matings, respectively. 



9.37. Correlated Responses to Selection for Radia- 

 tion Resistance in Mice. Thomas H. Rode- 

 rick, (Bar Harbor, U.S.A.). 



Selection for survival under daily doses of 

 X-rays was successful in two experiments: (1) 

 under lOOr/day, and (2) under 400r/day. Litter 

 size was a correlated response in both experi- 

 ments, and body weight only in the first experi- 

 ment. W 



The correlated response in body weight was 

 only in the low direction, and that response was 

 more pronounced in the males than in the fe- 

 males. For the females, the means and standard 

 errors for body weight were: 39.3 ± 1.0 g for 

 the high line, 39.4 ± 1.1 g for the unselected 

 control line, and 35.6 ± 1.0 g for the low line. 

 For the males: 41.8 ±0.5 g for the high line, 

 44.6 ± 0.6 g for the unselected control line, and 

 29.6 :; 0.8 g for the low line. 



The first selection experiment also produced a 

 correlated response in LDso:3o° LDso:3o means 

 and standard errors for the females were: 

 905 ± 18r for the high selected line, 833 ± 

 21r for the unselected control line, and 788 

 ± 21r for the low selected line. Survival under 

 400r/day was also a correlated response in the 

 direction of selection. 



This study suggests that, although the selected 

 trait appears to have responded symmetrically, 

 body weight (a highly correlated character) 

 has responded asymmetrically. These results in 

 turn suggest that the physiology of response 

 in the upward direction is very different from the 

 physiology of response in the lower direction. 

 It is possible that the loci involved in upward 

 selection of this trait are quite different from the 

 loci involved in downward selection. 



1. Roderick, Genetics 48, in press. 



9.38. Evidence that Selection for the Maintenance of 

 Developmental Homeostasis may Lead to the 

 Retention of Genetic Variability. H. T. Band 



(Vancouver, Canada). 



Changes in the lethal + semilethal frequencies 

 in the South Amherst D. melanogaster popula- 

 tion can be correlated with changes in environ- 

 mental variables (Band and Ives, 1961). Analysis 

 of homozygotes and random heterozygotes 

 containing second chromosomes from the 1960 

 population suggest that the gene pool is inte- 

 grated on two levels: (1) in the maintenance of 

 developmental homeostasis and (2) in the magni- 



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