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Testing for Over-Dominance and Epistasis1
Testing for Over-Dominance and Epistasis1 F . A. H A Y S
University of Massachusetts, Amherst (Received for publication January 11, 1954)
' T p H E Rhode Island Red flock at the -*• Massachusetts Experiment Station has been pedigree bred as a closed flock for more than 20 years. The chief objectives have been increased fecundity along with high viability. In recent years mean egg production has not increased at a satisfactory rate. It is conceivable that overdominance, epistasis, or decreased genetic variability may be operating here. Disease control measures have been effective, environmental conditions have been regulated, and inbreeding has been held below 5 percent. The last five generations have shown the mean first-year egg production to be about 235 for survivors with no significant fluctuation from year to year. Since progress has been greatly reduced, it seems desirable to search for some of the possible causes. Experimental Data Available. Birds hatched in nine generations from 1943 to 1951 were studied. Methods of management, feeding, and housing were kept constant through the period. All females were trapnested from first egg through 365 days, and only families of full sisters with five or more survivors were considered. Methods of Interpretation. Female breeders were divided into five classes as follows: (1) low, 200 to 224 eggs; (2) medium, 225 to 249 eggs; (3) high, 250 eggs or more; (4) below the flock mean of 235 eggs; and (5) above the flock mean. Only those males that were mated to two or more classes of females were included so that the effect of selection of dams might be measured by the performance of their daughters. Identical Males Mated to Dams of 1 Contribution No. 937 from the Massachusetts Agricultural Experiment Station.
Classes 1, 2 and 3. Four sires were mated to low, medium, and high dams. From four dams of the low group, 34 daughters averaged 197 + 5.91 eggs! These same males mated to 6 medium dams gave 43 daughters averaging 209 + 6.10 eggs. The four males when mated to 13 dams of the high group had 97 daughters with a mean of 224 + 3.44 eggs. The magnitude of the standard error of the means shows that the daughters from low and high dams differed significantly in egg production, and the differences between the means of daughters from medium and high dams were barely significant. The most probable explanation of these results would appear to lie in low heritability of fecundity, which requires high selection pressure to produce significant differences. Identical Males Mated to Dams of Classes 1 and 3. Four sires were mated to low dams and high dams. From 4 low dams 30 daughters were tested and had a mean record of 227 + 6.79 eggs. These sires when mated to 9 high dams produced 65 daughters with a mean production of 230 + 4.39 eggs. The mean difference in the daughters' egg record was only 3 eggs and was insignificant. Identical Males Mated to Dams of Classes 2 and 3. A total of 23 sires when mated to 32 medium dams gave 273 daughters with a mean egg record of 226 + 2.13 eggs. From the same 23 sires when mated to 53 high dams, 406 daughters were produced with a mean egg production of 232 + 1.71 eggs. The difference amounted to 6 + 2.73 eggs and was significant. Ten Males Mated to Dams Below and Above the Mean of the Flock. Ten sires were mated to 12 dams whose record was below
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the flock mean of 235 eggs. These 12 dams averaged 213 eggs. The same ten sires were mated to 32 dams whose record was above the flock mean. These 32 dams averaged to lay 269 eggs. From the low dams, 99 daughters averaged 212 + 4.04 eggs, and from the high dams 236 daughters averaged 227 + 2.25 eggs. The difference in egg production of the two groups of daughters was 15 + 4.62 eggs and was statistically significant. Selection differential in the dams that were above the flock mean was 269-235 or 34 eggs, whereas all the daughters from the two classes of dams averaged to lay 223 eggs. Daughters from above average dams had a mean production of 227 eggs. The difference between the mean of all daughters and daughters from dams above the flock mean was 227-223 or 4 eggs. If this difference of 4 is divided by 34, which is the selection differential, we obtain an estimate of heritability of about 12 percent for egg production. Phenotypic variation as measured by the coefficient of variation in daughters may be some indication of genetic variability. The standard deviation in production of daughters from dams that produced below the flock mean was 40.23 eggs. The coefficient of variation in production was 19.24 percent. The standard deviation in production of the daughters from dams that laid more than the flock mean was 34.64 eggs. The coefficient of variation for this group of daughters was 15.24 percent. Evidently genetic variability was not greatly reduced in the dams that produced above the flock mean. Inheritance of Fecundity. Methods of calculating the heritability of egg production have been developed by Lush (1945), Lerner (1950), and others. These methods are intended to bring out the fraction of the differences in egg production between individuals that is due to cumulative genes and not to environmental factors. For example, if the heritability of egg
production is 30 percent and the flock average is 225 eggs, what might be expected for the daughters of breeding hens that laid 275 eggs provided they were mated to equally good males? The selection differential is 275-225 or 50 eggs. The assumption is that 30 percent of this difference between the selected dams and the flock as a whole is genetic. Therefore, 50 X 30%= 15 eggs; 225 + 15 = 240, which might be expected for the mean in the next generation. Application to our Data. Using the data previously considered in which 31 males were mated to 121 dams to produce 948 daughters with egg records from 1943 to 1951, we can calculate the heritability of egg production in our flock. By using analysis of variance method suggested by Lush, Lamoreux and Hazel (1948) with slight modifications suggested by Lush through correspondence, we have calculated the heritability of egg production from our data. The fact should be noted that variance between years was high and its effect was removed. Using the t l method suggested by Lush and associates, the heritability turns out to be .1808. Breeding females have averaged 270 eggs, and good males have been used. Genetic variability is somewhat reduced in the high producing families, and environmental effects may be rather adverse, yet it appears to us that we still lack evidence on the course of the plateauing effect in egg production. The flock has been closed for more than 20 years and completely pedigreed, and calculations show no significant amount of inbreeding. REFERENCES Lerner, I. M., 1950. Population Genetics and Animal Improvement, Cambridge University Press, Cambridge, England. Lush, J. L., 1945. Animal Breeding Plans, Iowa State College Press, Ames, Iowa. Lush, J. L., W. F. Lamoreux and L. N. Hazel, 1948. The heritability of resistance to death in the fowl. Poultry Sci. 27:375-388.
University of Massachusetts, Amherst (Received for publication January 11, 1954)
' T p H E Rhode Island Red flock at the -*• Massachusetts Experiment Station has been pedigree bred as a closed flock for more than 20 years. The chief objectives have been increased fecundity along with high viability. In recent years mean egg production has not increased at a satisfactory rate. It is conceivable that overdominance, epistasis, or decreased genetic variability may be operating here. Disease control measures have been effective, environmental conditions have been regulated, and inbreeding has been held below 5 percent. The last five generations have shown the mean first-year egg production to be about 235 for survivors with no significant fluctuation from year to year. Since progress has been greatly reduced, it seems desirable to search for some of the possible causes. Experimental Data Available. Birds hatched in nine generations from 1943 to 1951 were studied. Methods of management, feeding, and housing were kept constant through the period. All females were trapnested from first egg through 365 days, and only families of full sisters with five or more survivors were considered. Methods of Interpretation. Female breeders were divided into five classes as follows: (1) low, 200 to 224 eggs; (2) medium, 225 to 249 eggs; (3) high, 250 eggs or more; (4) below the flock mean of 235 eggs; and (5) above the flock mean. Only those males that were mated to two or more classes of females were included so that the effect of selection of dams might be measured by the performance of their daughters. Identical Males Mated to Dams of 1 Contribution No. 937 from the Massachusetts Agricultural Experiment Station.
Classes 1, 2 and 3. Four sires were mated to low, medium, and high dams. From four dams of the low group, 34 daughters averaged 197 + 5.91 eggs! These same males mated to 6 medium dams gave 43 daughters averaging 209 + 6.10 eggs. The four males when mated to 13 dams of the high group had 97 daughters with a mean of 224 + 3.44 eggs. The magnitude of the standard error of the means shows that the daughters from low and high dams differed significantly in egg production, and the differences between the means of daughters from medium and high dams were barely significant. The most probable explanation of these results would appear to lie in low heritability of fecundity, which requires high selection pressure to produce significant differences. Identical Males Mated to Dams of Classes 1 and 3. Four sires were mated to low dams and high dams. From 4 low dams 30 daughters were tested and had a mean record of 227 + 6.79 eggs. These sires when mated to 9 high dams produced 65 daughters with a mean production of 230 + 4.39 eggs. The mean difference in the daughters' egg record was only 3 eggs and was insignificant. Identical Males Mated to Dams of Classes 2 and 3. A total of 23 sires when mated to 32 medium dams gave 273 daughters with a mean egg record of 226 + 2.13 eggs. From the same 23 sires when mated to 53 high dams, 406 daughters were produced with a mean egg production of 232 + 1.71 eggs. The difference amounted to 6 + 2.73 eggs and was significant. Ten Males Mated to Dams Below and Above the Mean of the Flock. Ten sires were mated to 12 dams whose record was below
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the flock mean of 235 eggs. These 12 dams averaged 213 eggs. The same ten sires were mated to 32 dams whose record was above the flock mean. These 32 dams averaged to lay 269 eggs. From the low dams, 99 daughters averaged 212 + 4.04 eggs, and from the high dams 236 daughters averaged 227 + 2.25 eggs. The difference in egg production of the two groups of daughters was 15 + 4.62 eggs and was statistically significant. Selection differential in the dams that were above the flock mean was 269-235 or 34 eggs, whereas all the daughters from the two classes of dams averaged to lay 223 eggs. Daughters from above average dams had a mean production of 227 eggs. The difference between the mean of all daughters and daughters from dams above the flock mean was 227-223 or 4 eggs. If this difference of 4 is divided by 34, which is the selection differential, we obtain an estimate of heritability of about 12 percent for egg production. Phenotypic variation as measured by the coefficient of variation in daughters may be some indication of genetic variability. The standard deviation in production of daughters from dams that produced below the flock mean was 40.23 eggs. The coefficient of variation in production was 19.24 percent. The standard deviation in production of the daughters from dams that laid more than the flock mean was 34.64 eggs. The coefficient of variation for this group of daughters was 15.24 percent. Evidently genetic variability was not greatly reduced in the dams that produced above the flock mean. Inheritance of Fecundity. Methods of calculating the heritability of egg production have been developed by Lush (1945), Lerner (1950), and others. These methods are intended to bring out the fraction of the differences in egg production between individuals that is due to cumulative genes and not to environmental factors. For example, if the heritability of egg
production is 30 percent and the flock average is 225 eggs, what might be expected for the daughters of breeding hens that laid 275 eggs provided they were mated to equally good males? The selection differential is 275-225 or 50 eggs. The assumption is that 30 percent of this difference between the selected dams and the flock as a whole is genetic. Therefore, 50 X 30%= 15 eggs; 225 + 15 = 240, which might be expected for the mean in the next generation. Application to our Data. Using the data previously considered in which 31 males were mated to 121 dams to produce 948 daughters with egg records from 1943 to 1951, we can calculate the heritability of egg production in our flock. By using analysis of variance method suggested by Lush, Lamoreux and Hazel (1948) with slight modifications suggested by Lush through correspondence, we have calculated the heritability of egg production from our data. The fact should be noted that variance between years was high and its effect was removed. Using the t l method suggested by Lush and associates, the heritability turns out to be .1808. Breeding females have averaged 270 eggs, and good males have been used. Genetic variability is somewhat reduced in the high producing families, and environmental effects may be rather adverse, yet it appears to us that we still lack evidence on the course of the plateauing effect in egg production. The flock has been closed for more than 20 years and completely pedigreed, and calculations show no significant amount of inbreeding. REFERENCES Lerner, I. M., 1950. Population Genetics and Animal Improvement, Cambridge University Press, Cambridge, England. Lush, J. L., 1945. Animal Breeding Plans, Iowa State College Press, Ames, Iowa. Lush, J. L., W. F. Lamoreux and L. N. Hazel, 1948. The heritability of resistance to death in the fowl. Poultry Sci. 27:375-388.