Selection on an index of two traits was represented graphically by means of a response ellipse or response circle. This procedure was used to find optimal index weights for uncorrelated traits, and to compare the efficiency of alternative indices.Profit was expressed as a reciprocal function of reproductive performance, from which it was shown that the higher the reproductive performance, the smaller the weight that should be applied to it in a selection index. It was found that in pigs and broilers the average commercial standard of reproductive performance is sufficiently high that selection on production traits alone is almost as efficient as selecting on the optimum index.Three alternative procedures for selecting from a single original line were compared graphically: A. Maintaining the single line and selecting all animals on the same index. B. Maintaining the single line but selecting males and females on separate indices. C. Splitting the original line into separate sire and dam lines, each selected on a specialised index. Method C was found to be the most efficient of the three; B was more efficient than A for one generation, but less efficient in subsequent generations.
SUMMARYGrowth rate of 12 groups of common carp was measured at five experimental environments. Three of the 12 tested groups were strains of the domesticated European race of the common carp, one group was a representative of the Big-Belly Chinese race, and the remaining eight groups were F1 crossbreds among the European strains and between the European and the Chinese races. The average growth rate over the five environments of the Chinese Big-Belly was considerably poorer than that of the European carp. All the inter-race crossbreds and the crossbreds among the European strains showed heterosis.When the genotype-environment interaction was presented as a linear function of the quality of the environment, the regression coefficient (the overall responsiveness parameter) assumed relatively low values in the Big-Belly and two to two-and-a-half fold higher values in the European carp. The overall responsiveness of crossbreds was, on the average, intermediate between the two parents. When, however, it was partitioned into a scale function of the average genotype and specific independent responsiveness, the two components showed a high degree of heterosis but in opposite directions. An explanation of this genetic system in terms of adaptive evolution to the diverse modes of carp domestication in Europe and China was given.
Gonad development was studied in young carp of the European and Chinese (big-belly) races. The effects of age and body weight on gonad and egg size were partitioned and the following findings were made: (i) Gonad weight was correlated, in both males and females, with body weight, but proportional gonad weight, i.e. the ratio gonad weight divided by body weight, was independent of body weight. (ii) Proportional gonad weight had reached its maximum value in one year old males, but it more than doubled in two year old females. (iii) Egg size of two year old females was more than twice the size of eggs of one year old females. (iv) The European and Chinese races of carp differed in several aspects of gonad development. In particular, the Chinese matured earlier and their relative gonad sizes were considerably larger. (v) Genetic variation in gonad development was also found within the European race, where faster rate of development is dominant over slower rate. The evolutionary implications of the above differences were discussed.
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