Performance of layer-type chickens as related to body conformation and composition. 1. A static analysis of shank length and body weight at 20 weeks of age.
The growth of an animal part (y) relative to the whole (x), is expressed in terms of an exponential equation, y = alpha x beta, where alpha and beta are growth constants. For the present study x is body weight and y is shank length determined for 10 lines of chickens at 20 weeks of age. The derived values of alpha and beta, assumed to be limiting relative growth constants for bone, were estimated by least squares of the log-transformed (linear) exponential equation. From the means of alpha and beta, calculated for each of seven Leghorn and three Fayoumi lines, individual values of alpha i and beta i were calculated for more than 24,000 pedigreed birds included in 9 years of records. The individual values (i) were calculated as: (formula; see text) If the initial growth constant, alpha, is the primary hereditary growth parameter governing conformation, then the parameter, beta, should not be as highly heritable as alpha. To test this hypothesis, the heritability (h2) of both alpha and beta were calculated for each of the 10 lines. For alpha sire component h2 values ranged from .12 +/- .05 to .45 +/- .10 and for beta from .05 to .41. The pooled h2 estimate of alpha over lines (.31 +/- .02) was not significantly different from that for beta (.31 +/- .02). Compared with the original data on body weight and shank length, the transformation significantly reduced the heritability estimated. We conclude that the parameters, alpha and beta, are useful only in describing phenotypic and not genetic differences in relative growth.
D000818:Animals; D001699:Biometry; D001835:Body Weight; D002645:Chickens / Q000033:anatomy & histology* / Q000235:genetics / Q000254:growth & development; D005121:Extremities / Q000033:anatomy & histology; D005260:Female; D008297:Male; D010641:Phenotype