Genetical studies on growth and form in the fowl 1. Phenotypic variation in the relative growth pattern of shank length and body-weight

Abstract

Body-weight and shank length from 2 weeks of age to adult (and from 8 weeks onwards, shank width) have been measured on 154 fowls all hatched on the same date, belonging to two F1breed crosses: White Leghorn × Rhode Island Red (L × R) and Indian Game × Light Sussex (G × S). After logarithmic transformation the data have first been analysed cross-sectionally (analysis of the age-means for each sex and cross). A longitudinal analysis (fitting a regression line to the data of each individual) has then been made of the approximately linear portions of the curves. The following conclusions are drawn.(1) Growth in shank length relative to body-weight between 2 and 10 weeks conforms closely to simple allometry. The coefficient of ontogenetic allometry (heter-auxesis),k, is approximately 0·4, being 0·02 higher in L × R than in G × S and 0·05 higher in males than in females. In femaleskdeclines (eventually to zero) after 10 weeks; the decline occurs about 4 weeks later in males. For shank width relative to body-weightkis about 0·25.(2) At a given body-weight males have longer and thicker shanks than females; L × R have longer but thinner shanks than G × S.(3) Within sexes and crosses there is highly significant individual variation ink, but the allometry lines do not pass, within the limits of error, through any single point. This implies that variation in relative shank length is complex in its ontogenetic origin.(4) There is no appreciable correlation within sexes and crosses between shank width and shank length at a given body-weight; this implies (as does (2)) that variation is also complex anatomically.(5) Differences in shape and rate-of-change of shape contribute only a small part of the total variation within sexes and crosses; most is due to differences ingeneralsize andgeneralgrowth rate.(6) Shank width at a given body-weight is positively correlated with body-weight at a given age (r= + 0·36 within sexes and crosses). This agrees with the finding that the coefficient of static allometry (allomorphosis) for shank width is much higher than the ontogenetic coefficient. For shank length the ontogenetic and static coefficients are approximately the same.