GENETIC ANALYSES OF BANTAM AND SELECTED LOW-WEIGHT WHITE PLYMOUTH ROCK CHICKENS AND THEIR CROSSES .2. ONSET OF SEXUAL MATURITY AND EGG-PRODUCTION

Two generations of crosses between a population of White Plymouth Rock Bantams and a line of White Plymouth Rocks selected for low body weight for 31 generations were produced. Characteristically, the Bantams had produced a high proportion of normal eggs, but hen-day normal egg production in this population was low. The hens selected for low body weight also produced a high proportion of normal eggs, but age at sexual maturity had been greatly delayed as a correlated response to selection. In the 16 populations involving parental lines and crosses (F1, F2 and backcrosses), age at onset of sexual maturity and egg production traits were measured. Comparisons of these populations suggested that age at sexual maturity was influenced by sex linkage, by one or more genes with major effects and by other genes with lesser effects. When compared to performance of the parental populations, some measures of reproductive fitness were improved by crossing (age at first semen production, age at first egg, hen-day normal egg production). Other traits were not changed by crossing (percent normal eggs, duration of fertility).