Genetic Variation influencing reproduction and growth of White Rock chickens was studied. The populations used were lines high (HH) and low (LL) selected for 56-day body weight, reciprocal F₁ crosses (HL and LH), F₂ crosses of the F₁ and dwarf populations (HD and LD) originating from lines HH and LL.

Inheritance of egg production and egg composition was examined (Experiment 1). Although reciprocal effects were generally unimportant, nonadditive genetic variation was evident for reproductive traits and egg component measures of shell and yolk. Comparisons in Experiment 2 involved dwarf and normal chickens from lines HH and LL. Yolk weight increased while the ratio of albumen to yolk decreased with age. Line and genotype differences were evident for egg shell and yolk traits while line by genotype interactions showed that genetic background influenced expression of dw.

Embryo growth of dwarfs was similar to that of nondwarfs at all ages (Experiment 3). Reduced egg size of dwarf pullets did not reduce embryonic weights. Embryo length and weight and yolk sac weight were greater for line HH than LL. In Experiment 4, allometric growth and cellular content of brains were measured in normal and dwarf chickens from lines HH and LL and reciprocal F₁ crosses from hatch to maturity. Allometric slopes were similar for lines HH and LL with significant heterosis. Within an age, DNA, RNA and protein content (mg/g) were similar for all populations. Brain weight differences were due to equal filling of a greater number cells for line HH than LL. Final brain size was influenced more by embryonic hyperplasia than postembryonic hypertrophy.

The final experiment measured growth and cellular content of pectoralis and gastrocnemius muscles in populations HH, LL and F₁ from hatch to day 273. HH chicks had a smaller DNA unit size and a greater unit number than LL chicks at hatch. During rapid muscle growth, cell size was larger for HH than LL chickens. Pectoralis muscle grew at a faster rate than gastrocnemius muscle. Also cellular filling as measured by the DNA unit size was higher for pectoralis than gastrocnemius muscle through day 4. By day 10 the pattern reversed with values greater for gastrocnemius than pectoralis muscle.