The number and size of muscle fibres and the DNA concentration in m. pectoralis superficialis of normal (Dw+/-) and dwarf (dw/-) White Plymouth Rock females were investigated at the ages of 9 and 52 weeks. 2. Live weight and total meat was reduced by 37.4 and 42.8% in dw/- as compared with Dw+/- females at the age of 9 weeks. The suppressive effect of the dwarf allele on the weight of m. pectoralis superficialis was 40.6% (67.8 v. 40.3 g in Dw+/- and dw/- genotypes). This reduction in weight was accompanied by a similar decrease of DNA content (28.8 v. 17.1 mg, i.e. 40.6%), a decrease in number of muscle fibres (829 x 10(3) v. 654 x 10(3), i.e. 21.2%) and fibre diameter (30.80 v. 28.95 microns, i.e. 6.0%). In mature hens, the differences between genotypes Dw+/- and dw/- were slightly smaller. 3. The different relationships between the muscle growth traits studied are discussed.
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