Biological and economic efficiencies of major genes for feather reduction (naked neck and frizzle) and body size reduction (dwarf) in a dual purpose layer strain were evaluated in the subtropical coastal region of South-East Africa (Maputo, Mozambique). The experimental material consisted of eight different genetic groups, two diets (14.4 and 16.2% crude protein) and two climatic seasons. Birds were caged individually in an open-side shelter with natural light. Traits measured were: temperature and relative humidity, body weight, age at first egg, egg weight, egg production, egg quality, feed intake and mortality. The following were calculated: temperature-humidity index (THI), growth rate, persistence, egg mass, feed efficiency, feed conversion, biological efficiency (EM/BWO.75) and productivity (EN/BWo. 75). The main results show that: (1) none of the feather-reduced genes significantly improved egg production or the efficiency of feed utilization, although the naked neck (Na) excelled in terms of the number and mass of eggs produced per metabolic body weight; (2) the dwarf gene (dw) was associated with delayed sexual maturity, production of fewer and lighter eggs, higher persistence, better feed conversion and higher survivability; (3) climatic seasonal effects were observed in all traits analysed, with elevated temperatures restraining body weight gain, the number and weight of the eggs produced, and voluntary feed intake; (4) the lower dietary protein content resulted in decreased egg weight. It was concluded that the normal feathered dwarf is the most suitable genetic group for peri-urban and rural production systems in this region. / Thesis (PhD (Animal Science))--University of Pretoria, 2007. / Animal and Wildlife Sciences / unrestricted