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Dwarfism in chickens is an inherited condition found in chickens consisting of a significant delayed growth, resulting in adult individuals with a distinctive small size in comparison with normal specimens of the same breed or population. The affected birds show no signs of dwarfism in the first weeks of age.〔Hutt, F.B. ''Genetics of the fowl''. McGraw-Hill Book Co. N.Y. 1949.〕 Differences in size due to dwarfism appear slow and progressively along the growing stage. Poultry breeders begin to distinguish gradually dwarfs from normal birds by their shortest shanks and smallest body size.〔Strong, C.F. and Jaap, R.G. 1974 Chick size and early growth rate of dwarf broiler-type chickens. ''Poultry Science'' Vol. 53:1982-1983〕〔Leenstra, F.R. and Pit, R. 1984 The autosomal dwarf as broiler sire mated to normal and sex-linked dwarf broiler dams:Performance of progeny. XVII World's Poultry Congress and Exhibition (August 8–12) Finland.〕 Depending on the breed, most types of dwarfism in chickens begin to be recognized when the birds reach 8–10 weeks of age,〔Hutt, F.B 1959 Sex-linked dwarfism in the fowl. ''Journal of Heredity'' 50:209-221〕〔Cole, R.K. 1973 An autosomal dwarfism in the fowl. Poultry Science vol.52:2012-2013〕 but classification is more precise when the chickens are five months old or more. At this point differences between normal and dwarf sibs is evident in all males and in 98% of the females.〔 Dwarfs chickens reach sexual maturity and reproduce normally. Dwarfism in chickens has been found to be controlled by several simple genetic factors. Some types are autosomic while others are sex-linked, but when poultry breeders make reference to 'dwarf chickens' they usually refer implicitly to sex-linked recessive dwarfism due to the recessive gene dw, located on the Z chromosome. As sex-linked dwarf broiler breeder hens can bring about normal sized broiler chickens, sex-linked recessive dwarfism found application in poultry industry since the last decades of the 20th century. These hens require less food and less housing space. Their feed intake do not need to be restricted. They also have more tolerance to heat (see: Advantages...). So the use of sex-linked dwarf broiler female parent stocks helps to save costs and to improve animal welfare and economic efficiency in European broiler industry (see: Use of...). But in spite of the proven advantages of raising dwarf breeder hens, their use is not generalized in broiler industry. ==Autosomal types of dwarfism== Two different types of autosomal dwarfisms have been found in chickens. These types of dwarfism are controlled by genes located on the autosomal chromosomes so inherits the same way in both sexes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dwarfism in chickens」の詳細全文を読む スポンサード リンク
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