HEREDITARY HYPOTHYROIDISM IN THE DOMESTIC FOWL

Abstract

OPULATIONS of fully pedigreed chickens often contain individuals which 'deviate considerably in one or more respects from the average for the flock.When pedigrees reveal close genetic relationship among those similarly affected, further studies are warranted.Such was the case for the trait now recognized as hypothyroidism.Affected 5 %-month old White Leghorn pullets, which were somewhat smaller in skeletal size than normal, possessed long silky feathers, and were rather obese, began to lay before there was a development in size and redness of the comb.Reported egg production by two such birds had to be witnessed in person by the author before he would believe!The fact that the few such individuals all came from one of three strains and that they often were halfor full-sisters prompted a study to determine the role of heredity in the development of this trait.Individuals with similar phenotype have now been observed in several other unrelated stocks.It is apparent that the trait may be more common than at first beljeved but, because of the associated defects in normal physiology, affected individuals rarely survive to reach the age of maturity. Reuiew of pertinent literature:As far as the author knows, no similar trait has been described in chickens or other species, save possibly man.The most obvious deviation from normal phenotype is the obesity which is especially pronounced during the period of from 8 to 16 weeks of age.Obesity of hereditary origin has been described in mice, rats, and dogs, and is obvious in man.Obese (ob ob) (INGALLS, DICKIE and SNELL 1950) and adipose (ad ad) (FALCONER and ISAACSON 1959) are simple Mendelian recessive traits but nonallelic, while the inheritance of the New Zealand obese mice (BIELSCHOWSKY and BIELSCHOWSKY 1956) is not a simple Mendelian type.The ob ob and New Zealand obese traits are associated with hyperglycemia and hyperinsulinism but differ in some respects, including reproduction and general activity.The famous yellow mouse of CUBNOT (1905), a heterozygote, AYA, is also characterized by obesity (DANFORTH 1927) ; the homozygote, AYA Y, being the first known genetic lethal.The fatty mutation in rats, f a f a (ZUCKER and ZUCXER 1961), is associated with abnormal lipid metabolism, while obesity in MAYER'S Shetland Sheepdogs (1 954) may be associated with disturbed hypothalamic functioning.In all these cases the obese individuals are heavier than the normals and in most cases, if not all, the thyroid gland is essentially normal.(The obese chicken, on the other hand, is definitely smaller in size than its normal siblings if measured in terms of body weight, and even more so if skeletal size rather than body weight is considered.)IQ hereditary type of dwarfism in the fowl was described by LANDAUER (1929) with genetic evidence for its inheritance as a simple Mendelian recessive provided by UPP (1934) who included some of WARREN'S data.Rhode Island Red dwarfs, possibly of similar origin, were also studied by PAYNE (1944).In LANDAUER'S case the thyroids were enlarged, while in PAYNE'S birds they were small, and in both instances with little or no colloid.In these cases no special mention