"Dominant Rumplessness . . . Among the various skeletal mutations of the fowl, one of the more striking is the rumpless condition. In the past, this condition has been a characteristics of such breeds as the Persian Fowl, rumpless Game Bantams, rumpless Polish, and others (Dunn, 1925). At the present time it is a breed characteristic of the Araucana. Dunn and Landauer have studied the morphology and inheritance of this trait extensively. Their studies indicated that there are three types of rumplessness: complete, intermediate, and accidental.

In the normal chicken there are 16 synsacral vertebrae which are fused with the pelvis, five free caudal vertebrae, and six vertebrae which are fused with the pygostyle. The normal chicken also gas 14 or 16 tail feathers and a uropygial oil gland. Landauer and Dunn (1925) and Landauer (1928) determined the morphological differences among the three types of rumplessness and the normal state. The complete an accidental forms appeared outwardly to be the same. The posterior of their bodies were round, and they lacked an oil gland and tail feathers. Examination of the vertebrae, however, showed that they were different. The complete rumpless condition was characterized by a lack of all but the last two pygostyle vertebrae, all the free caudal vertebrae, and one or two vertebrae form the center of the five synosacrocaudal vertebrae. The accidental form differed in that all of the pygostyle was absent, and the last two synsacrocaudal vertebrae were missing instead of one or two from the center. Du Toit (1913) studied the musculature of these birds in detail, and reported that nearly all muscles were present although some were displaced. Landauer and Dunn (1925) felt that some of the muscles were actually missing. Du Toit (1913) also examined the embryonic development of the complete rumpless condition and found that the tail vertebrae were absent from the beginning. The intermediate rumplessness differed from both of the others in that all the free caudal vertebrae were present although they were fused together irregularly. As with complete rumplessness, the pygostle was lacking except for the last two vertebrae, and one or two vertebare were missing from the center of the synsacrocaudal vertebrae. Rudimentery oil glands appeared on about 25 percent of this type, and varying numbers of tail feathers (2-17) were usually present.

Davenport (1906,1909) was one of the first to report genetic studies with rumpless fowl, concluding that a dominant gene was involved. Dunn (1925) verified the dominant status of the complete type, and determined that the accidental type was not genetic. In Dunn’s stocks, accidental rumplessness appeared at the rate of about one per 1000 chicks. Dunn and Landauer (1936) assigned the gene Rp to the complete type. The genetic basis of the intermediate or modified rumplessness was studied by Dunn and Landauer (1934, 1936) after it appeared following the mating of rumpless to normal. Additional outcrosses as well as selection for the intermediate type resulted in heterozygotes and some homozygotes of the intermediate type. This phenotype appeared to be due to multiple recessive modifying genes which are carried by normal birds and which tend to suppress abnormal development.

The rumpless condition also had an influence on viability and fertility (Dunn and Landauer, 1934). The proportion of rumpless embryos dying between the 17th to 21st days of incubation was always higher than expected, and the proportion of rumpless chicks that hatched was lower than expected. This applied to both the complete and intermediate types. For the complete type this reduction in viability was about five percent, They were unable to determine if viability was better with the intermediate type. Increased mortality continued posthatch with 12.1 percent of rumpless and intermediate chicks dying during the first two months as opposed to 7.2 percent of the normal chicks.

Fertility problems resulted purely mechanical reasons. Lack of tails which act as balancers in the copulation act resulted in incomplete copulations. Also very heavy fluff about the cloaca was an additional handicap. Fertility was as low as 17 percent for rumpless x rumpless, and as high as 71.2 percent for normal female x intermediate male. Fertility was always better when the females were normal . . . "

Excerpt from ‘Google Books Results’: Poultry Breeding and Genetics (pages 218-219) - R. D. Crawford (1990)

Expression of this trait varies widely, being bilateral as well as unilateral, and having variation in size, direction of peduncle projection, and location of attachment (Figure 6.5).  Expression of the ear tuft gene includes an effect on structural rearrangement of the ear. The external opening is quite irregular in shape and the external auditory canal is either reduced in length or completely absent (Somes 1978b). Studies of the embryonic development of ear tuft and ear canal suggest that the structural abnormalities are derived from incomplete fusion of the hyoid and mandibular arches over an area from the distal part of the ear opening to the neck. (Pamblonia and Somes, 1981, 1983).

The Araucana ear tuft trait is inherited as an autosomal dominant trait that is lethal in homozygotes. It has been assigned the gene symbol Et. Prenatal death is also increased in heterozygotes varying from 20-42 percent, and penetrance is reduced 4-14 percent.  Posthatch mortality is significantly greater among tufted chicks than non-tufted ones (Somes, 1978b, Somes and Pabilonia, 1981). Occasionally an Et/Et bird will escape the lethal effects of this gene. Such a bird was reported by Somes and Pablionia (1981) but this male was unable to pass his 'escaper' ability to the next generation."

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