Here is some information regarding the Frizzle Gene that I was able to find when searching the internet early on in my effort to develop the Sizzle. Unfortunately I did not save the entire text and cannot give the author credit.
Description.
Without careful breeding it is somewhat dlifficult to keep Frizzles free from one or more modifying genes which prevent full impressions of the character and which are widespread in domestic poultry. Among unmodified frizzles there is considerable variation, but in general two types are quite distinct. They are described in detail by Hutt (1930) and by Landauer aThd Dunn (1930). Unmodified heterozygotes have the shafts of contour feathers recurved, the outer surface being concave. Barbs of the rectrices and remiges curl and eventually wear off, particularly in the outer primaries. This effect is less conspicuous in the inner primaries and still less so in the secondaries.
The birds cannot fly and at night squat on the floor unless given low roosts. Un-modified homozygotes have the rachis extremely recurved in all feathers. The barbs are much curled. No feather has a flat vane, and all are narrow. As a result, the adult in full plumage has a somewhat woolly appearance. The feathers are broken off by the crowding of the birds at night and by the treading of the males in the breeding season.
Consequently, the homozygotes frequently appear quite bare in the summer. Modified heterozygotes have less extreme frizzling in all parts of the body, and at maturity it may even be difficult to distinguish such birds from those not frizzled. However, even the most modified birds still show some curling of the barbs in the outer primaries, and the feathers on the neck are slightly raised (Hutt, 1936). Secondaries of such birds are usually almost normal, and the inner ones are least affected.
Modified homozygotes lack the woolly appearance of those not modified, and the plumage seems somewhat velvety, almost as if the barbs of the feathers constituted a deep pile on an enveloping rug. Each feather is less extremely curled than in unmodified birds and is somewhat wider. Though all the barbs are curled, the curling begins farther from the rachis than in homozygotes not modified, and, as a result, there is a narrow strip on each side of the shaft where the web is flat as in normal feathers.
Genetics.
Frizzling is caused by the incompletely dominant auto somal gene, F (Hutt, 1930; Landauer and Dunn, 1930), and the n~6 1 e types apparently result from the interaction of F with an autosomal recessive modifier7 mf. According to Landauer (1933), all birds homozygous for the modifier are affected by it, but in the material of Hutt (1936), though all Ff genotypes were modified by it, only about 40 per cent of the FF birds showed the modified plumage. The latter investigator FIG. 34. A heterozygous Frizzle (left) and a homozygous one (right), both unmodified (From Hutt in J. Genetics, 1930.) found the modifier in 11 different breeds and varieties and in most of the non-frizzled fowls tested for its presence. Prior to genetic analyses of frizzling, complaints of breeders that Frizzles did not breed true but always produced some birds with non frizzled plumage led to the suspicion that homozygosity for frizzling might be lethal, leaving only heterozygotes available for breeding. After discovery that the extremely curled feathers were found only in homozygotes, the reason for the fanciers complaint was made clear., With Frizzles, as in Blue Andalusians, the preference in the showroom is for the phenotype of the heterozygote, and accordingly the breeder saves only these and discards the curly-feathered, homozygous birds. His matings of the desired type, therefore, inevitably produce progeny of which only half have the kind of plumage desired in the showroom. The remainder in this 1:2:1 segregation are either not frizzled at all or homo zygous for F and hence frizzled too much. The gene F is linked with those for crest and for dominant white Action of the F Gene. The question arises whether frizzling results from some abnormality of metabolism, such as abnormal functioning of an endocrine gland, or from direct action of the F gene in the developing feather. In reciprocal transplants of skin between frizzled and normal fowls Landauer and Aberle (1935) found that the grafts always grew the plumage typical of the donor and never that of the host. A male with modified, heterozygous frizzling that is recognizable only by the slightly raised feathers of the neck and breast. Frizzled fowls produces only frizzled feathers on birds normally not frizzled, it is clear that the action of the gene is localized in the feather follicles and that the condition does not result from some disorder of metabolism. One graft of normal skin on an FE male persisted to 15 months of age, at which time the host moulted and subsequently produced simultaneously frizzled feathers over most of the body and normal feathers on the grafted skin.
Classification.
Except in extremely slow-feathering birds, the presence or absence of F can be recognized with almost complete accuracy at 14 days of age and even earlier. Effects of the modifier are less conspicuous in young chicks than in other birds, and for most chicks of 3 weeks or so it is difficult to say with certainty whether they are modified or not. Secondary Effects of F. Since homozygous frizzles are usually more or less naked, except when a new coat of plumage has just been acquired, it is to be expected that loss of the normal insulation would cause some disturbances of the physiology in such birds. These have been studied in detail by Landauer and his associates.
Metabolism.
Benedict, Landauer, and Fox (1932) calculated that even at 28CC. the heat production of frizzles was greater than in normal fowls. At 170C. the difference was much more pronounced, and in some of the homozygous frizzles the heat produced was more than twice that of normal fowls. The loss of heat from the body surface in homozygous frizzles was partially offset by an abnormally low heat loss from vaporization of water, the amount of water thus lost being 30 to 35 gin. in 24 hours per kilogram of body weight, compared with 50 to 56 gin. in normal fowls. The rectal temperatures of heterozygous frizzles did not differ from those of normal fowls, but at environmental temperatures below 1500. the average rectal temperature for 10 homozygotes was slightly lower than in controls. One would naturally expect the homozygous frizzle fowls to eat more feed than normal ones in order to compensate for their extra heat loss, and some evidence was found that they do so. Other Effects. In mature, homozygous frizzles, the heart is larger and beats more rapidly than in normal fowls (Boas and Landauer, 1933, 1934). The difference in females was 72 beats per minute, an increase of 27 per cent over the rate for normal fowls. It was attributed to the higher rate of metabolism in frizzles. Landauer and Upham (1936) found that in homozygous frizzles there was an increase over normal in the relative weight of the heart, blood, spleen, kidneys, adrenals, pancreas, crop and gizzard and in the relative capacity of the duodenum, small intestine, caeca, and large intestine. However, since the normal birds were Leghorns and the frizzled ones were not, it is possible that some of these differences might be associated with the several physiological traits by which Leghorns differ from other breeds (Hutt, 1941) and not caused entirely by the deficiency of plumage in the frizzled birds. According to Landauer and Aberle (1935), the thyroid gland and adrenals of homozygous frizzles are not normal in structure. The thyroid shows exhaustion atrophy attributed to overwork in the effort to maintain body temperature by a higher metabolism. Landauer (1932) maintains that the hatchability of eggs laid by heterozygous frizzles is subnormal and that it is still lower in eggs from homozygotes, but the evidence on this point is hardly conclusive.
Significance of F and mf in Evolution.
The gene_F iet not a desirable one from the standpoint of a species trying to survive in competition with others. Apart from the fact that unmodified frizzles are unable to fly and tend to squat on the ground instead of roosting, it is clear that the secondary effects of the mutation are more likely to shorten life than to lengthen it. There are some indications that even under optimum conditions of domestication the mortality among frizzled fowls is higher than in normal ones. Landauer found the males to be somewhat slow in reaching sexual maturity. For all these reasons, it is highly improbable that the mutation could persist long in nature. Its preservation by fanciers is quite a different matter. On the other hand, the modifying gene, rnf, which to a considerable extent overcomes the action of F and makes the plumage almost normal, is a very desirable one. Perhaps this accounts for its presence in at least 11 different breeds and varieties (Hutt, 1936). According to Fishers theory of the evolution of dominance, this modifying gene is exactly of the kind that would be preserved and accumulated under natural selection. It is possible that a widespread distribution of it among the ancestors of our domestic fowls, even before Jungle Fowls were distributed to the four corners of the world, may be responsible for its presence and prevalence in our modern breeds.
