Kⁿ (extremely slow-feathering)

Kⁿ ("naked") is the most dominant of the allelic series which controls feather growth rate. It is a gene whose action on feather growth is quite extreme. Birds carrying this gene are nearly naked during juvenile life and in some cases females remain nearly naked well into adult life. One-day-old chicks lack primary and secondary remiges (they may appear as extremely small pinfeathers, but always shorter than the coverlets).^[4]

Kⁿ also has other pleiotropic effects, as a reduction in comb size and hypertrophy of the uropygial gland.^[5]

K^s (slow-feathering)

K^s ("slow") is another dominant allele of the allelic series which controls feather growth rate. In one-day-old chicks primary remiges are shorter than the coverlets. Plumage growing is greatly delayed during the first weeks of juvenile life, but birds of both sexes completely finish plumage growing at twelve weeks of age. This allele has no effect on adult plumage.^[3]

K (delayed-feathering)

K is another dominant allele of the allelic series which controls feather growth rate. One-day-old chicks have primary remiges and coverlets of the same length (see Figure 2B). In 8- to 12-day-old chicks rectrices are not yet developed (see Figure 1, chick on the right).

Figure 2. Feathering types Diagram of the wing of a one-day-old chick in dorsal view: A fast normal-feathering with the primary remiges (1a) longer than the coverlets (2a). B sex-linked delayed-feathering with the primary remiges (1a) of the same length than the coverlets (2a). C modified delayed-feathering with the primary remiges. (1a) shorter than the coverlets (2a).

Feather growth rate is generally slower in K-carriers than in fast normal-feathering birds. But this allele has not any effect on adult plumage.^[6] Sex-linked delayed-feathering is typical in many meat-type breeds.^[7]

k⁺ (fast normal-feathering)

k⁺ is the most recessive of the allelic series. One-day-old chicks pure for this allele have the primary remiges evidently longer than the coverlets (see Figure 2A) while 8- to 12-day-old chicks show their rectrices well developed (see Figure 1, chick on the left), and they later complete feather growing long before those chicks carrying K.

k⁺ is responsible for the fast normal-feathering found in most egg-type breeds like Leghorn, Minorca, Ancona and many others.^[7]

Practical use of the sex-linked variation

When fast normal-feathering k⁺k⁺ cocks are mated to delayed-feathering K hens, the sex of all the descendants can be easily recognized at hatch: resulting female chicks k⁺ have fast normal-feathering (see Figure 2A) while male chicks Kk⁺ have delayed-feathering (see Figure 2B and 2C).

This happens because female birds have a single Z sexual chromosome, while males carry two,^[8][9] contrarily to what happens in humans, where the male is the heterogametic sex, as occurs in most diploid species.^[10]

k⁺ carrier females are always of the fast normal-feathering type, while heterozygous Kk⁺ males are of the delayed-feathering type because of the dominance of K over k⁺.

This regularity found practical use in commercial poultry breeding to separate chicks of different sexes in the hatcheries, thus avoiding the inconveniences associated with vent sexing and making easier, faster, harmless and cheaper the sex identification in one-day-old chicks.

Through controlled matings and selection, K gene was put into egg-type breeds like White Leghorn, where sex-linked delayed feathering did not originally occur, thus obtaining genetically delayed-feathering female parent breeders to be mated with fast normal-feathering male parent breeders in order to obtain sexable female chicks of the fast normal-feathering type.

In meat-type breeds, feather sexing was not so extended to commercial broiler production because resulting poorly-feathered males are very prone to scratches in the skin of the belly, which can discredit carcass quality. After all, unlike egg-laying poultry, meat-type male chicks are not culled, so broiler males can be set apart later, once sexual differences become evident.

However, feather-sexing was found to be very useful in the reproduction of female parent stocks, which are usually raised separated by sexes. This way, both sexes of the resulting broiler are of the fast normal-feathering type.

"Retarded" t^st^s chicks

Fast normal-feathering one-day-old chicks have six primary remiges and at least another six secondary remiges (see Figure 3A). While "retarded" t^st^s one-day-old chicks have normal primary remiges but only the first three (external) secondary remiges.

At 10 days of age, differences between normal and "retarded" chicks are even more evident. "Retarded" chicks show the first three or four primary remiges of normal length, with each new feather being shorter than the preceding one (see Figure 3B). At three weeks of age, "retarded" chicks have even fewer primary remiges than normal chicks, and only the first three secondary remiges reach the same length as primary remiges.

Figure 3. Effect of autosomal genes of T series on 10-day-old chicks.
(A) Normal-feathering, with a T⁻ genotype. (B) "Retarded"-feathering, with t^st^s genotype. (C) "Tardy"-feathering, with tt genotype.

"Tardy" tt chicks

"Tardy" tt chicks are more abnormal than "retarded" chicks. They show normal primary remiges at hatch, but lack secondary remiges. At ten days of age they still lack rectrices and show small or no development of secondary remiges (see Figure 3C). Rectrices do not appear until at least eight weeks of age. However, adult plumage is normal.