![407x208px-emplume comparativo](https://upload.wikimedia.org/wikipedia/commons/3/35/407x208px-emplume_comparativo.jpg)
Delayed-feathering in chickens is a genetically determined delay in the first weeks of
feather
Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and a premier ...
growing, which occurs normally among the chicks of many
chicken breed
There are hundreds of chicken breeds in existence. Domesticated for thousands of years, distinguishable breeds of chicken have been present since the combined factors of geographical isolation and selection for desired characteristics created reg ...
s and no longer manifests itself once the
chicken
The chicken (''Gallus gallus domesticus'') is a domesticated junglefowl species, with attributes of wild species such as the grey and the Ceylon junglefowl that are originally from Southeastern Asia. Rooster or cock is a term for an adult m ...
completes adult
plumage
Plumage ( "feather") is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, ...
.
The difference between fast normal
feather development
Feather development occurs in the epidermal layer of the skin in birds. It is a complicated process involving many steps. Once the feathers are fully developed, there are six different types of feathers: contour, flight, down, filoplumes, semi ...
and delayed-feathering can be recognized in one-day-old chicks but is always more evident in 10- to 12-day-old chicks.
Female chicks have a slightly faster feathering than males. Barely seen in breeds with fast normal-feathering, this characteristic is better observed in breeds with delayed feathering, like
Barred Plymouth Rock
The Plymouth Rock is an American breed of domestic chicken. It was first seen in Massachusetts in the nineteenth century, and for much of the early twentieth century was the most widely kept chicken breed in the United States. It is a dual-pu ...
.
[Hutt, F.B. Genética Avícola. Salvat Editores,S.A. 1ra.ed. España (1958)]
Natal down color is not related to feathering speed, but in chickens of full-black adult plumage, chicks normally have shorter natal down than those from breeds of any other plumage color pattern this shortening being more obvious in the head and back.
Feathering growth in the first weeks of life is genetically controlled by a few autosomal genes and a sex-linked gene.
Most of the phenotypical variation among breeds can be explained by the sex-linked gene ''K''. Autosomal genes retarding feathering growth are not typical in any particular breed. Because those genes are
recessive
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
, they need to be in homozygosity to manifest themselves, but as poultry farmers dislike bad-fledged birds, autosomal genes only subsist in very low frequencies.
Contrarily, sex-linked delayed-feathering is very frequent, particularly in meat-type breeds like
Brahma
Brahma ( sa, ब्रह्मा, Brahmā) is a Hindu god, referred to as "the Creator" within the Trimurti, the trinity of supreme divinity that includes Vishnu, and Shiva.Jan Gonda (1969)The Hindu Trinity Anthropos, Bd 63/64, H 1/2, pp. 21 ...
,
Cornish,
Cochin
Kochi (), also known as Cochin ( ) ( the official name until 1996) is a major port city on the Malabar Coast of India bordering the Laccadive Sea, which is a part of the Arabian Sea. It is part of the district of Ernakulam in the state of K ...
,
Rhode Island Red
The Rhode Island Red is an American breed of domestic chicken. It is the state bird of Rhode Island. It was developed there and in Massachusetts in the late nineteenth century, by cross-breeding birds of Oriental origin such as the Malay with ...
,
Wyandotte,
Plymouth Rock
Plymouth Rock is the traditional site of disembarkation of William Bradford and the ''Mayflower'' Pilgrims who founded Plymouth Colony in December 1620. The Pilgrims did not refer to Plymouth Rock in any of their writings; the first known writt ...
and
Orpington
Orpington is a town and area in south east London, England, within the London Borough of Bromley. It is 13.4 miles (21.6 km) south east of Charing Cross.
On the south-eastern edge of the Greater London Built-up Area, it is south of St Ma ...
.
Sex-linked genetic variation has proven to be very useful for the
sex identification of one-day-old chicks in commercial hybrids.
Sex-linked feathering
Sex-linked feathering is controlled by locus ''K'' on the
sexual chromosome, with four known alleles in the following
dominance order: ''K''
n > ''K''
s > ''K'' > ''k''
+.
[McGibbon, W. H. A sex-linked mutation affecting rate of feathering in chickens. Poultry Science 56, 872-875 (1977)]
''K''n (extremely slow-feathering)
''K''
n ("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
Flight feathers (''Pennae volatus'') are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the wings or tail of a bird; those on the wings are called remiges (), singular remex (), while those on the tail ...
(they may appear as extremely small pinfeathers, but always shorter than the coverlets).
''K''
n also has other pleiotropic effects, as a reduction in
comb
A comb is a tool consisting of a shaft that holds a row of teeth for pulling through the hair to clean, untangle, or style it. Combs have been used since prehistoric times, having been discovered in very refined forms from settlements dating ba ...
size and hypertrophy of the uropygial gland.
''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.
''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
Flight feathers (''Pennae volatus'') are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the wings or tail of a bird; those on the wings are called remiges (), singular remex (), while those on the tail ...
are not yet developed (see Figure 1, chick on the right).
![672x278px-tipos_de_emplume](https://upload.wikimedia.org/wikipedia/commons/5/5e/672x278px-tipos_de_emplume.png)
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. Sex-linked delayed-feathering is typical in many meat-type breeds.
[Somes, R.G. International Registry of Poultry Genetic Stocks. A Directory of Specialized Lines and Strains, Mutations, Breeds and Varieties of Chickens, Japanese Quail and Turkeys. Storrs Agricultural Experiment Station, The University of Connecticut, Storrs, Bulletin #460, (1981)]
''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
Menorca or Minorca (from la, Insula Minor, , smaller island, later ''Minorica'') is one of the Balearic Islands located in the Mediterranean Sea belonging to Spain. Its name derives from its size, contrasting it with nearby Majorca. Its capita ...
,
Ancona
Ancona (, also , ) is a city and a seaport in the Marche region in central Italy, with a population of around 101,997 . Ancona is the capital of the province of Ancona and of the region. The city is located northeast of Rome, on the Adriatic S ...
and many others.
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, contrarily to what happens in humans, where the male is the
heterogametic sex
Heterogametic sex (digametic sex) refers to the individuals of a species in which the sex chromosomes are not the same.
For example, in humans, males with an X and a Y sex chromosome would be referred to as the heterogametic sex, and females ...
, as occurs in most diploid species.
''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
A broiler is any chicken (''Gallus gallus domesticus'') that is bred and raised specifically for meat production. Most commercial broilers reach slaughter weight between four and six weeks of age, although slower growing breeds reach slaughte ...
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
In biology, culling is the process of segregating organisms from a group according to desired or undesired characteristics. In animal breeding, it is the process of removing or segregating animals from a breeding stock based on a specific tr ...
, 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.
Advantages of feather-sexing
Identification of sex in one-day-old chicks by feather length has a few advantages over vent sexing:
:* Special sorting personnel is not required. While vent sexing can only be carried out by highly trained personnel, feather-sexing can be carried out by personnel without training.
:* Risk of infection via sexing operative is completely eliminated, especially risk of infection by ''Escherichia coli'' due to manipulation.
:* There is a lower probability of mistakes: when one-day-old females are sexed for breeding purposes there is less risk of males slipping through.
:* Feather sexing is faster than vent sexing, reducing time required to dispatch the chicks out of the hatcheries. One can hatch more chicks per hour because one is not hampered by the drawback of vent sexing operatives being capable of sexing only a restricted number of chicks per hour.
:* Some paraphernalia may be dispensed with, such as tables and lamps.
:* The cost of sexing is much lower.
Disadvantages of feather-sexing
Feather-sexing can only be carried out in chicks resulting from the mating of fast normal-feathering males with delayed-feathering females, while vent sexing can be applied to any chicks. This means that feather-sexing cannot be applied to chicks of the same breed, unless geneticists preserve that breed segregating for both types of feathering and birds are strictly controlled for feathering type.
The preparation of new breeds to produce feather sexable crosses implies considerable previous genetic work which is completely beyond the means of most poultry farmers.
Multifactorial autosomal slow-feathering
There are some multifactorial autosomal genes which affect only chicks with delayed-feathering and have no effect on fast normal-feathering chicks, nor on adult plumage.
The sole effect of multifactorial slow-feathering is to reduce the length of the primary remiges of one-day-old chicks without a concomitant reduction of the primary coverlets.
That is, one-day-old chicks with primary remiges shorter than coverlets carry the ''K'' gene of sex-linked delayed-feathering (see Figure 2C).
Autosomal ''T'' allelic series
Contrary to multifactorial slow-feathering genes, which affect only chicks carrying ''K'', the autosomal recessive alleles ''t'' ("tardy") and ''t''
s ("retarded") of the ''T'' allelic series affect only those chicks of the fast normal-feathering type.
The dominance order is ''T'' (normal) > ''t''
s ("retarded") > ''t'' ("tardy").
"Retarded" ''t''s''t''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''
s''t''
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.
"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.
Displastic remiges
There is an autosomal recessive gene ''dr'' which affects remiges and rectrices. Between the 8th and 28th day the feather follicles on several remiges and rectrices become enlarged and blood filled. The tissue of the developing feather becomes necrotic, and these feathers are soon lost.
[Poultry Science 59, 1667 (1980)]
References
{{Reflist
Bird anatomy
Feathers
Aviculture
Poultry
Chickens