Dwarfism in chickens

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Dwarfism due to the sex-linked recessive gene dw. Comparative size of two full-sib roosters. Left: Normal sibling (genotype Dw/dw). Right: Dwarf sibling (genotype dw/dw). Dwarf rooster.jpg
Dwarfism due to the sex-linked recessive gene dw. Comparative size of two full-sib roosters. Left: Normal sibling (genotype Dw/dw). Right: Dwarf sibling (genotype dw/dw).

Dwarfism in chickens is an inherited condition found in chickens consisting of a significant delayed growth, resulting in adult individuals with a distinctive small size in comparison with normal specimens of the same breed or population.

Contents

The affected birds show no signs of dwarfism in the first weeks of age. [1] Differences in size due to dwarfism appear slow and progressively along the growing stage. Poultry breeders begin to distinguish gradually dwarfs from normal birds by their shortest shanks and smallest body size. [2] [3] Depending on the breed, most types of dwarfism in chickens begin to be recognized when the birds reach 8–10 weeks of age, [4] [5] but classification is more precise when the chickens are five months old or more. At this point differences between normal and dwarf sibs is evident in all males and in 98% of the females. [1] Dwarfs chickens reach sexual maturity and reproduce normally.

Dwarfism in chickens has been found to be controlled by several simple genetic factors. Some types are autosomic while others are sex-linked, but when poultry breeders make reference to 'dwarf chickens' they usually refer implicitly to sex-linked recessive dwarfism due to the recessive gene dw, located on the Z chromosome.

As sex-linked dwarf broiler breeder hens can bring about normal sized broiler chickens, sex-linked recessive dwarfism found application in poultry industry since the last decades of the 20th century.

These hens require less food and less housing space. Their feed intake do not need to be restricted. They also have more tolerance to heat (see: Advantages...). So the use of sex-linked dwarf broiler female parent stocks helps to save costs and to improve animal welfare and economic efficiency in European broiler industry (see: Use of...). But in spite of the proven advantages of raising dwarf breeder hens, their use is not generalized in broiler industry.

Autosomal types of dwarfism

Two different types of autosomal dwarfisms have been found in chickens. These types of dwarfism are controlled by genes located on the autosomal chromosomes so inherits the same way in both sexes.

Thyrogenous dwarfism, td

In 1929 a type of dwarfism was described in different breeds of Rhode Island Red chickens. [6] [7] [8] [9] This type of dwarfism produced individuals which showed a general growth delay, which was recognizable since two or three weeks of age. The outer toe was curved backwards. The skull was high and wide in relation to its length and the upper beak was curved downwards. Tongue was shortened and tumescent. Legs shortened, more in the metatarsal than in the femur. The condition was semi-lethal. None of the affected birds reached sexual maturity. It was considered as the result of a dysfunction of the thyroid gland, similar to the human pathological condition known as myxoedema infantilis. Dwarfs of this type were homozygotes for an autosomal recessive gene td (thyrogenous dwarfism).

Autosomal dwarfism, adw

Another body size mutation was found in the experimental Cornell K-strain of chickens. Body size was reduced by about 30% and the affected birds were recognizable by 6 to 8 weeks of age. Their sexual maturity was somewhat delayed and rate of lay was about 90% that of the K-strain. Viability of the carriers was good but hatchability was poor. The condition was due to an autosomal recessive gene designed adw. [5]

The ultimate goal of the modern genetic studies is to find out the underlying genes involved in these traits. To achieve this, the so-called positional candidate gene approach is gaining in importance. This approach is based on the genetic localization of a trait using genetic linkage analysis. Subsequent comparative mapping of the trait locus with the gene-rich maps of the human and the mouse may reveal candidate genes for the trait in question. Comparative mapping revealed that autosomal dwarfism in the chicken (adw) is located in a chromosomal region that is conserved between chicken, human and mouse. In the mouse the phenotype "Pygmy", similar to chicken adw is also located in that region. The Pygmy mouse phenotype arises from the inactivation of the High Mobility Group I-C (HMGCI-C). In the human the HMGCI-C gene is also located in the same conserved chromosomal segment. Fluorescent in situ hybridization of chicken metaphase chromosomes using the chicken HMGI-C gene as a probe, showed that the chicken HMGI-C gene is indeed located in the region of the adw locus. [10] Insulin-like growth factor 1 (IGF1) is another candidate for adw in the chicken. [11]

Sex-linked dwarfism

In birds, female is the heterogametic sex, that is, it has one Z and one W sexual chromosome (genotype ZW), while male (the homogametic sex) carries two Z chromosomes (genotype ZZ). Thus, reciprocal crosses between normal and dwarf specimens may give rather different results.

Sex-linked recessive dwarfism, dw

Hutt studied in the 1940s a remarkable type of dwarfism caused by only one sex-linked recessive gene to which he assigned the symbol dw. [1]

This mutation reduces body weight in females by 26 to 32%, but the effect is still greater in homozygous males, by about 42-43%. Chicks are normal size. [12] [13] This is the best studied type of dwarfism in chickens. Sex-linked dwarfism in meat type breeds are first recognized by the shortening of the shanks than by the lowering body weight in the rearing period [14]

There are no signs of sex-linked dwarfism in the first weeks of age. Some individuals can be identified as dwarfs at 8–10 weeks of age, but classification is more precise when the chickens are five months old or more. At this point differences between normal and dwarf sibs is evident in all males and in 98% of the females. [1] These dwarfs reach sexual maturity and reproduce normally.

Normal females are always of genotype Dw/-, while dwarf females are always of genotype dw/-, because female is the heterogametic sex having only one Z chromosome. That is, females carrying a sex-linked gene of dwarfism are always pure and exhibit the trait. On the other hand, normal males may be either homozygous Dw/Dw or heterozygous Dw/dw, but dwarf males are always homozygous dw/dw.

Double dose of dwarf gene causes the dwarfism to be much more evident in males than in females. The above picture illustrates the comparative size of two full-sib roosters born the same day: Left: Normal sibling of genotype Dw/dw. Right: Dwarf sibling of genotype dw/dw.

Hormonal causes of dwarfism

Among the main factors involved in growth regulation, thyroid hormones tiroxine (T4) triiodothyronine (T3), growth hormone (GH), and its related growth factor, Insulin-like growth factor-I (IGF-I), were the most studied in dwarfs. [15]

Sex-linked dwarf chickens are characterized by low circulating levels of T3 and IGF-I in spite of normal or even increased levels of T4 and GH. The T3 deficiency is explained by a lower peripheral activity of T4 monodeiodination which could be related to an abnormal T4 uptake by the cell, particularly the hepatocyte. The low production of IGF-I could be related to a deficient GH receptor, as suggested by the decreased GH binding observed in the liver of dwarf birds. Both T3 and IGF-I synthesis may share common pathways since thyroidectomy also decreases IGF-I level while a GH injection stimulates the T4 to T3 monodeiodination in the normal embryo but not in the dwarf. Further studies are needed on the GH receptor and the T4 uptake in the hepatic cell to identify the common point where the dwarf gene could act. Ovulation rate and lipomobilisation are decreased in adult dwarfs but these findings are not yet easily related to the endocrinological changes observed during growth [16]

Administration of triiodothyronine (T3) in the diet, from day of hatch until 8 weeks of age, to sex-linked dwarf chickens stimulates growth but can not fully restore a normal growth rate. [17] [18]

Sex-linked dwarfism in chickens is a form of growth hormone resistance that resembles the Laron syndrome in humans, characterized by reductions in stature and plasma insulin-like growth factor-I (IGF-I) levels. [19]

Variants in chicken growth hormone receptor (GHR) gene lead to sex-linked dwarf chickens, but effects of different variants are distinct. [20]

Bantam dwarfism, dwB

Bantam dwarfism is a variety of dwarfism existing in many breeds of bantam chickens. Bantam chickens are also called miniatures. These birds are popular as pets, but Bantam hens are also renowned for hatching and brooding because they are very protective mothers and will attack anything that gets near their young.

The reciprocal crosses made between normal sized chickens with Bantams revealed that Bantams carry one or several sex-linked dominant genes that reduce body size. [21] [22] This mutation is present in Sebright Bantams and probably other bantams. This mutation is thought to be an allele at the Dw locus and to be different from the allele dw. [23]

In genetics, the common convention is that most dominant alleles are written as capital letters and recessive alleles as lower-case letters (see: Dominance (genetics)). In spite of this, literature refers to the gene of Bantam dwarfism with the lower-case symbol dwB.

MacDonald dwarfism, dwM

A second type of sex-linked recessive dwarfism was found in a sex-linked dwarf chicken population. This mutation is thought to be an allele or the Dw locus and to be different from the dw allele. This conclusion is based on the fact that males heterozygous for dwM/dw produce female offspring which fall into two populations with respect to shank length. [24] The evidence is inconclusive as to whether this allele is different or the same as the dwB allele.

Up to the present level of knowledge the dominance order of the alleles of locus Dw is: dwB > Dw > dw.

In other words, allele dwB for Bantam dwarfism is dominant over the normal allele Dw, and the last is dominant over the sex-linked recessive allele dw. The existence of a second recessive allele has not yet been confirmed.

Use of sex-linked dwarfism in poultry industry

Sex-linked recessive dwarfism found application in poultry industry in the last decades of the 20th century. The application in broiler production is based on the known fact that dwarf female parents give 100% normal progeny when they are mated with normal male parents. The resulting progeny males are 100% heterozygotes Dw/dw and the females are hemizygotes Dw/-.

In commercial broiler chicken production the use of dwarf female parents acquired a great importance. [25] Nowadays, the majority of broiler breeders in Europe are the standard, fast growing genotype, but 18-20% of the broiler breeders are dwarf parental females that produce standard and alternative (medium or slow growing) broilers. [26] The use of dwarfism also found application in commercial egg production. Dwarf Leghorn layers need less housing space, so the main advantage is to allow a more efficient use of housing space producing more eggs per unit of surface, but the smallest size of their eggs imposes a serious drawback to this purpose.

Advantages of broiler breeder hens

Under current practice, normal parent poultry breeding stock potentially face welfare problems. Intensive selection for production traits, especially growth rate, is associated with increased nutritious requirement and thus feed consumption, but also reproductive dysfunctions and decreased sexual activity in broiler breeders. A first resulting serious welfare problem is the subsequent severe feed restriction which is applied during rearing, in order to prevent health problems and to reach better egg production. This severe feed restriction has negative effects on bird welfare as it causes chronic stress resulting from hunger. [27] The use of normal fast growing broiler breeder hens require dedicated programmes of feed restriction, both to maximise egg and chick production and secondly to avoid metabolic disorders and mortality in broiler breeders. The negative correlation between muscle growth and reproduction effectiveness is known as the "broiler breeder paradox". [28] [29] Using dwarf broiler breeder hens is a good alternative, because dwarf hens combine relatively good reproductive fitness with ad libitum feeding. [28]

With respect to bird welfare the use of slow growing birds is a viable alternative to reduce the negative effects of feed restriction. Dwarf broiler breeders do not need to be (severely) feed restricted. [27] [30] The resulting male progeny grow as fast as progeny from normal female parents. [31] [32] The use of female broiler female parent breeders improves feed efficiency [33] and allows a reduction of feed costs up to 33%. [34] They take up 20-30% less housing space and have more tolerance to heat. [35] Comparative performance tests proved that these are important advantages in tropical environments. [36]

See also

Related Research Articles

The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific gene depends on the number of copies of each chromosome found in that species, also referred to as ploidy. In diploid species like humans, two full sets of chromosomes are present, meaning each individual has two alleles for any given gene. If both alleles are the same, the genotype is referred to as homozygous. If the alleles are different, the genotype is referred to as heterozygous.

<span class="mw-page-title-main">Dominance (genetics)</span> One gene variant masking the effect of another in the other copy of the gene

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 the second is called recessive. This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child. Since there is only one copy of the Y chromosome, Y-linked traits cannot be dominant or recessive. Additionally, there are other forms of dominance, such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, and co-dominance, in which different variants on each chromosome both show their associated traits.

<span class="mw-page-title-main">Broiler</span> Chicken bred for meat

Breed broiler is any chicken 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 slaughter weight at approximately 14 weeks of age. Typical broilers have white feathers and yellowish skin. Broiler or sometimes broiler-fryer is also used sometimes to refer specifically to younger chickens under 2.0 kilograms, as compared with the larger roasters.

<span class="mw-page-title-main">Sex linkage</span> Sex-specific patterns of inheritance

Sex linked describes the sex-specific reading patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). In humans, these are termed X-linked recessive, X-linked dominant and Y-linked. The inheritance and presentation of all three differ depending on the sex of both the parent and the child. This makes them characteristically different from autosomal dominance and recessiveness.

<span class="mw-page-title-main">Human genetics</span> Study of inheritance as it occurs in human beings

Human genetics is the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics, and genetic counseling.

Chondrodystrophy refers to a skeletal disorder caused by one of myriad genetic mutations that can affect the development of cartilage. As a very general term, it is only used in the medical literature when a more precise description of the condition is unavailable.

<span class="mw-page-title-main">Japanese Bantam</span> Japanese breed of chicken

The Japanese Bantam or Chabo is a Japanese breed of ornamental chicken. It is a true bantam breed, meaning that it has no large fowl counterpart. It characterised by very short legs and a large upright tail that reaches much higher than the head of the bird.

<span class="mw-page-title-main">Allan–Herndon–Dudley syndrome</span> Medical condition

Allan–Herndon–Dudley syndrome is a rare X-linked inherited disorder of brain development that causes both moderate to severe intellectual disability and problems with speech and movement.

<span class="mw-page-title-main">Hereditary carrier</span> Organism with a recessive genetic allele that does not display the recessive trait

A hereditary carrier, is a person or other organism that has inherited a recessive allele for a genetic trait or mutation but usually does not display that trait or show symptoms of the disease. Carriers are, however, able to pass the allele onto their offspring, who may then express the genetic trait.

<span class="mw-page-title-main">Naked Neck</span> Breed of chicken

The Naked Neck is a breed of chicken that is naturally devoid of feathers on its neck and vent. The breed is also called the Transylvanian Naked Neck, as well as the Turken. The name "Turken" arose from the mistaken idea that the bird was a hybrid of a chicken and the domestic turkey. Naked Necks are fairly common in Europe today, but are rare in North America and very common in South America. The trait for a naked neck is a dominant one controlled by one gene and is fairly easy to introduce into other breeds, however these are hybrids rather than true Naked Necks, which is a breed recognized by the American Poultry Association since 1965, it was introduced in Britain in the 1920s. There are other breeds of naked necked chicken, such as the French naked neck, which is often confused with the Transylvanian, and the naked necked gamefowl.

<span class="mw-page-title-main">Zygosity</span> Degree of similarity of the alleles in an organism

Zygosity is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.

<span class="mw-page-title-main">Lavender (chicken plumage)</span>

Lavender or self-blue refers to a plumage color pattern in the chicken characterized by a uniform, pale bluish grey color across all feathers. The distinctive color is caused by the action of an autosomal recessive gene, commonly designated as "lav", which reduces the expression of eumelanin and phaeomelanin so that black areas of the plumage appear pale grey instead, and red areas appear a pale buff.

<span class="mw-page-title-main">Broodiness</span> Behavioral tendency to sit on a clutch of eggs to incubate them

Broodiness is the action or behavioral tendency to sit on a clutch of eggs to incubate them, often requiring the non-expression of many other behaviors including feeding and drinking. Being broody has been defined as "Being in a state of readiness to brood eggs that is characterized by cessation of laying and by marked changes in behavior and physiology".. Broodiness is usually associated with female birds, although males of some bird species become broody and some non-avian animals also show broodiness.

<span class="mw-page-title-main">Delayed feathering in chickens</span>

Delayed-feathering in chickens is a genetically determined delay in the first weeks of feather growing, which occurs normally among the chicks of many chicken breeds and no longer manifests itself once the chicken completes adult plumage.

<span class="mw-page-title-main">Hen feathering</span>

Hen feathering in cocks is the occurrence of a genetically conditioned character in domestic fowl. Males with this condition develop a female-type plumage, although otherwise look and respond as virile males.

<span class="mw-page-title-main">Solid black (chicken plumage)</span>

Solid black plumage color refers to a plumage pattern in chickens characterized by a uniform, black color across all feathers. There are chicken breeds where the typical plumage color is black, such as Australorp, Sumatra, White-Faced Black Spanish, Jersey Giant and others. And there are many other breeds having different color varieties, which also have an extended black variety, such as Leghorn, Minorca, Wyandotte, Orpington, Langshan and others.

<span class="mw-page-title-main">Solid white (chicken plumage)</span> Breed of chicken

In poultry standards, solid white is coloration of plumage in chickens characterized by a uniform pure white color across all feathers, which is not generally associated with depigmentation in any other part of the body.

Sex-linked barring is a plumage pattern on individual feathers in chickens, which is characterized by alternating pigmented and apigmented bars. The pigmented bar can either contain red pigment (phaeomelanin) or black pigment (eumelanin) whereas the apigmented bar is always white. The locus is therefore often referred to as an ‘eumelanin diluter’ or ‘melanin disruptor’. Typical sex-linked barred breeds include the Barred Plymouth Rock, Delaware, Old English Crele Games as well as Coucou de Renne.

<span class="mw-page-title-main">Creeper chickens</span>

Creeper chickens are characterised by abnormally short legs, so short that the body is carried a few centimetres from the ground. This chondrodystrophy (dwarfism) is caused by a dominant lethal allele, Cp. A number of breeds display the characteristic, among them the Chabo and Jitokku breeds of Japan, the Courte-pattes of France, the Krüper of Germany, the Luttehøns of Denmark, and the Scots Dumpy. They have been called by many names, among them bakies, brevicrews, corlaighs, crawlers, creepers, creepies, dumpfries, dumpies and jumpers.

<span class="mw-page-title-main">Auto-sexing</span>

Auto-sexing breeds of poultry are those in which the sex of newly-hatched chicks can be determined from the colour and markings of the down. Some breeds of chicken, of goose and of domestic pigeon have this characteristic. The idea of such a breed is due to Reginald Punnett, who created the first auto-sexing chicken breed, the Cambar, at the Genetical Institute in Cambridge in 1928.

References

  1. 1 2 3 4 Hutt, F.B. Genetics of the fowl. McGraw-Hill Book Co. N.Y. 1949.
  2. ↑ Strong, C.F. and Jaap, R.G. 1974 Chick size and early growth rate of dwarf broiler-type chickens. Poultry Science Vol. 53:1982-1983
  3. ↑ Leenstra, F.R. and Pit, R. 1984 The autosomal dwarf as broiler sire mated to normal and sex-linked dwarf broiler dams:Performance of progeny. XVII World's Poultry Congress and Exhibition (August 8–12) Finland.
  4. ↑ Hutt, F.B 1959 Sex-linked dwarfism in the fowl. Journal of Heredity 50:209-221
  5. 1 2 Cole, R.K. 1973 An autosomal dwarfism in the fowl. Poultry Science vol.52:2012-2013
  6. ↑ Landauer, W. 1929 Thyrogenous dwarfism (myxoedema infantilis) in the domestic fowl. Am. J . Anat. 43, 1-43.
  7. ↑ Mayhew, R. L. and Upp, C. W. 1932 Inherited (?) dwarfism in the fowl. J. Hered. 23, 269-276.
  8. ↑ Upp, C. W. 1932 Notes on a form of dwarfism encountered in Rhode Island Red fowls. Poult. Sci. 11, 370-371.
  9. ↑ Upp, C. W. 1934 Further data on the inheritance of dwarfism in fowls. Poult. Sci. 13, 157-165.
  10. ↑ Ruyter-Spira, C.P. 1998 Analysis of the chicken genome : mapping of monogenic traits (PhD thesis)Wageningen UR Library Catalogue. Record #956960
  11. ↑ Ruyter-Spira, C.P. et al. 1998 Nucleotide sequence of the chicken HMGI-C cDNA and expression of the HMGI-C and IGF1 genes in autosomal dwarf chicken embryos. Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression. Volume 1399, Issue 1, 30 July 1998, pp. 83–87
  12. ↑ Guillaume, J. 1976 The Dwarfing Gene dw: Its Effects on Anatomy, Physiology, Nutrition, Management. Its Application in Poultry Industry World's Poultry Sci. J. 32,285-304.
  13. ↑ 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).
  14. ↑ Pampín, M. et al 1988 Comparison of dwarf and normal White Plymouth Rock birds in the growing stage (original in Spanish) Revista Cubana de Ciencia Avícola Vol.15:31-37
  15. ↑ Scanes, C.G. 2009 Perspectives on the endocrinology of poultry growth and metabolism. General and Comparative Endocrinology Vol. 163, Issues 1–2, 1 September 2009, pp. 24–32
  16. ↑ Tixier-Boichard, M. et al. 1989 Physiological studies on the sex-linked dwarfism of the fowl: a review on the search for the gene’s primary effect. (Review article) Genet. Sel. Evol. Vol.21:217-234.
  17. ↑ Leung, F.C. et al. 1984 Effects of Dietary Thyroid Hormones on Growth and Serum T3, T4, and Growth Hormone in Sex-Linked Dwarf Chickens. Exp Biol Med vol. 177(1):77-81
  18. ↑ Tixier-Boichard, M. et al. 1990 Effects of dietary T3 on growth parameters and hormone levels in normal and sex-linked dwarf chickens. Domestic Animal Endocrinology Vol. 7, Issue 4, pp. 573–585
  19. ↑ Huang, N. et al. 1993 Overexpression of a truncated growth hormone receptor in the sex-linked dwarf chicken: evidence for a splice mutation. Molecular Endocrinology vol. 7(11):1391-1398
  20. ↑ Ouyang, Jian-hua et al. 2012 The Effects of Different Sex-Linked Dwarf Variations on Chinese Native Chickens Journal of Integrative Agriculture Vol. 11(9):1500–1508
  21. ↑ Jull, M.A. and Quinn, J.P. 1931 The inheritance of body weight in the domestic fowl
  22. ↑ Maw, A.J.G. 1935 The inheritance of skeletal dimensions in the domestic fowl. Sci. Agr. 16, 85-112.
  23. ↑ Custodio, R.W.S. and Jaap, R.G. 1973 Sex-linked reduction of body size in Golden Sebright Bantams. Poultry Science 52:204-210.
  24. ↑ Hsu, P.L., Buckland, R.B. and Hawes, R.O. 1975 A new dwarf isolate in the chicken: a possible new allele at the dw locus Poultry Science 54:1315-1319.
  25. ↑ Whitehead, C.C. et al. 1987 Reproductive performance of dwarf broileer breeders giving different food during the rearing and breeding periods and two lighting patterns. British Poultry Science Vol. 2:415-427
  26. ↑ de Jong, Ingrid C. and Swalander, M. 2011 Housing and management of broiler breeders and turkey breeders. Proceedings of the 30th Poultry Science Symposium, University of Strathclyde, Glasgow, Scotland, 2011. ISBN   978-1-84593-824-6
  27. 1 2 de Jong, Ingrid. C. and Guémené, D. Welfare issues in broiler breeders. Poultry Welfare Symposium (Main lecture) Cervia, Italy, 18–22 May 2009
  28. 1 2 Decuypere, E. et al. 2010 The Broiler Breeder Paradox: ethical, genetic and physiological perspectives, and suggestions for solutions. British Poultry Science Vol. 51(5):569-579
  29. ↑ Meijerhof, Dr. Ron (11 Oct 2011). "Broiler breeders: Managing the paradox between reproduction and growth". WorldPoultry.net. Retrieved June 25, 2012
  30. ↑ Dawkins, M.S. and R Layton, R. 2012 Breeding for better welfare: genetic goals for broiler chickens and their parents. Animal Welfare Vol. 21:147-155, ISSN   0962-7286
  31. ↑ Zlochevskaya, K.V. y Markov, Y. 1987 Experiencia de la firma ISA con la minigallina del propósito de carne. Ptitzevodsvo Vol. 8:39-42
  32. ↑ Godínez, O. et al. 1992 Evaluation of new dwarf broiler breeders and their hybrids (original in Spanish) Revista Cubana de Ciencia Avícola Vol 19 p.9
  33. ↑ Renden, J.A. 1984 Características del huevo y eficiencia en la producción de gallinas White Leghorn enanas divergentemente seleccionadas de acuerdo al peso corporal. (original in English) Poultry Science Vol.63(2):214
  34. ↑ Willard, R.C. 1986 Seleccione el macho que le quede a su mercado. Industria Avícola 3:12-16
  35. ↑ Kousiakis, O. et al. 1985 Comparison of dwarf and normal broiler breeder hens. Poultry Science Vol. 64:795-802.
  36. ↑ Sarker, M.S.K. et al. Profitability and Meat Yield Traits of Different Fast Growing Broiler Strains in Winter. Journal of Biological Sciences Vol. 2(6):361-363. ISSN   1727-3048
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