The Role of Genetics in Hybridizing for Production, Adaptability and Sustainability in Poultry
The intent of this essay was to answer the question of whether or not it would be possible, using genetic research, to conduct a series of hypothetical gene crosses in order to arrive at a final hybrid chicken with specific desired traits. The scope of the investigation was to research the genetic aspects of improving egg production, meat production, adaptability and sustainability in chickens, and subsequently apply that information in an exploration of hypothetical crosses in an attempt to derive a hybrid chicken which possessed all of the traits corresponding to the four categories mentioned above.
Research was conducted to provide sufficient knowledge of genetics in broad-scope, genetics relating specifically to poultry, and the characteristics of three different chicken breeds that each possessed traits which corresponded to the desired purposes of the final hybrid (egg and meat production, adaptability to environmental change, and sustainability). These breeds included the Leghorn (for egg-production), the Cornish (for meat-production), and the Silkie (for adaptability and sustainability). Specific traits and genes corresponded to each of these abilitiesegg frequency for the Leghorn, broad-breastedness for the Cornish, and broodiness along with several feathering genes for the Silkie. Each of these genes was explored using Punnett squares (charts used to predict offspring outcome) through five generations.
The conclusion which can be taken from the research and genetic exploration conducted in this investigation is that the possibility of creating a hybrid chicken with exhibition of efficient egg production, meat production, adaptability and sustainability is certainly viable, but would require further experimentation and application to live breeding stock (rather than hypothetical) to be considered concrete.
The examination of poultry genetics is especially important not only for farmers and fanciers looking to improve the efficiency and appearance of their laying, meat or show birds, but it is also extremely important in the world of food production. For the backyard chicken raiser, genetics can help to slowly improve the appearance of a bird for show, or increase the efficiency of the laying hens and meat birds that put food on the table. On a much larger scale, however, the exploration of poultry genetic science has already changed, and will continue to change the way that eggs and meat are produced for the population. Through eliminating unwanted mutations and crossing the most proficient breeds to produce hybrids, the efficiency of food production can be dramatically increased. By extension, these efficient egg and meat producing breeds can be genetically improved simply through breeding, in order to eventually produce a self-sustainable breed that can provide food while requiring little maintenance. If and when genetic exploration can result in a hybrid chicken which possesses many optimal traits, the extreme importance of understanding genetics in poultry can reach its maximum potential.
In order to achieve this optimal hybrid chicken, four necessary traits must be present after multiple crosses:
- Efficient egg production
- Adequate meat product after egg-laying has ceased
- Hardiness/adaptability in cold weather and confinement
- Sustainability (ability to breed naturally to increase population)
With over 120 breeds of chicken recognized by the American Poultry Association, all of these characteristics can be found within at least a portion of breeds ("American Poultry Association"). However, there is a superior breed that epitomizes each of these specific traits and would have dominance in passing them to further generations in a breeding cross. The breeds corresponding most precisely with each trait are as follows:
- Egg-laying= Leghorn
- Meat-Production= Cornish
- Adaptability/Sustainability= Silkie
The intent of this essay is to examine each of these breeds and their ideal traits, and by extension, explore genetic crosses and factors in order to answer the following question: Is it possible, by means of breeding, to arrive at a final predicted hybrid chicken in which the above characteristics are present in a balanced manner?
II.Genetics in Poultry
In order to be able to fully understand and experiment with genetics of fowl, it is important to first explore the overall topic of genetics in the field of biology.
When looking into genetics as a part of breeding, factors such as genotype, or the genetic makeup of an individual, and phenotype, or the physical characteristic displayed as a result of that genotype, are taken into account (O'Neil, "Glossary of Terms"). When genotypes and phenotypes of parents are crossed, the genes and alleles, or components of those genes, essentially determine the genotypes and phenotypes of the offspring (O'Neil, "Probability of Inheritance"). A gene contains two alleles which are either dominant (meaning they are expressed phenotypically) or recessive (meaning they are masked). For example, the gene in humans for brown eyes is dominant, while blue eyes are recessive. Outcomes of the offspring can be determined by analyzing the combination of parent genes with the use of Punnett squarescharts that help to determine what the outcomes of the cross of maternal and paternal genes will look like (O'Neil, "Probability of Inheritance"). Paternal genes are at the top of the chart, maternal genes on the left, and offspring genes in each of the squares. The offspring outcomes from a cross can yield a certain number of results that are either homozygous dominant, heterozygous dominant or homozygous recessive (O'Neil, "Probability of Inheritance"). Homozygous dominant offspring carry two dominant alleles (represented by capital letters) and express the dominant trait, thus exhibiting the dominant phenotype; homozygous recessive offspring carry two recessive alleles (represented by lowercase letters) and thus exhibit the recessive trait and phenotype; heterozygous dominant offspring carry one dominant allele and one recessive allele, but exhibit the dominant trait (O'Neil, "Glossary of Terms"). An example cross is shown in Figure 1 below. Crosses can also involve determining all possible combinations for genes with more allelesthis is called a dihybrid cross, shown on the right in Figure 1 below.
Punnett squares are key in predicting outcomes of genetic crosses. However, in production poultry breeding they are most often used as a tool to experiment with a variety of different crosses until the desired outcome offspring are derived.
In poultry, the outcome of offspring is greatly impacted by the presence of sex-linked traits, or genes carried on the sex chromosome that can affect male and female offspring in different ways (Stevens 50). In humans, two X's for females and an X and a Y in males represent these types of traits for genetic analysis. For poultry, a similar concept is used, however the hen is heterogametic (possessing two different gametes), thus represented by a Z and a W, while the rooster is homogametic (possessing two of the same gamete), and represented by two Z's (Stevens 50). Sex-linked traits are carried only on the Z gamete for birds, meaning the female carries one, while the male carries two. Certain traits are sex-linked, thus passing to either males or females in each cross. Generally with sex-linked traits, a 'criss-cross' inheritance takes placewhen a male possesses a sex-linked trait, it passes to the female offspring, and when a female possesses a sex-linked trait, it passes to male offspring (Stevens 55). For example, the barred (black and white striped) feather pattern in certain breeds such as the Dominique and Plymouth Rock, is a sex-linked trait (Schmid et al. 171). If the dam (mother) of a cross has the barred feather pattern and the sire (father) does not, the male offspring will have a barred feather pattern while the female offspring will not. This takes place because of the way in which the sex-linked genes are carried by the parent birds. For this trait, the genotype of the male would be Zb-Zb, meaning he is homozygous recessive, and neither exhibits or carries the gene for barred feather pattern. The female genotype would be ZB-W, meaning she is homozygous dominant for the barred feather pattern. A cross for sex-linked barred feather pattern is shown in Figure 2 below.
Sex-linked genes in poultry often affect feather pattern, egg production and body size, making them essential for the hybridization of a production bird (Schmid et al. 171). For egg production specifically, they play a role in that the genes influencing egg color, size and laying frequency are often passed down from just one parent. Egg color is most often influenced by the genes of the father, while egg production (laying frequency) is attributed the genes of the mother (Schmid et al. 171).
In production strains of poultry, genetic alteration is based on the needed outcome. For example, if prolific egg production is the goal, certain genetic crosses (the genetic background) will provide the most efficient strain of egg-laying birds. The genetic background of any bird is key to its abilities as a producer and as a breeder. The challenge lies in combining genetic backgrounds of multiple birds to produce an amalgamation of desired traits while eliminating unnecessary or unwanted traits. If this hybridizing could be achieved, however, the outcome would be an adaptable breed of chicken suited for efficient or even prolific production of eggs and meat which would also possess qualities of appearance and docile personality.
Each of the three chicken breeds mentioned in the introduction of this investigation were chosen due to their outstanding efficiency at producing, or due to a trait which puts them apart from other breeds. All three breeds are unique in that they have their own individual appearances, behaviors, and abilities which will ultimately contribute to a combination of these superior characteristics and thus an ideal hybrid bird.
a.Egg Producing Breeds - White Leghorn
The White Leghorn chicken is one of the most popular breeds among commercial producers and small-scale flock owners. The breed is known for its exceptional laying abilities, and low maturity age, which allows for faster and more efficient breeding, and by extension, improved production (Ekarius 58-59).
The Leghorn, thought to have been developed first in the 1870's in Italy and then imported to North America in 1950, is a base breed for many other egg-producing and dual-purpose breeds developed later (Ekarius 58). Leghorn traits were considered in the breeding for development of several producer breeds common today such as the Rhode Island Red and some varieties of Wyandotte and Plymouth Rock. Their excellent laying abilities and rapid maturity serve as their most valuable traits, and place them as perhaps the most efficient egg-laying chicken breed ("History of Breeds"). A White Leghorn hen was one of the first to set a record of 357 eggs in 52 weeks in 1930. In 1945, twelve Leghorn sisters had an average of 312.1 eggs in 51 weeks (Hutt 501).
In attempting to genetically produce a strain of chickens with optimal egg-laying abilities as a main trait, Leghorns would serve as the carrier of the necessary genetic components for this outcome.
b.Meat Producing Breeds - Cornish
The Cornish chicken is the breed which lies at the basis of all strains of commercial and small-scale meat producing breeds. The Cornish, originally developed in England, was the result of a cross between several game birds. These included the Aseel, which provided the original bloodlines for the breed, the Old English Game, and the Malay game (Ekarius 81; "History of Breeds"). The outcome of this cross was a bird with wide and deep breast (characteristic of meat birds), thick legs widely set apart, and a back that slopes downward (Ekarius 81). The breed also has narrow feathers which lack the fluff, or soft component at the base of a feather, making meat processing simpler. For these reasons, the Cornish itself is an excellent meat-producer. However, when crossed with a Dual-Purpose breed (Plymouth Rocks are the most common for this purpose) can produce hybrid offspring who possess all of the necessary and ideal qualities of a meat bird (Ekarius 81-82).
c.Non-Producing Breeds - Silkie
The Silkie, though not a food producing breed, possesses optimal qualities such as docile nature, broodiness (the natural instinct to hatch and raise chicks), adaptability and appearance. The origin of the Silkie is not certain, however they are said to have Asian derivation. They were admitted the American Poultry Association Standard of Perfection in the late 1800's as an ornamental breed (Ekarius 158).
Perhaps the most well-known characteristic of the Silkie is its broody instinct (Ekarius 158-159). This trait allows for the natural hormones in a hen to cause periods of broodiness, or instinctive hatching and raising of chicks. In commercial egg production, this trait is completely undesirable because it causes the hen to cease laying eggs for a period of up to three months (Romanov 1). However, in moderation this instinct can be beneficial in that it allows for natural population increase within a flock.
Silkies also possess some undesirable traits in terms of production, making them a harder breed to incorporate in hybridization. They are known for their unique featheringtheir feathers lack barbs to hold them together, giving them the appearance of hair (Hollander). However, these feathers do not trap heat and do not repel water like the feathers of other birds, making them slightly less adapted to extreme weather. On the contrary, their small combs, muffs and beards (facial feathers) and feathered feet make them less susceptible to frostbite. Silkies have a melanotic gene resulting in fibromelanosis, or black skin and meat, making them an undesirable bird for meat production (Ekarius 158).
IV. Hybridization-Genetic Crossing for Optimal Qualities
Hybridization to include all desired qualities (egg production, meat production, adaptability, appearance, and minor traits such as broodiness, small combs and feathered feet) would have to take place over multiple generations. If the process was started from parent stock of three different breeds, it could be hypothesized that at least four different generation crosses would be essential.
In order to predict likely outcomes, it is necessary to examine each of the characteristics and their genetic tendencies in relation to their corresponding breeds.
a. Examination of Characteristics
The genetics behind egg production in chickens are complicated, consisting of many factors which are important to a varying degree. In commercial egg production, factors are monitored with a great degree of detail. These factors include the age of a hen at sexual maturity (when laying begins), the rate of lay before and after molting, egg weight, egg shell color, egg shell strength (thickness), and egg inclusions (defects such as blood or meat spots in the egg) among others (Muir 1,3). However, for more small-scale breeding and production the essential traits to monitor and select would most likely include rate of production, egg size, age at maturity and perhaps egg color.
Egg producing offspring must come from a strong line of prolific producing birds. If prolific egg production is the goal, certain genetic crosses will provide the most efficient strain of egg-laying birds. When looking to improve efficiency of egg production, a significant number of factors play a role. According to F.B. Hutt, Professor of Animal Genetics at New York State College of Agriculture, Cornell University, these factors include, but are not limited to, environmental influences on the birds, anatomical and physiological differences in birds, hormone levels, and breeding factors such as domestication and strain (491-500).
Perhaps the most significant of these factors in relation to genetics and breeding is that of strain. The precise number of genes that directly and indirectly affect egg production is unknown. However, numerous studies have been conducted by researchers since the beginning of the 20th century to determine which, if any, specific genes alter the production of eggs. Raymond Pearl conducted several of these studies in the early 1900's, and hypothesized that two pairs of genes, one autosomal and one sex-linked, determined the egg-laying ability of any given hen (114-119) . Numerous other studies concluded similar theories of two or three gene pairs carrying the egg-laying traits (Hutt 500-509).
With Pearl's hypothesis of one autosomal and one sex-linked trait affecting egg producing abilities of a hen, it is apparent that both parents will have an effect on the laying abilities of the offspring of any given cross. Thus, to have a prolific egg-producer, the beginning parent stock must both be from egg-laying breeds and strains.
Egg production itself, in the genetic sense, is thought to be influenced by a sex-linked trait from the sire (Bowis). However, strain (essentially the family history) of the dam is also greatly important. When choosing parent stock for a cross of egg-laying birds, the male is the dominant factor because egg production is partially a sex-linked trait (Bowis). If the rooster carries dominant genes for efficient egg-production, the hen offspring (who will be responsible for future egg production) will possess the same trait, regardless of the mother's genotype because of the 'criss-cross' inheritance (Stevens 55).
Multiple gene crosses (Punnett squares) can be used to predict the outcomes of a cross between two productive birdsa Leghorn rooster (to pass production genes) and a Leghorn hen (to influence strain). The first cross involves a dam and a sire of the same breedthis is to ensure strong egg production abilities before a first hybrid is bred. In the Punnett squares below, the allele 'E' represents prolific egg production. The genotype of the rooster is shown as ZE-ZE, because in this situation, the rooster would be chosen from a strain of productive hens, and would have been hatched from the egg of the most productive hen, thus having dominance. The genotype of the hen is shown as ZE-W, also meaning she was chosen for her production abilities. Figure 3 below shows the cross of the sex-linked gene for egg production, with all male and female offspring expressing a homozygous dominant genotype. As mentioned above, the male has dominance in this sex-linked cross, meaning the female offspring will carry a dominant gene for superior egg production even if the mother does not. This hypothetical cross is shown in Figure 4 below, with male offspring expressing a heterozygous dominant genotype, and female offspring still possessing a homozygous dominant genotype.
When crossing autosomal genes for egg production, results are similar to those of the sex-linked crosses. Regardless of whether the female carries dominance for efficiency of egg production, all offspring still carry dominance from the sire. In a cross where the dam carries partial dominance (heterozygous), 50% of the offspring also carry partial dominance. In a cross where the dam is recessive, all offspring carry heterozygous dominance. These crosses are shown in Figure 5 and Figure 6 below.
As perhaps the most valuable quality in a production chicken, egg-producing abilities are extremely important. From the hypothetical crosses, it can be predicted that as long as the father comes from a strain with strong egg production, the offspring will carry that trait as well.
Commercial meat poultry, usually crosses of Cornish and White Rock, are bred for rapid growth, efficient feed conversion, broad-breastedness for more meat and limited feathering for easy plucking in processing (Fanatico and Polson 1). They are extremely specialized to produce sufficient meat at a rapid rate. For purposes of small-scale meat production in a hybrid chicken, these factors change. Since meat is not the hybrid bird's sole purpose, factors such as rapid growth rate and efficient feed conversion are not necessary to consider, as the birds would ideally live a full egg-production cycle (usually one to two years) before being used for meat (Mercia 70). This means that just a Cornish cross would possess too many dominant qualities of a meat producer, such as weak legs, high risk of heart attacks and poor foraging ability that would decrease its viability as an egg-layer (Fanatico and Polson 1). Consequently, it would be necessary to first breed a Cornish cross, and then introduce that cross into the egg-laying strain. The first cross would most likely come from parent stock with a meat producing breed and an egg-laying breed, perhaps a Cornish sire and a Leghorn dam. A cross of the autosomal gene for broad-breastedness (a recessive trait) is shown below. All offspring are heterozygous dominant for this trait, making them excellent meat-producers.
At this point, the second generation consists of Leghorns (all homozygous dominant or heterozygous dominant for egg production) and Cornish-Leghorn crosses (heterozygous dominant for meat production). To incorporate the final traits of adaptability and sustainability, Silkie genes must be integrated in the next generation crosses. Due to the unique genes of the Silkie, this generation cross takes more careful consideration. Certain undesirable traits can be avoided by properly sex-linking (choosing a sire or a dam). The traits which will be incorporated in the final hybrid are those of feathered legs (a dominant sex-linked trait), walnut combs (a dihybrid autosomal cross) and muffs/beards (muffs are a dominant autosomal trait, beards are recessive autosomal) all for adaptability to cold temperature (Hollander). Broodiness, a dominant autosomal trait, will be included for natural population growth (Romanov 1). The fibromelanosis (a dominant sex-linked trait) is undesirable in meat-producing birds, and will thus be eliminated. Due to their complicity, the Silkie traits are organized in Figure 8 below, along with their crosses and outcomes. The crosses are the results of a breeding whose parent stock was a Silkie sire and a non-Silkie dam (one of the hens
Offspring= Feather Legged
WALNUT COMBS (dihybrid of Rose and Pea combs)
Sire=Walnut Comb (RRPP)
Dam=Rose Comb (RRpp)
Offspring= Walnut Comb
Homozygous Dom. (Rose Comb)
Heterozygous Dom. (Pea Comb)
MUFFS AND BEARDS
Sire= Muffed and Bearded (MMbb)
Dam= Clean-faced (mmBB)
Offspring= Muffed and Bearded
Heterozygous Dominant (Muffs)
Heterozygous Dominant (Beard)
Sire= Broodiness (RR)
Dam= No Broodiness (rr)
Sire= Fibromelanosis (ZFm-ZFm)
Dam= Yellow Skin (Zf-W)
Male-Heterozygous Dom. (Fibromelanosis)
Female- Homozygous Dom. (Fibromelanosis)
The undesired trait, fibromelanosis, remains dominant in both male and female offspring. A fourth generation cross needs to take place to begin to eliminate this trait. The success in eliminating the dominance of fibromelanosis in the fourth generation (final hybrid) depends on the correct selection of dam and sire due to the sex-linked nature of the gene. The third generation females carried dominance, and therefore could not serve as dams. However, the males only carry partial dominance. If a third generation male (recessive for the trait) was crossed with one of the second generation females (heterozygous), at least a portion of offspring would be recessive for the fibromelanosis gene. This cross is shown in Figure 9 below.
Sire= Fibromelanosis (ZFm-Zf)
Dam= Yellow Skin (Zf-W)
Offspring= 50% Fibromelanosis, 50% Yellow Skin
Male-50% Heterozygous Dominant (Fibromelanosis), 50% Homozygous Recessive (Yellow Skin)
Female- 50% Homozygous Dominant (Fibromelanosis), 50% Homozygous Recessive (Yellow Skin)
With a fifth generation cross using either a dam or sire from the fourth generation, the gene could potentially be completely eliminated. However, the risk of eliminating other desired traits increases with each subsequent cross, unless the individuals chosen carry full dominance for the traits which need to remain in the genetic makeup. A hypothetical cross between a fourth generation recessive sire and a second generation recessive dam is shown below in Figure 10.
After the fifth generation cross, the fibromelanosis gene can be successfully eliminated, as all male and female offspring are homozygous recessive.
Through research of breed purposes and characteristics, and the specific genetic makeup of those characteristics, the intent of this investigation was successfully fulfilled. It can be concluded from the data that it is possible to derive a hybrid chicken which possesses traits for egg production, meat production, adaptability and sustainability. After five generations of crosses, a hybrid was developed. This hybrid carried the genotypes and phenotypes listed in Figure 11 below.
Although the results are viable in evaluating a hypothetical breeding program, it is necessary to note that these crosses show the genetic combinations at their most simplistic level. Without specific breeding stock and individual birds, it is impossible to precisely and accurately determine the exact outcomes of offspring. Mutations and masked genes could play a role in the outcomes, but were not seen in the predictions of hypothetical crosses. However, these crosses provide exploration of the genes so that a fairly accurate prediction of possible outcomes can be derived.
Ultimately, complete conclusions cannot be derived from this investigation without further application to live breeding stock. The conclusion which can be taken from the research and genetic exploration conducted is that the possibility of creating a hybrid chicken with exhibition of efficient egg production, meat production, adaptability and sustainability is certainly viable, but would require further experimentation and application to be considered concrete.
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