Chapter 6 Pig genetics.
Pig improvement through genetic selection relies on tools and knowledge developed over centuries of effort. This chapter describes the biological basis on which genetic changes are made and introduces methods of selection for various traits of economic importance. Emerging tools involving molecular genetics and other new technologies are explored and described.
THE CHANGING PIG
For centuries, animal breeders have capitalized on the impressive variation in the genome of the domestic pig. Imagine two pigs standing side-by-side: a European wild boar and a modern meat-type pig. It is hard to imagine that one derives from the other. But, in fact, the recombination of genes from the wild ancestor results in the current pig found on farms around the world--from the fattest to the leanest and the most to the least prolific.
This chapter reviews procedures for the gradual improvement of biological
traits through genetic selection. More modern techniques that will undoubtedly dominate the industry in the near future include tools of molecular biology that will spread and hasten genetic change at rates that far exceed the older techniques. The following describes the progression of genetic selection over the centuries.
THE PROGRESSION OF GENETIC SELECTION * 11,000 yr ago through Breed like to like; propagate same about mid 1980s * Mid 1900s through 1980s Breed for desired genotypes using carefully measured phenotypes * 1980s through 1990s As above, plus use of sophisticated Best Linear Unbiased Predictor (BLUP) computer models for selection * 1990s As above, plus marker-assisted selection and concentration of certain genes with known effects * Future Transgenic pigs with specialized traits or features of interest to food production
The effects of genetic selection have not changed from prehistoric times. What has changed, as new tools for selection are incorporated, is the rate of genetic change. Using mathematical procedures like best linear unbiased prediction (BLUP), animal breeders can effect change in the pig genotype in much less time than with previous methods. Applying modern tools of marker-assisted selection will speed genotype improvement even further. The international effort called the Pig Genome Project will help to elucidate important genes as the pig genome is mapped. New genotypes created in research laboratories such as transgenic technologies will be applied to farm animal production in ways similar to the application of animal models to biomedical problems.
THE U.S. GENE MAPPING PROGRAM CAN BE VIEWED ON THE WORLD WIDE WEB AT: http://www.public.iastate.edu/~pigmap/pigmap.html
ADDITIVE VERSUS NONADDITIVE GENE ACTION
Gregor Mendel, the Czechoslovakian monk, bred plants in the late 1800s and carefully recorded the outcome. Mendel's work was lost for decades, then rediscovered during the birth of the field of quantitative genetics. Quantitative genetics resulted from the assignment of traits to genes transmitted through selective breeding.
Mendelian genetics refers to the simple quantitative inheritance of genes that express themselves in defined and predictable phenotypes. Animals of AA genotype, bred to AA, will yield all AA offspring. This is the outcome of breeding like to like. If an animal with the genes AA is bred with an animal bearing aa alleles, all offspring will be Aa. However, when two animals, both with the genotype Aa are bred (to the wonder of early animal breeders), breeding "like to like" does not yield like offspring.
If pigs with the genotype Aa are bred with a mate bearing the Aa genotype, the offspring genotypes fall in the predictable, Mendelian distribution of:
Genotype Genotype Distribution Phenotype AA 1/4 A-like Aa 1/2 A-like aa 1/4 a-like
The phenotype distribution falls in only two categories: those like A and those like a. Certain aspects of coat color in pigs have this general genetic mechanism. If a purebred white sow is bred to a boar of any other color, the offspring are all white. Thus, the purebred could be said to have the AA genotype and a white-coat-color phenotype. All offspring would have a white coat color and would be either AA or Aa, regardless of the boar's coat color in the boar's genes.
In practice, some traits are controlled by one or just a few genes and are said to follow Mendelian genetics. Still other traits are controlled by many genes, and the distribution of individuals follow a normal distribution. These traits are polygenic and, thus, may have several genes that code for a trait. Most traits of economic importance to the pig industry are polygenic.
When more and more genes code for a particular trait, the distribution changes from just three phenotypes to more and more genotypes depending on the number of genes. The distribution of genotypes and phenotypes approaches a normal distribution with just three pairs of genes, segregating at random in a population. Common, economically important traits in commercial pork production have a normal distribution and, thus, many genes are involved, each contributing an incremental effect on a given trait. From time to time, a major gene is discovered that contributes a much greater effect on the trait.
The objective in genetic improvement is to move the mean of the population in a desired direction (to increase or decrease). For some traits, like lean gain, producers seek to increase the value. For other traits, like backfat thickness (or % body fat), producers seek to decrease the mean of the herd (although some modern lines may be too lean). The objective for the animal breeder is to shift the population either left or right.
Figure 6-1 shows an example herd profile adapted from PIC USA, Inc. To affect genetic change, sires and dams must be superior to the present herd average. By eliminating the poor-performing animals through culling (discarding lower-valued animals) and by adding genetically superior animals, the herd average improves.
Some traits have more than two possible genes at a given locus. For example, instead of just A and a genes there may be A1, A2, A3, or A4 genes. Such a polygenic trait would be more difficult to select. It is quite possible that traits of economic importance are controlled by many genes.
A highly inbred genetic line is one in which relatives have been mated to create uniform animals that are homozygous (AA or aa) for many traits at many loci. Purebred livestock were selected over many generations to be uniform in certain traits and only using certain families of animals to start with. Thus, although pure breeds of livestock are more homozygous than crosses, breeds are not as homozygous as inbred lines.
[FIGURE 6-1 OMITTED]
Certain phenotypic traits change more consistently in incremental steps from several genes. In an oversimplified example of gene effects on average daily gain (ADG), the unit of growth for animals containing these genes might be:
Genotype ADG, lb/d ADG, kg/d GG 1.6 0.72 Gg 1.5 0.68 gg 1.4 0.64
Currently, there is no known, single gene that would have such an effect on ADG, but it could be discovered. Simple Mendelian genetics can improve ADG. As pigs with faster ADG are selected, producers are more and more likely to be selecting for one or two copies of the G gene. This type of incremental gene action is called additive gene action--the small contribution of many genes to a trait. Traits that respond well to the effects of additive gene action are those associated with growth and body composition (especially fat content). For traits that respond through additive gene action, the offspring trait is the average trait of the parents. Heritability estimates are measures of additive gene action. Heritability refers to the proportion of the variation that is attributed to genetics relative to the total trait variation (due to genetics and environment). Table 6-1 gives heritability estimates for common traits.
Heterosis has its greatest effect on traits that tend to be low in their heritability estimates. Thus, reproductive performance and health tend to be thought of as traits that respond with greater heterosis. Table 6-2 shows measures of heterosis that represent the views of many authors.
Some traits do not respond as a simple addition of individual gene effects. Traits that express a greater effect for one or more genes than the sum of the individual gene effects are influenced by nonadditive gene action. When the average of the offspring differs from the average of the parents, the trait is said to express heterosis, or hybrid vigor.
There are several types of heterosis:
a. Individual heterosis--The improvement in performance traits in a pig such as growth rate, feed efficiency, survival, etc., due to its crossbreeding relative to that of its pureline parents.
b. Maternal heterosis--The improvement in performance in sows and their offspring from using a crossbred dam (e.g., improved pre- and post-natal environment, larger litter size, greater rebreeding rate, etc.).
c. Paternal heterosis--The improvement in performance in boars and their offspring from using a crossbred sire (e.g., improved libido, persistent breeding, longevity, etc.).
Estimates for heritability for economically important growth and carcass traits are given in Table 6-1. These estimates are adapted from the National Pork Producers Council 1995 Genetic Evaluation Program (NGEP). All traits have a heritability estimate that is greater than zero. With these moderate heritability estimates, producers could make genetic progress in these traits through selection.
The data for meat quality traits such as tenderness and water-holding capacity have lower heritability estimates and, thus, slower genetic improvement is possible for these traits. Still another problem with measures of tenderness is the need to kill animals to test for tenderness. This is not necessary for measures such as ADG and body fat percentage. Because slaughter eliminates the ability of high-quality animals to reproduce, genetic improvement in meat quality may be slower. Identification of genetic markers or genes that control pork quality will greatly hasten the search for improved pork quality. Progeny tests are conducted to evaluate performance traits of breeding lines.
Breeds of pigs vary in meat quality, especially intramuscular (within the muscle) fat percentage (Table 6-3). Packers who seek quality pork products have encouraged use of the Duroc and Berkshire breeds to add flavor and tenderness as well as intramuscular fat.
Seedstock producers and commercial producers, who make their own replacement seedstock, seek constant genetic improvement. The rate of genetic improvement ([DELTA]G) is determined by three factors:
* Heritability estimate ([h.sup.2]) of the trait
* Selection differential (SD)--the difference of the selected animals from the herd average, calculated as: (average of selected animals--herd average)
* Generation interval (GI)--the average age of breeding stock when replacements enter the herd
The formula for genetic improvement is:
[DELTA]G = [h.sup.2] x SD/GI = rate of genetic improvement
For example, assume a producer wishes to lower the trait for backfat thickness. With replacement seedstock averaging 0.7 in and the herd average at 1.0 in of backfat, an [h.sup.2] of 0.46 (see Table 6-1) and a generation interval of 2.0 yr, the formula works out like this:
[DELTA]G = 0.46 x (0.7 - 1.0)/2.0 = - 0.069 in/yr
At the end of one year, the producer expects the herd to lower its backfat thickness by 0.069 in (starting from 1.0 in). The year-end backfat thickness would be 0.931 in (not quite a 0.10-in improvement in backfat thickness). The [h.sup.2] is fixed and the generation interval, for a given herd, is difficult to change. The easiest way to increase the rate of genetic progress is to have a greater SD.
Many animal breeding projects have quantified genetic improvement over time. Figures 6-2 and 6-3 show data from herds that have been selected for changes in economically important traits. The USDA scientists at Beltsville, Maryland selected genetic lines to produce boars and gilts with increasing or decreasing backfat thickness. Over the 12 generations, the lean line had 50% less backfat than the fat line. Fat pigs tended to be shorter in body length and had a shorter height. Fat pigs were more round-shaped with a larger circumference. Breeding stock companies use the same process to reduce backfat thickness of its commercial seedstock. In addition to growth and body composition, reproductive traits like ovulation rate can be improved with selection, even though the [h.sup.2] is lower and genetic progress is slower.
Crossbreeding systems are used to capitalize on traits controlled by both additive and nonadditive gene action. If the maternal line is selected for the desired body composition and meat quality, using crossbred females, advantages in health, reproduction, and mothering ability (especially milk production) may be captured by the heterosis (Table 6-3). The amount of heterosis obtained by crossbreeding systems is outlined in Table 6-4. A similar logic can be used to determine the amount of heterosis for any matings of combinations of pure lines.
[FIGURE 6-2 OMITTED]
[FIGURE 6-3 OMITTED]
The sire line has traditionally been a purebred animal because producers felt there was little advantage to use of crossbred boars. This view is largely changed today. Producers now favor use of crossbred boars, which are thought to have better health, more sperm production, and better libido. These are reasons enough to use crossbred boars, even if the benefits are limited.
Crossbreeding systems seek to obtain as much of the available heterosis as possible. For a simple two-way cross such as A x B, all the offspring have 100% of available heterosis, while the parents have 0% of the possible heterosis. Use of rotational crossing systems has the advantage that only boars (or semen) must be brought onto the farm, thus reducing biosecurity concerns. The disadvantage of rotational systems is that less that 100% of the available heterosis is obtained in the progeny. With terminal crossing systems, 100% of the available heterosis is obtained. The biosecurity risk is increased because new females and males must enter the farm.
The modern business of pork production must use the best genetics and animal breeding systems that are affordable. Nearly all of the large-scale, commercial producers today use a terminal breeding system. To handle their biosecurity concerns, large farms use multiplication units that receive grandparent or great-grandparent stock, thus minimizing the number of animals that enter the herd (and thus reducing risk of disease entry). Biosecurity is improved because of the low number of animals entering the herd and the ability to test isolated breeding stock in the multiplication unit before possible contamination of the core herd. Thus, genetic progress and biosecurity represent a balance that must be struck for attainment of the herd goals--healthy animals of good market value.
BALANCE BETWEEN GENETIC PROGRESS AND BIOSECURITY Genetic progress and biosecurity must be weighed carefully. Genetic progress Biosecurity [DELTA]
TWO-WAY CROSSES--TERMINAL AND ROTATIONAL
The simplest crossbred animal is a two-way terminal cross. Crossing a maternal line with a meat line makes a great deal of sense--for example, a Landrace sow with a Hampshire boar. The resulting Y x H pigs would be sold as (mostly white) crossbred market animals. None of the offspring would be kept as replacements. Replacement maternal lines and sire lines would either have to be created from internal purebred herds or purchased. Parents have 0% of the available heterosis while offspring have 100% of the available heterosis.
The two-breed rotational cross requires that the producer purchase only replacement purebred boars of two genotypes. The sows (line A, for example) are first mated to boars (line B, for example). The next generation is selected from among the offspring and would be mated to line-A boars. The third generation is mated to line-B boars, and the cycle continues. Sows are always crossbred and boars are always purebred in this scheme. The boars have 0% of the available heterosis; the sows and piglets have an average of 68% of the available heterosis because the dam has some of the sire's breed in her genes.
THREE-BREED CROSSES--TERMINAL AND ROTATIONAL
Crossbreeding programs that use three breeds have the advantage that the maternal line is a cross, so females will exhibit heterosis in their health, reproduction, and maternal traits. The three-breed terminal cross captures 100% of the possible heterosis in the market hogs, while the three-breed rotational cross market hogs capture less than 100% (an average of about 86%).
Although some heterosis is lost by use of rotational crosses, they are still used. One major advantage of the rotational cross is that replacement gilts do not need to be purchased. Aside from their expense, bringing replacement gilts onto the farm poses a biosecurity risk to herd health (see Figure 6-4).
[FIGURE 6-4 OMITTED]
Three other crossbreeding programs are the rotaterminal, the four-breed rotational, and the four-breed terminal cross systems. The three- or four-breed rotaterminal uses a rotational system for the maternal side but uses a terminal sire. The four-breed terminal system uses a two-way cross sow (it could even use a three-way cross sow) and a two-way cross boar. This is the first time in the examples of breeding systems that a crossbred boar is used. The crossbred boar may have advantages in sexual drive, sperm production, longevity, and health.
Commercial lines of pigs resemble a five-way cross in many cases. The sow lines are most commonly composed of three breeds: Yorkshire, Landrace, and Duroc. The sire lines may be Hampshire, Pietrain, Duroc, or other breeds.
From a meat quality effect, the Hampshire breed has the disadvantage of adding a factor for poor meat quality, known as the "Hampshire effect." Note in Table 6-3 that meat from the Hampshire breed has the lowest ultimate pH and the highest Minolta color score (indications that the pork may be pale and lack water-holding capacity). These values indicate a greater tendency toward poor meat quality. The added benefits of the Hampshire for lean gain may offset some of the meat quality problems.
Many crossbreeding systems use Duroc in the maternal and/or the sire lines. The Duroc adds extra intramuscular fat--a desirable effect for eating quality of pork--and fewer meat quality problems. The Berkshire breed has been associated with greater pork quality by some pork buyers as well, but the data in Table 6-3 do not support use of the Berkshire over the Duroc for meat quality traits.
Several recent examples highlight the potential applications of molecular genetics in commercial pork production. The industry will continue making progress in applying technology in molecular genetics to the improvement of commercial lines of pigs.
Traditional selection relies on measuring the phenotype as a predictor of the animal's genotype. Environmental effects are often highly significant determinants of the observed phenotype, so more precise selection technologies that improve the odds of actually selecting genetically superior animals are always desired.
Short of having identified the actual causative gene, the pig breeder would like to have a gene that is closely linked to the desired trait. This selection technique is called marker-assisted selection by use of quantitative trait loci (QTL). QTL are known genes that are either (1) physically close on the chromosome to the desired genes or (2) will be shown later to be directly involved in the expression of the desired gene.
For example, assume that a gene that serves as a marker for lean growth was found. This gene would not, by itself, be the sole determinant of fast, lean growth. Rather, this gene would be closely linked to some of the genes that are positive (or negative) for lean growth. When animals with this gene are retained, lean growth increases. Thus, this marker will add to the phenotypic information available to the animal breeder. The selection, therefore, can be more rapid.
THE HAL GENE
Early scientists anesthetized pigs with the gas anesthetic halothane and found some pigs developed a rapid increase in body temperature and often died. This condition, known as Porcine Stress Syndrome, or PSS, is identical to the human condition known as malignant hyperthermia. PSS pigs often have poor meat quality (known as PSE pork--pale, soft, and exudative or watery).
When Canadian scientists first located the gene for PSS/malignant hyperthermia, they called it HAL-1843 (HAL-1843 is a registered trademark of Innovations Foundation, Toronto, Canada). The trait is a recessive gene that is considered a mutation. Nn pigs are normal and show few signs of stress susceptability. Nn pigs are called mono-mutants or carriers because they have one copy of the gene. These pigs are leaner, but have some meat quality problems. Pigs with the nn genotype are called dimutants because they have two copies of the gene. Dimutants are at risk when they experience even mild stress. Pigs of nn genotype can have significant meat quality problems.
Most geneticists and industry leaders feel the mutated HAL gene should be eliminated from the pig herds of the world. Consumers do not like the meat from [HAL.sup.mm] pigs, and these pigs are more stress susceptible and therefore are at risk from an animal welfare point of view. The HAL gene is not needed to get lean growth and lean pork.
Pig breeders who use the HAL gene do so in a controlled manner. The sow herd would be negative for the gene (HAL--or NN). The boar lines would be either Nn or nn. If the boars are Nn, one-half of their offspring will be of the Nn genotype. If the boars are nn, 100% of their offspring will be Nn.
THE ESR GENE
Iowa State University scientists, in collaboration with PIC USA, Inc., discovered a gene that influences ovulation rate in pigs. The gene is a variation of the estrogen sulfate receptor (ESR) gene. Pigs can have one or two copies of this gene and each copy is reported to add about one pig born alive/litter. The gene is a registered trademark of PIC USA, Inc. under the name [PIClit.sup.+]. PIC USA, Inc. is reportedly multiplying this gene in its seedstock.
THE K88 ADHERENCE GENE
Pigs, like other animals, are susceptible to various strains of E. coli that cause intestinal problems, including diarrhea. The E. coli attach to an intestinal receptor by a protein called K88. Some pigs lack the K88 antigen and, therefore, E. coli do not attach to the intestinal wall and colonize. Pigs that lack the K88 antigen have less problems with scours from E. coli. The relationship between E. coli adherence and disease susceptibility is not perfect. The genetic mechanisms of this condition are not fully worked out, but this is a good example of how genetics can play a significant role in disease resistance.
THE PIG SKIN COLOR GENE
After knowing for centuries that white coat color is dominant in pigs, molecular biologists identified the gene that codes for white skin in pigs. The gene is located on chromosome 8 and codes for white only. Other genes code for other skin colors. Swedish scientists identified the gene and the test for the color gene is licensed outside Sweden exclusively to PIC. The trademark is called PICment. Sows with PICment have all-white offspring no matter the color of the sire. Because packers have a slight preference for white-colored pigs, tracking this trait should have an economic impact.
This chapter described genetic selection, the biological basis on which genetic changes are made, and the methods of selection for various traits of economic importance. Mendelian genetics, the simple quantitative inheritance of genes, were explained. Some traits that are controlled by one or a few genes follow Mendelian genetics. However, other traits are controlled by many genes and follow a normal distribution; these are polygenic traits. Most traits of economic importance to the pig industry are polygenic. Some traits, particularly reproductive and health show hybrid vigor or heterosis.
The objective of genetic improvement is to move the mean of the population in a desired direction. Producers seek genetic improvement in traits such as percent of body fat, average daily gain, feed efficiency, conception rates, and litter size and weight.
The chapter explained heterosis, heritability estimates, and genetic improvement. Crossbreeding systems, including two-way crosses, three-way crosses, and others, were also discussed.
Molecular genetics has great potential in improving commercial lines of pigs. The industry will continue making progress in applying technology to pig production.
QUESTIONS AND ACTIVITIES
1. Assume a pork producer wants to make his herd more lean. He finds that his herd is too fat to capture the best price on his pork processor's buying grid (based on weight and backfat thickness), so he sets about to lower his herd backfat thickness. As a start, he measures backfat thickness at the last rib and finds that his pigs average about 1.2 in of backfat. His herd boars presently have a backfat thickness of 1.0 in and his sows (at market weight) have 1.4 in of backfat. If he now selects boars and replacement gilts that average 0.8 in of backfat, what will his backfat thickness be after one year? Assume a generation interval of 1.5 yr. State the heritability estimate to be used.
2. Boars are leaner than castrated males (barrows). From most to least fat, the pig genders rank barrow, gilt, and boar. In one paper, boars had 30% less backfat than gilts and 40% less backfat than barrows with the same time on feed (Nold et al., 1997). If boars are assumed to be 35% leaner than barrows and gilts (on average), then what must the replacement boars and gilts average backfat be in question 1? What level of backfat thickness do you recommend the pork producer purchase in his replacement breeding stock?
3. In Chapter 1, the Iberian pig was discussed.This pig is fatter, has a different fatty acid profile, and has the perception of better taste than conventional pork. In addition, pork from Iberian pigs commands a premium in some European markets. This trait's heritability estimate is unknown. However, if the heritability of the desired fatty acid profile is 0.10, 0.25, or 0.50, how quickly can a "pork flavor" sensory score be improved starting with an average of 3.0 units and seeking to increase the herd average to 4.0 (on an 8-point scale with 1 being the worst flavor)? (See Perez-Enciso et al., 2000).
4. Perez-Encisco et al. (2000) reported a QTL regarding the flavor of pork from Iberian pigs. If the QTL was used in a marker-assisted selection program, how many generations would it take to concentrate two copies of the desired gene through successive backcrosses with an American breed of pig? Show the results by diagram.
5. If you had the funds, which pig traits would you spend money on in a search for major genes? Put them in order of economic importance. Some examples include litter size, coat color, pork tenderness, boar taint odor, ease of handling, aggressive behaviors, and others.
Hetzer, H. O. and R. H. Miller. 1972. Rate of growth as influenced by selection for high and low fatness in swine. J. Anim. Sci. 35:730-746.
Nold, R. A., J. R. Romans, W. J. Costello, J. A. Henson, and G. W. Libal. 1997. Sensory characteristics and carcass traits of boars, barrows, and gilts fed high- or adequate-protein diets and slaughtered at 100 or 110 kilograms. J. Anim. Sci. 75:2641-2651.
National Pork Producers Council. 1995. Genetic Evaluation Program. National Pork Board. Des Moines, IA.
Perez-Enciso, M., A. Clop, J. L. Noguera, C. Ovilo, A. Coll, J. M. Folch, D. Babot, J. Estay, M. A. Oliver, I. Diaz, and A. Sanchez. 2000. A QTL on pig chromosome 4 affects fatty acid metabolism: Evidence from an Iberian by Landrace intercross. J. Anim. Sci. 78:2525-2531.
Zimmerman, D. W. and P. J. Cunningham. 1975. Selection for ovulation rate in swine: population, procedures and ovulation response.
Pig Genome references:
TABLE 6-1 Heritabilities and Genetic Correlations Estimated from the NPPC National Genetic Evaluation Program. The Heritability Estimates Are Given on the Diagonal and the Genetic Correlations Are below the Diagonal. BF ADG LMA IMF MIN PH WHC INST Backfat .46 Average daily gain .14 .50 Loin muscle area -.61 -.13 .48 Intramuscular fat .30 .06 -- .25 .47 Muscle color (Minolta) .08 .11 .02 .11 .25 Ultimate pH .03 -.11 -.11 0 -.49 .38 Water-holding capacity -.05 .07 .13 -.02 .52 -.92 .19 Tenderness (Instron) -.17 -.07 .15 -.17 .18 -.42 .22 .20 Source: National Pork Producers Council, Genetic Evaluation Program (1995). TABLE 6-2 Average Estimates of the Effects of Crossbreeding on Performance Traits in Pigs. TRAIT HETEROSIS (%) Maternal heterosis Number of embryos 7 Litter size At birth 7-10 At weaning 20 Litter weight At birth 8 At weaning 20 Paternal heterosis Testicle weight 20 Total sperm 30 Improved conception 10-14 Individual traits Average daily gain 5 Feed efficiency 5 TABLE 6-3 Least Squares Means for Muscle Quality Traits Estimated from NGEP Data. SIRE LINE INTRAMUSCULAR MINOLTA ULTIMATE FAT % REFLECTANCE PH Berkshire 2.41 (bc) 22.6 (a) 5.91 (a) Danbred HD 2.33 (c) 23.0 (a) 5.75 (cd) Duroc 3.03 (a) 23.2 (a) 5.85 (ab) Hampshire 2.57 (b) 25.3 (b) 5.70 (d) NGT Large White 2.15 (c) 23.4 (a) 5.84 (ab) Nebraska SPF Duroc 2.71 (ab) 23.1 (a) 5.88 (ab) Newsham Hybrid 2.25 (c) 22.7 (a) 5.82 (bc) Spotted 2.35 (c) 23.3 (a) 5.83 (bc) Yorkshire 2.33 (c) 23.0 (a) 5.84 (ab) (a, b, c, d) Means with different superscripts are statistically different (P < .05). Source: National Pork Producers Council Genetic Evaluation Program (1995). TABLE 6-4 An Example of the Computation of the Expected Heterosis in a Crossbreeding System. SIRE DAM LINE LINE COMPOSITION MATING A B A x B A 1/2A + 1/2B A x 1/2A; A x 1/2B B 3/4A + 1/4B B x 3/4A; B x 1/4B C 3/8A + 5/8B C x 3/8A; C x 5/8B A 3/16A + 5/16B + 8/16C A x 3/16A; A x 5/16B A x 8/16C SIRE PERCENT PROGENY LINE LINE HETEROSIS COMPOSITION A 100 1/2A + 1/2B A 50 3/4A + 1/4B B 75 3/8A + 5/8B C 100 3/16A + 5/16B + 8/16C A 81 19/32A + 5/32B + 8/3C Source: Adapted from Farmers Hybrid literature by H.I. Sellers (1994).
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||SECTION II Biology Of The Pig|
|Publication:||Pig Production, Biological Principles and Applications|
|Date:||Jan 1, 2003|
|Previous Article:||Chapter 5 Reproductive biology.|
|Next Article:||Chapter 7 Modern breeds of pigs.|