NSIF-FS3-W

Fact Sheet #3

Purdue University

Cooperative Extension Service

West Lafayette, IN 47907



Genetic Parameters and Their Use in Swine Breeding



William R. Lamberson, University of Missouri
Erik R. Cleveland, University of Hawaii




Reviewed by

Larry Young, MARC.

Charles Christians. University of Minnesota

Max Waldo, Nebraska Duroc breeder

Bruce Leman, Illinois Hampshire and Yorkshire breeder

Introduction

Efficient production is essential for a pork producer to survive in uncertain economic times. Continuous genetic improvement will allow the seedstock producer to reduce production costs and to compete for sales In the breeding stock market by providing a genetically superior animal. New tools are becoming available to producers for evaluation of the genetic merit of hogs. Knowledge of genetic principles will allow producers to take full advantage of computer programs used in estimation of genetic merit. This publication presents the key concepts of breeding value, heritability and genetic correlation.

Breeding Value and Transmitting Ability

Genetic improvement of a seedstock herd is dependent upon the producer's ability to select breeding stock with superior breeding values. The breeding value of an animal is defined as the animal's value as a parent considering the effect of its genes on all relevant traits whether or not they can be directly measured. The breeding value for a particular trait encompasses the effect of all genes affecting that trait. For example, a producer choosing a boar to be used as a terminal sire should be concerned with the boar's breeding value for traits such as growth rate, backfat thickness, and feed efficiency. Many genes affect each of these traits. The total effect of all genes influencing these traits contribute to the breeding value of the boar. Genes occur in pairs and a sample one-half of a selected animal's genes is transmitted to its offspring. Since a sample one-half of an animal's genes is transmitted to the offspring, on the average, one-half of the breeding value is transmitted to the offspring. One-half of the animal's breeding value is referred to as the animal's transmitting ability. The expected merit of progeny from a particular mating is equal to the average of the parents' breeding values or the sum of their transmitting abilities.

Heritability

Unfortunately, it is impossible to know the true breeding value of an animal. To make selection decisions we must estimate the breeding value of an animal based on information available on that animal or its relatives for traits of interest. The animal's own phenotype or its performance is one indicator of its breeding value. The usefulness of performance data on the individual as a predictor of breeding value is dependent on the heritability of the trait of interest. The heritability of a trait can be defined as the proportion of differences in performance between animals that are due to differences in breeding value. This concept is illustrated diagrammatically in Figure 1. Heritability can, in theory, range from 0 to 1.0. A heritability of zero indicates that all differences between animals are due to nongenetic causes. A heritability of one indicates that all differences between animals are due to genetic causes. Estimates of the heritabilities of some relevant traits are presented in Table 1. For traits that are highly heritable, e.g., backfat thickness, the performance of an animal is a good indicator of its breeding value. For traits with lower heritabilities, e.g., litter size, the phenotype is a poorer indicator of breeding value.

Figure 1. A diagrammatic representation of the proportion of phenotypic differences, which are expected to be due to differences in breeding value for a trait with a heritability of 0.25.

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Table 1. Heritability estimates of some traits of interest to swine seedstock producers.

Trait                  Heritability
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Litter size              .10
Litter birth weight      .30
21-day litter weight     .15
Number weaned            .05
Survival to weaning      .05
Average daily gain       .40
Feed conversion          .30
Days to 230 lb.          .35
Backfat thickness        .40
Loin eye area            .50
Carcass length           .55
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The concept of heritability can also be examined from the standpoint of selection response. The heritability represents the proportion of parental superiority in performance (termed selection differential) that is expected to be realized as superiority of the selected animal's progeny. This concept is diagrammed in Figure 2.

Figure 2. A diagrammatic relationship of the performance of parents and the expected performance of their progeny relative to the population mean for traits with differing heritabilities.

The cause of the likeness between parent and offspring is because genes are passed from the parent to the offspring. Sibs tend to be alike because they share genes in common which are received from their parents. There is a greater likeness between relatives for a highly heritable trait than for a lowly heritable trait. For example, a group of sibs is more likely to be similar for backfat thickness than for number born in their first litter. Genes, as compared to management, have a greater effect on performance for a highly heritable than for a lowly heritable trait.

Differences among animals can have both genetic and environmental causes. The heritability of a trait is the ratio of variation due to genetic causes to the variation due to genetic plus plus environmental causes. Producers differ in their ability to control the production environment. Poor control of the production environment can disguise genetic differences among animals. For that reason heritabilities can differ between herds. To help increase heritabilities and improve the accuracy of breeding value estimates producers should: 1) treat animals the same, 2) take complete and accurate records and 3) adjust records for nongenetic sources of variation such as number reared in a litter or age at weighing.

Genetic Correlation

Extremes in performance for certain traits tend to be associated. An animal that is shorter than average at a given age is also likely to be lighter than average. These associations are known as phenotypic correlations. The association between traits can be caused by environmental factors that affect both traits similarly. For example, availability of food or exposure to disease organisms probably affect both of these traits similarly. Association resulting from environmental factors is referred to as environmental correlation. Association between two traits can also be caused by genes that affect both traits simultaneously. This effect results in a genetic correlation. Genetic correlations are of greater importance to the animal breeder. The genetic correlation can be thought of as the correlation between the breeding values of two traits. Genetic correlations can range from - 1.0 to + 1.0. They indicate the strength of the genetic relationship between two traits. An example of a gene that causes association between two traits is the halothane sensitivity gene, which results in improved meatiness but a decline in carcass quality. A negative genetic correlation indicates that animals with breeding values that are negative for one trait are likely to have breeding values that are positive for the other and vice versa. A positive correlation indicates that breeding values tend to have the same sign.

The sign of the genetic correlation does not indicate the favorability of the relationship between traits, only the statistical relationship. For example, the genetic correlation between feed conversion and average daily gain is negative (Table 2). Because fast gains tend to be associated with low feed required per unit of gain, this illustrates a negative statistical relationship but favorable economic relationship.

Table 2. Genetic correlations among performance traits among sow productivity traits.

Item                        Feed conversion           Backfat probe
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Days to 230 lb.                  60                      -.25
Average daily gain             -.60                       .25
Feed conversion                                           .30
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                        Litter         Number            Litter
Item                  birth weight     weaned        21-day weight
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Number born              .65            .71              .48
Litter birth weight                     .67              .69
Number weaned                                            .93
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The consequence of having a genetic correlation between traits is a correlated response to selection. A favorable correlation results in selection for one trait improving another. An unfavorable correlation between traits increases the difficulty of making simultaneous improvement in the traits.

The relationships of average daily gain with feed conversion and backfat (Table 2) can illustrate this concept. Direct selection for only increased average daily gain would be expected to result in selection of replacements with favorable breeding values for feed conversion because of the positive genetic correlation between the traits. Selection for growth rate is thus expected to improve feed utilization because faster growing animals tend to be more efficient. If selection is solely for average daily gain, a tendency for selected animals to have unfavorable breeding values for backfat would be expected. In this situation care must be taken to choose animals with favorable breeding values for both traits.

A plot of breeding values for backfat and average daily gain are presented in Figure 3. The two animals with the highest breeding values for average daily gain (1 and 2) have unfavorable breeding values for backfat. Animals better than average for both traits (6, 8, and 9) fall in the lower left portion of the diagram. Selection decisions among the animals described depend upon the relative importance of the traits. With knowledge of the concept of the genetic correlation, selection decisions can be aided by computerized multiple trait breeding value estimation programs.

Figure 3. A plot of breeding values for backfat thickness and average daily gain with a genetic correlation between traits of .25.

Summary

The need for production efficiency in the today's uncertain economic times makes accurate evaluation of breeding values essential. It is desirable to simultaneously evaluate traits such as growth rate, body composition, efficiency, and reproductive traits in potential replacement animals. Reproductive traits and feed efficiency are difficult to measure directly, so information from relatives and from other correlated traits are used when estimating breeding values. Aids in breeding value estimation are available to today's producer in the form of centrally operated computer programs such as S.T.A.G.E.S. and the Nebraska S.P.F. performance testing program, and in microcomputer programs available for on farm use.

Application of output of these programs requires a basic knowledge of genetic principles and an understanding of the concepts of breeding value, transmitting ability, heritability, and genetic correlation.


RR 5/91

Cooperative Extension work in Agriculture and Home Economics, State of Indiana, Purdue University and U.S. Department of Agriculture cooperating: H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an equal opportunity/equal access institution.