INHERITANCE OF SCROTAL HERNIA IN SWINE 1 W. T. MAGEE 2. Iowa State College

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1 S INHERITANCE OF SCROTAL HERNIA IN SWINE 1 W. T. MAGEE 2 Iowa State College CROTAL hernia is the protrusion of the intestine or of any other organ into the scrotum, usually through the inguinal canal. Herewith are presented some pertinent findings from a study of the incidence of scrotal hernia among the inbred lines of swine maintained by the Iowa Agricultural Experiment Station. Warwick (1926, 193 l) conducted the most extensive breeding experiments yet reported on hernia in swine. After selecting for hernia for 3 generations, he concluded that inguinal hernia is definitely heritable. Material and Methods A herd of inbred swine was established at the Iowa Agricultural Experiment Station in The herd was expanded to twelve Poland China lines and one Danish Landrace line by The data from 1938 to and including the fall of 1947 were used in this study. During this period 1597 litters were produced which contained one or more male pigs alive at weaning age. The number of male pigs alive at weaning was studied, rather than the number born, because some cases of hernia were not observed until the pigs were examined carefully at weaning time. These litters contained 4135 male pigs. Two hundred and ten of these pigs were herniated as follows: left, 155; right, 36; double hernia or side not recorded, 19. The inbreeding of the lines averaged about 15 percent in 1938, and about 40 percent in Selection against hernia was always practiced in this herd. No herniated boars were ever used as herd sires. Only rarely were litter mates of herniated pigs used as breeding stock. The statistical procedures used in the analysis were chiefly the chisquare analysis and the analysis of variance. Each herniated pig was given a value of one and each normal male pig was given a value of zero. This method was used by Lush, Lamoreux and Hazel (1948) in studying mortality in poultry. For estimating the variance corn- 1 Journal Paper No of the Iowa Agricultural Experiment Station, Ames, Iowa. Project No The assistance of Dr. J'. L. Lush and Dr. L. N. Hazel is gratefully acknowledged.

2 SCROTAL HERNIA IN SWINE 517 ponents and heritability, the method of C. R. Henderson (1948) was used. Results The incidence of hernia for the herd during this period averaged 5.1 percent. It was about 9 percent in 1938 and about 6 percent in the spring of 1947, but has ranged between 2 and 9 percent in different years. A linear regression showed a decline of only 0.04 percent per TABLE 1. INCIDENCE OF HERNIA BY LINES Number of Number Fraction Line Boar Pigs with Hernia with Hernia A B C D E F G O H I J K S L year. Since this is very small and statistically insignificant, the incidence of hernia probably has not really changed during this period. The average incidence of hernia among the 13 lines varied from 0 to more than 15 percent (table 1). These differences between lines are almost certainly real because the chi-square of 149 with 12 degrees of freedom is far beyond the 0.01 level of significance. Variance components were calculated for lines, years, line-year interaction, sires, and dams (table 2). These estimates were denoted as r ax 2, ~AT", ~ and ~s -~, respectively. It is easy to solve for ire 2, as s and (rb 2 in table 2 because the equations containing only these may be solved by substituting values found in the equations below the equation to be solved. However, the nonorthogonality of the line-byyear classification causes some of the year effects to be in the equation for line effects. To solve for ~x 2, ~T 2, aat 2, the three equations

3 518 W.T. MAG.EE containing them were solved by iteration, after the estimated value of ge 2, as 2 and gb "~ were substituted into the equations. The answers obtained by the iteration were ~rt'-'~.00167, ~Aw~ , ~A2~ This shows that the year effect and year-line interaction are very small, and could easily be due to sampling errors. Therefore, ~rw 2, TABLE 2. COMPOSITION OF THE MEAN SQUARES Source of Mean Variation d.f. Square Componcnts in the Mean Squares Total 4134 Between lines 12 Between years 19 Line X year interaction Between boars (within lines) 218 Between SOWS (within boars) 1162 Within litters o.e~+ 3.2~rs~ o,,.r" aaY + 5 1~.,, o'er+ 3.3O'se o'i~ a^ a, aT a.,, ~.0743 o'er+ 3.0~s2+ 10.Oan or^T2--.7~rT~--.3*^ ae-~+2.8*s~+8.5ob "~.0586 af3+2.5~s a~ = asz~ ~ an2~ (8.5) (.0088) ~ which expresses the size of the real differences between lines, was calculated on the assumption that aa~ 2 and o42 were zero and was found to be only smaller than when oa 2 and ~AT 2 were considered. Heritability of individual differences within lines was estimated from the relative size of av. 2, as S and ~B 2, but these were measured on an intra-line basis and the inbreeding which had been practiced in making these lines had probably extinguished part of the genetic variability which was within these lines initially. To make these findings more applicable to purebred but non-inbred populations, it

4 SCROTAL HERNIA IN SWINE 519 seems advisable to allow for the probable effect of inbreeding on the genic variance in the actual data. This can be shown as: Observed Variance Components Total Between lines Within lines Between sires Amount of the Original Genic Variance Contained in Each (lnuf) V(G) 2F. V(G) (l--f) V(G) lq-f'--2f V(G) 4 Between dams 1-pF'--2F V(G) 4 I--F' Between litter mates V(G) 2 Where V(G)=original genic variance F and F'=inbreeding in the present and preceding generations, respectively. Two methods of calculating from these data on partially inbred lines what the heritability would be in the random bred population from which they arose and of which they may be considered as samples, modified by some inbreeding, are as follows: From gn 2 Heritability = 0.15 From the genetic differences between the inbred lines Heritability=0.07 The former is much more dependable, since there were 218 degrees of freedom between boars within lines but only 12 between lines. An estimate of maternal effect, over and above the genes a sow transmits to her offspring, is obtained by assuming that the sire effect, an", is all additively genetic and that the sire and dam have equal additively genetic effects. Then the maternal effect, the extra effect of the dam, is estimated as: Gs2--O'B 2 O.p2 ~ O.E2._t_ Or $2 _]_ O.n2 _~_ O.T2 _~_ O.A2 _.t_ O.AT 2 =0.157 This means that 15.7 percent of the variance in the incidence of scrotal hernia is due to the extra effect of the dam. It may be genetic or environmental. It would be genetic to the extent that some inherited qualities in the dams make them provide variable environments some of which favor while others prevent the occurrence of hernia in the

5 520 W.T. MAGEE pig which develop in their bodies or are nursed by them. It could also be genetic, in the broad sense, to the extent that dominance or overdominance deviations contribute anything to litter-mate correlations. In these data the maternal effect could not be separated further into genetic and environmental portions, since the maternal half sibs were too rare to be useful for comparison with paternal half sibs and with full sibs. The correlation between litter mates is,,s~+o-~, 2 o.s"+o.,z+,,-~ on an intra-line basis, or '-,-- '-,-- "-, in the whole set ~s --J-orB -l-~re -i-eft of data. What such a correlation means in terms of herniated pigs may be illustrated by the frequencies of hernia in those litters which had two male pigs at weaning. The actual frequencies and those expected, if the actual frequency of hernia were the same but there were no correlation between litter mates, were as follows: If Littermates Actual Were Uncorrelated Differences Both herniated 6 p2n~ One herniated 39 2pqn~ Neither herniated 369 q'~n~ The tetrachoric correlation was 0.19 for these litters of two. Perfect correlation would exist only if all litters were either wholly herniated or lacked hernia completely. Naturally such a perfect correlation never occurs, but the correlations found between litter mates show that there is a real excess of litters free of hernia and of litters with more than a random share of hernia. Breeding Plan The rate of genetic improvement that could be expected under the different methods of selection can be calculated following the procedure outlined by Lush (1947). Combination selection is ~/ (r--t) 2 (n--l) 1+ (l--t) [l+(n--1)t] times as effective as mass (individual) selection. This is completely determined by r, the correlation between breeding values of members of a family; t, the phenotypic correlation between members of a family; and n, the number of individuals in the family.

6 SCROTAL HERNIA 1N SWINE 521 In a population of inbred lines one might select among the lines. When the individuals in a line have an average F of 0.28 and the average F of their offspring is 0.30, the average genetic correlation between line mates is: 2 (.30) r= (1.2s) (1.2a) The phenotypic correlation between individuals in a line is: ~rt 2 t= =0.035 ab2 +as" +at2-4-cre2 The average number of boar pigs born in a line per season in this herd has been sixteen. Therefore, the relative value of combination selection as compared with individual selection is: ~/ ('434)2 15 = (.035) This indicates that selection on the basis of the best combination of attention to line average and to individual phenotype is about 1.7 times as effective as selection on individual phenotype, However, it will take several generations to form these inbred lines. Therefore, on a per generation basis utilizing line differences will not have as large an advantage over mass selection as this appears. By selecting against the dams which have produced herniated pigs, a breeder would effectively lower the incidence of hernia if the maternal effect is completely genetic. How much of the maternal effect is genetic was not determined in this study, but is definitely needed to determine the effect of selecting against dams of herniated pigs. Summary Over the last ten years approximately 5 percent of the male pigs weaned in this herd have had scrotal hernia. The differences between lines are greater than could be explained by chance alone. These differencs have fluctuated from year to year, but little if any more than would be expected by chance. The yearly differences in the incidence of scrotal hernia were not statistically significant. The estimates of heritability for scrotal hernia were about 0.15 when based on differences among sires within lines. A less dependable esti-

7 522 W.T. ~IAGEE mate, based on differences among lines, was about These figures estimate the amount to which the additive effects of his genes determine whether a male pig which reaches weaning age had scrotal hernia or not. Whether or not a male pig had scrotal hernia was also 15 percent determined by a maternal effect. It was not determined to what extent this unexpectedly large maternal effect was genetic or environmental. The large maternal effect indicates that a breeder should practice strong selection against sows that have produced herniated pigs. If the maternal effect can be identified as some environmental effect and if the environment which causes hernia can be easily corrected, that would offer a way to lower the incidence of hernia. Since scrotal hernia is a threshold characteristic with low heritability, hernia could be reduced by forming many inbred lines, selecting those which have the least hernia and crossing them together to form new lines. Scrotal hernia is not important enough by itself to warrant forming inbred lines but, since many lines are being formed for other reasons, the incidence of hernia should be considered when selecting among the lines. Literature Cited Henderson, C. R Variance components. Lecture Series, Animal Breeding Subsection. Iowa State College, Ames. Ia. Lush, J. L Family merit and individual merit as bases for selection. Amer. Nat. 81:241, 362. Lush, J. L., W. F. Lamoreux, and L. N. Hazel, The heritability of resistance to death in the fowl. Poul. Sci. 27:375. Warwick, B. L A study of hernia in swine. Wisc. Agr. Exp. Sta. Res. Bul. 69:1. Warwick, B. L Breeding experiments with sheep and swine. Ohio Agr. Exp. Sta. Bul. 480:25.