Environment and Poultry Breeding Problems

462

GOWE, JOHNSON, DOWNS, GIBSON, MOUNTAIN, STRAIN AND TINNEY Animal Improvement. Cambridge University Press, Cambridge, 342 p. Lerner, I. M., and L. N. Hazel, 1947. Population genetics of a poultry flock under artificial selection. Genetics, 32: 325-339. Pearl, R., and F. M. Surface, 1909. A biometrical study of egg production in the domestic fowl. I. Variation in annual egg production. U.S. Dept. Agr. Bur. Animal Ind. Bui. 110. Proudfoot, F. G., R. S. Gowe and B. F. Cheney, 1957. Studies on the design of comparative poultry tests. 1. A comparison of three strains of White Leghorns housed in replicated fiftybird pens and intermingled in a large pen. Can. J. Animal Sci. 37 : 168-178. Robertson, F. W., and E. Reeve, 1952. Studies in quantitative inheritance. I. The effects of selection of wing and thorax length in Drosophila melanogaster. J. Genetics, 50: 414-448. Skaller, F., 19S6. The Hagedoorn "nucleus-system" of breeding—a critical evaluation based on an experiment with poultry. Proc. Australian Soc. Animal Prod. 1: 165-176.

Environment and Poultry Breeding Problems 5. THE DESIGN OF POULTRY CONTROL STRAINS R. S. GOWE,1 ALAN ROBERTSON2 AND B. D. H. LATTER3 Institute of Animal Genetics, Edinburgh, Scotland (Received for publication August 14, 1958)

INTRODUCTION

D

URING the past decade, the value of control populations to provide standard material for the evalution of the level of management in selection programs has become increasingly apparent. Control strains have been used extensively in laboratory investigations over a much longer period, and their usefulness amply dem1

On leave from Animal and Poultry Science Division, Central Experimental Farm, Ottawa, Canada. 2 Member of Staff of the Agricultural Research Council, Great Britain. "Walter and Eliza Hall Agricultural Research Fellow, University of Sydney, Australia.

onstrated. Only recently, however, has the expense involved in their maintenance been considered worthwhile in breeding research involving poultry. Goodwin, Dickerson and Lamoreux (19SS), Gowe and Johnson (1956), Skaller (19S6) and King (19S8) have given limited information on particular control flocks currently in use in selection programs. A detailed analysis of the results for one particular control flock has been presented by Gowe et al. (1959). It is the purpose of this paper to discuss in theoretical terms various problems arising in the maintenance of control flocks, and to set out the relative merits of two general types of design.

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several locations. Poultry Sci. 35: 1146. Gowe, R. S., A. Robertson and B. H. D. Latter, 1959. Environment and poultry breeding problems. 5. Design of poultry control strains. Poultry Sci. 38: 462-471. Guhl, A. M., 1953. Social behaviour in the domestic fowl. Kansas Agr. Expt. Sta. Tech. Bull. 73. Hall, G. 0., 1934. Breeding a low producing strain of Single Comb White Leghorns. Poultry Sci. 13: 123-127. Hutt, F. B., and R. K. Cole, 1953. The interaction of genetic and environmental influences affecting the incidence of avian leucosis. Science, 117: 695-697. Hutt, F. B., and R. K. Cole, 1947. Genetic control of lymphomatosis in the fowl. Science, 106: 379-384. Lamoreux, W. F., F. B. Hutt and G. O. Hall, 1943. Breeding for low fecundity in the fowl with the aid of the progeny test. Poultry Sci. 22: 161169. Lerner, I. M., 1950. Population Genetics and

DESIGN OF POULTRY CONTROL STRAINS

GENERAL CONSIDERATIONS

Genetic changes in a control flock may be of two kinds: (a) random changes of gene frequency due to genetic sampling or "drift"; (b) directional changes due to natural selection. The effects of natural selection will depend on the population concerned. In a population initially in genetic equilibrium, they might be expected to be on the whole conservative; i.e. to tend to maintain the gene frequencies at their equilibrium values and hence to reduce the effect of drift. But in a population which had been under artificial selection which was now relaxed, the forces of natural selection are far more likely to act unidirectionally. Of these two agencies which can alter gene frequency, we can make a reasonable prediction of the absolute magnitude of the first, at least when the degree of inbreeding is small, but we can only predict the relative magnitude of the second in comparing different types of control populations. Now the restriction in parental population size which is implicit in any closed breeding program brings two consequences in its train. It increases the degree of homozygosis of members of the fllock above that initially present, and causes the gene frequencies to drift from their initial values because of the sampling of a small number of genes each generation. As a consequence of the latter, the mean performance of the population under constant environment will slowly alter. These two consequences of inbreeding are closely related, but in their mathematical formulation slightly different, and we shall be interested in the change in the mean. The distinction between the two has been well discussed by Crow (1954) and Crow and Morton (1955). The overall effect of the restriction in population size can be discussed in terms of the effective number of parents, Ne, such that the expected difference in any gene frequency, q, from one generation to the

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The function of control populations has been threefold: (i) to assess the magnitude of short-term fluctuations in environment and to furnish a means of correction; (ii) to maintain genetic constancy over a period of time, thereby enabling the evaluation of long term trends in the environment; and (iii) to serve as a gene pool with known genetic parameters for use as base material in selection experiments. The first two functions are of course inter-related in that in (i), some degree of constancy of genetic material is assumed. But the main problem in the smoothing-out of short term fluctuations will be to measure enough birds to get an accurate estimate of the environmental level. It would seem that if control flocks are to be of value in random-sample testing, they should be given more space than they are in some tests (e.g. New York Random Tests, see Marble, 1957). If control material is to function accurately as such, then its own mean performance must be known accurately. Let us suppose that for the purpose a standard error of the mean of the control of ± 5 eggs in hen-housed production is accepted as reasonable. As the individual variance is in the neighborhood of 5000, it follows that a sample size of 200 birds would be desirable. A further feature of the control flock of importance from this short-term aspect would be its representativeness. Any degree of reaction to specific environmental changes of a kind peculiar to the control line would be a disadvantage. It follows that one obvious type of control, a cross between two long-inbred lines, might not be as useful as it might at first appear because it represents a replication of only one genotype. This would however not be such a disadvantage in use as a long-term control providing the inbred lines could be maintained without serious genetic changes.

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R. S. GOWE, A. ROBERTSON AND B. D. H. LATTER

_ =_ + — •

(1)

Ne 4M 4F But the premises lying behind this formula have not been generally realized and have only been adequately brought into the light in the papers by Crow. It is assumed that each parent has an equal chance of contributing to the next generation and not that each leaves an equal number of offspring. In fact the assumption of "equal chance" implies that the actual distribution of progeny number per parent will be of the Poisson type. Any real differences among the parents in reproductive ability will lead to an increase in the variance in progeny per parent and to a decrease in the effective number of parents. On the other hand, any deliberate balancing of the number of offspring left per parent will decrease the variance of progeny number and increase the effective number of parents. The problem is most simply looked at by considering the drift in gene frequency from generation to generation as made up of two parts—the sampling of genes within individuals and the sampling of individuals. In a truly random breeding population, these two contribute equally to the variance in gene frequency in the next generation (because in fact of diploidy) and also to the genetic sampling between generations. Any distribution of progeny numbers per individual which is not Poisson will then

affect the second part only. At one extreme, the exact balancing of individual differences in progeny number reduces to zero the variance due to individual sampling, so that the drift variance is one half that derived from the usual formula. Algebraically, the situation is summed up in the following formula. Suppose we have N potential parents from which we sample nl, n2, . . . to nN gametes respectively. Then the expected drift variance (the square of the change in gene frequency) between gametes and parents for a gene which has frequency q in the parents is T 1 2

N(T2n "I

E(Aq) = q ( l - q ) -\ (2) HJ HV 4; L 2 2 n 2 (Sn) 2 J where n2D is the variance of n. The first term comes from sampling within individuals and the second from sampling between individuals. If the distribution of gametes among the parents is Poisson 2n (s 2 n = ) then the two terms are equal. But if the contributions are balanced (a2n = 0) the last term is zero. In his papers, Crow has discussed various modifications of this formula to suit different situations. In applying formula (2), we have to evaluate the expected drift in the four different samplings of gametes which take place in reproduction (male to male, male to female, female to male and female to female) bearing in mind that the contributions of individuals to the progeny of both sexes may be correlated. The total expected q (i - q) drift variance is then equated to 2 Ne to give the effective number of parents, Ne. The existence of natural selection implies genetic differences between individuals in viability and reproductive ability. But the effect of these differences on changes in gene frequency are to some extent under the

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q ( i - q) J - , and very 2 Ne roughly the change in genetic mean performance from one generation to the next has variance equal to where
DESIGN OF POULTRY CONTROL STRAINS

THE ALTERNATIVE PLANS

From the point of view of genetic constancy, it is obvious that a control population of unlimited size is the ideal. For given resources and labor, however, the practical problem is to specify the mating design which leads to maximum efficiency in the sense that genetic drift is reduced to a minimum. There are two principal alternatives that will be considered here. These will be referred to as the "random-breeding" and "pedigreed" controls, the former exemplified by the strain maintained by Gowe et al. (1959), and the latter by that proposed by the North East Poultry Breeding Advisory Committee on Population Genetics (U.S.A.) in February, 1954 (King, 1958), and currently in use at several institutions in the U.S.A. and Canada. (a) The Random-Breeding Flock. Chicks are randomly chosen at hatching within each sex from the population of chicks available (unpedigreed), and the required M males and F females to be used as parents taken at random from the survivors

available for breeding at maturity. These parents are mated at random by artificial insemination using semen from individual males (semen collected in a separate vial for each cock) preferably mating twice weekly, the females being reassigned to the males at random at each insemination. A proportionate number of females would be inseminated with the semen of each male at each insemination. Usually there would be fewer males than females but this need not necessarily be so. Since 1954 the control flock described by Gowe et al. (1959) has been maintained by the above mating system. Forty males were used each year from 1954 to 1956, and 50 males in each of the succeeding two years. The numbers of females actually alive and laying during the breeding season have varied between 170 and 250. Lacking precise information of the importance of non-random mating due to differences in social order and in libido of males in large flocks, artificial insemination was considered a useful precaution. Hatching eggs to produce the replacement flock were saved over a 28 day period so that each hen was mated 8 times. Proportionate numbers of males and females were randomly saved from each hatch and proportionate numbers of males from each hatch used in breeding the next generation. (b) The Pedigreed Flock. Mates are as/F\ signed at random, a fixed number I — 1 \M/ of the F selected female parents being allocated to each male and repeatedly mated to him over the breeding season. Every effort is then made to ensure that each sire shall contribute one male to the succeeding set F of male parents and — females to the M selected set of female parents, one from F each of the — dams to which he himself M

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experimenter's control. There can be no natural selection for viability if there is no mortality. It follows then that every effort must be made to increase hatchability and to reduce mortality to breeding age as much as possible. If it is not possible to prevent some individuals dying, it may nevertheless be possible to ensure that fullsib groups are equally represented amongst birds used as breeders. As natural selection will then be acting on the variance within families, it might be expected from Fisher's fundamental theorem of natural selection to be having only half the effect. In the case of male fertility and egg-laying ability, it may be possible to produce an equal number of offspring from all but the most infertile of parents, and even the effect of this can be halved by the substitution of fertile full-sibs.

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R. S. GOWE, A. ROBERTSON AND B. D. H. LATTER

RELATIVE MERITS OF THE DESIGNS

In comparing the two types of control population we have two points of view to keep in mind:firstly their efficiency in maintaining genetic constancy over a period of time; and secondly their "operational" efficiency, reflected in the resources and manpower necessary to maintain each at a

given level of "genetic" efficiency. We will be concerned for the moment with the former viewpoint, returning to the more practical considerations at a later stage. (i) Effective numbers of parents. Suppose as a basis for comparison that 50 males and 250 females are to be used as parents each generation. In the pedigreed flock genetic sampling between individual parents is reduced to a minimum, and is in fact only involved in the sampling of females for the breeding of males. Of the 250 female parents, 50 will contribute one maje each to the next set of male parents and the remainder will not be represented. The sampling involved is not in fact strictly random, but is "sampling without replacement," and application of formula (2) shows the expected drift variance to be q(l-q) T 3 IT E(Aq)2 = — ~ + 2 L16M 1 6 F J

(3)

where M males and F females are used as parents each generation. The effective number of parents is therefore given by Ne 16M 16F When equal numbers of males and females are used (M = F), the effective number of parents Ne is equal to (M + F) i.e. twice the actual number used, since there is no sampling among individuals involved. With F = 5M as in the flock we are considering (and for the design described by King, 1958) the effective number of parents is equal to the number of females employed, i.e. 250. For a random-breeding control flock with M males and F females we have already seen that if all parents have an equal chance of contributing offspring to the following generation, the expected distribution of progeny number per parent will be one in which the mean and variance are equal (i.e. a Poisson distribution) whatever the values

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was mated. In most flocks fewer male than female parents would be used each generation, although equal numbers of each sex could be used to advantage in certain situations to be described later. Artificial insemination would be again desirable to ensure that each female is fertile (as far as possible) and contribute to the next generation. Individual sire mating pens would be required if artificial insemination were not practiced. In the control flock design described by King (1958), fifty males are mated to 250 females. The females are randomly assigned to the males and repeatedly mated to the same male; therefore all progeny are pedigreed as to sire and dam. By hatching and rearing several daughters per dam and 2 or 3 males per sire (each from a different dam randomly designated to produce males) each female parent would be expected to have one daughter alive and laying eggs and each male parent would have one son alive and producing semen at the next breeding season as far as possible. If all the progeny of one family were missing due to low fertility or mortality or a combination of circumstances, the number of males and females mated each generation would be kept constant by randomly choosing from amongst the surplus individuals, avoiding more than two full sibs from each half-sib family for females, and for males avoiding full sibs completely and more than two half-sibs from each family. When mating at random the precaution was taken of avoiding all half-sib and full-sib matings.

467

DESIGN OF POULTRY CONTROL STRAINS

of the mean. The expected drift variance is then known to be q(l-q) r 1 11 E(Aq)2 = — + — 2 |_4M 4 F j

(5)

1

1 /

o-2m — fl„A

Me_M \

ii2m

Fe

n2,

F \

/ ' /

An estimate of the magnitude of this reduction in the effective number of parents for their particular flock was obtained by Gowe et al. (1959). The reduction was small (0.97F and 0.97M). However, it should be emphasized that it was impossible to include in this estimate the variation that resulted from non-random mortality from hatching to breeding age. This flock was also characterized by high fertility, hatchability, and rate of lay at least during the breeding season. AH these factors would tend to minimize the non-Poisson variation in progeny numbers per parent. It should also be stressed here that this estimate is only applicable to the period when the flock was actually artificially inseminated, as was the practice for the Ottawa control flock since 1954. The reduction in effective numbers of parents in flocks naturally mated may be quite large. The meagre evidence 2 L4M\ fi2m / available (Guhl, 1953) suggests that this is 2 so in small flocks but there is no evidence 1 / o- £-nA "I of the magnitude of these effects in larger + (1+ -J (6) 2 flocks. For Drosophila, Crow (1954) has 4F \ nf / J shown the effective number of males to be A detailed discussion of this equation is 40% of the actual number because of the presented by Latter (1959). With no remating habits of this species. An estimate of productive differences among the parents, 2 2 the magnitude of this effect (non-random o- m — nm and <7 f = nt, and the formula mating) in poultry flocks of more than 100 reduces to (5) as expected. Variability in birds would be quite useful. reproductive potential among the parents, or between-family differences in survival to (ii) The Operation of Natural Selection. maturity, will be expected to swell the The use of artificial insemination in a ranvariances in progeny number (a2m > fim, dom breeding control reduces the possibility fif) thereby increasing the magnitude of natural selection for male fertility. We of the expected drift variance. The effective can therefore expect no genetic change of number of parents is given by importance from this agency. On the female side, the low level of variability be1 1 1 —= + • (7) tween hens in egg production reported Ne 4Me 4Fe

E(Aq)s = i ^ r i - (

1+^ZM

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and the effective number of parents is given by equation (1). With M = 50 and F = 250, the effective number of parents would therefore be 167. We know, however, that real differences in reproductive ability among the parents will lead to increased genetic drift, reflected in a reduced effective number of parents. Suppose then that the M males and F females of one generation contribute with unequal probabilities to the group of adult progeny which are to be sampled at random within each sex to furnish the next set of M male and F female parents. Let this group of progeny be such that the mean number of birds per sire is nm, and the variance in number of birds per sire is u2m; and such that the mean and variance of progeny number per dam are fif, cr2£. Following random sampling to provide the required parents, the drift variance is

where

468

R. S. GOWE, A. ROBERTSON AND B. D. H. LATTER

TABLE 1.—Effective

(iii) Practical Considerations, (a) Control flocks: The poultry breeder may have a variety of reasons for maintaining stable gene pools or control strains. To some degree the reasons for maintaining the strain will determine the flock structure and the size of the flock. However, it may be useful to comment on some of the more obvious practical alternatives in the light of the foregoing theoretical discussion. In Table 1 the effective number of parents and the change in inbreeding per generation expected with different numbers of parents are contrasted for the two flock structures described. The advantage of the pedigreed flock in increasing the effective number of parents is evident. It is also apparent that the use of equal numbers of males and females will minimize inbreeding and the genetic drift for a population of any given size. It will also be readily recognized that to take full advantage of the pedigree flock structure described as far as the effective number of parents is concerned, it will be necessary to have available in reproductive condition one representative of each family

number of parents for two mating plans and flocks of various sizes

Mating Plan*

Effective number of parents (Ne)

Change in inbreeding coefficient (F) per generation (%)

Maximum expected drift in mean henhoused egg production after 20 generations**

50 100 200 73 167 240 300

1.00 0.50 0.25 0.68 0.30 0.21 0.17

±25 ±18 ±13 ±21 ±14 ±12 ±10

100 200 400 103 250 384 480

0.50 0.25 0.12 0.48 0.20 0.13 0.10

±18 ±13

Actual parents Males

Females

25 50 100 20 50 80 100

25 50 100 200 250 210 3Q0

R R R R R R R

25 50 100 20 50 80 100

25 50 100 200 250 240 300

P P P P P P P

'

± 9 ±18 ±11

± 9 ± 8

* R and P refer to the random breeding flock and the pedigreed flock described in the text. ** See text.

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(Gowe et al., 1959) for Ottawa control (little more than the Poisson variation expected) would suggest that natural selection is not likely to be of importance for this strain, particularly if the strain is reproduced at a time when most birds are in an optimum state of reproductive fitness. It can still act on survival to breeding age, but will be unimportant where mortality is low. In the "ideal" pedigreed control flock, there would be no selection for fertility or egg production apart from the incidence of extremely infertile birds of both sexes and birds completely out of production which would be replaced by full-sibs. As the flock structure involves equal representation of families of progeny, it follows that natural selection during rearing can act only on the variability within families, and will therefore have only half its effect. In any practical situation it will be difficult and costly to hatch and rear sufficient individuals from every family to eliminate entirely natural selection between families but it could be reduced to an insignificant level, particularly if precautions are taken to rear under optimal conditions.

DESIGN OF POULTRY CONTROL STRAINS

years (see Table 1). Some idea of what this means in terms of genetic drift of the mean can be obtained in the following way. On the assumption of additive gene action, the expected drift variance in the mean is given by / "V* \ 2F(7g2 I or per generation I , \ Ne / where F is the inbreeding coefficient and
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bred the previous season. When the reproductive rate is low (due to poor fertility, hatchability or rate of lay) or the viability is low (or the exposure to disease high) several individuals would have to be reared and housed from each family to be sure that one was alive and laying at the next breeding season. This would mean that under some circumstances it might be easier to double the flock size and use the random mating procedure without pedigreeing. Although this approach has some advantage it should be remembered that one of the important characteristics of the pedigreed control flock is that natural selection among families can be minimized and ideally restricted to operate within families. In practice, it probably would be more economical to increase the number of families by 10% over what would be required for any given effective number of parents in the ideal one-to-one replacement program, and then hatch and rear two females per dam parent and 2 males per male parent. Some families would be without representatives alive to breed at the next breeding season and would have to be replaced by sibs of individuals already represented in the breeding flock. This would decrease the effective number of parents, but if the number of families with missing progeny were not over 5% the effect would not be large. Since all progeny would be pedigreed, the exact effective number of parents could be computed when required. It would seem that under almost any circumstances when the flock was to act as a control on selection studies, the pedigreed flock structure would be preferable. As a rough guide it is suggested that not less than 300 effective parents should be used to reproduce control flocks for long term selection studies that are expected to proceed 20 generations or more. With flocks of this size the inbreeding in the control strain would amount to 3.3 percent after 20

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R. S. GOWE, A. ROBERTSON AND B. D. H. LATTER

(c) Flocks for the maintenance of gene pools: Poultry breeders are frequently faced with the maintenance of large numbers of strains of various breeds that may be of value in the breeding programs of the future, but which are currently being stored. Except for the most important of these strains there is usually no need to reduce genetic drift to the degree necessary in a control flock. The main principles to observe here would be (1) to breed equal numbers of males and females (2) to

pedigree-mate and follow the pedigreed program outlined, keeping as close as possible to a one-to-one representation of each family in each generation, and (3) to brood, rear and house, at least until the breeding season, under optimum conditions with minimum losses due to mortality, perferably on isolated premises. It is difficult to state how large the populations should be since it would depend on how important the actual or potential value of the strain might be to the particular breeder. However, thirty to fifty birds of each sex per strain would only allow 0.50 to 1.00 percent inbreeding per generation (depending upon the number of families lost) which would probably be satisfactory for strains to be held for only a few years and of no immediate importance. These estimates assume the pedigree flock program would be followed and that 2 or 3 individuals per family would be raised at least for the first 6 months, so that there would be not too many families lost prior to the next breeding season. SUMMARY

A theoretical discussion of the maintenance of control flocks of poultry is presented with particular reference to the problem of minimizing genetic change over a period of time. Two basic designs are considered. (a) non-pedigreed—matings not being identified and eggs being taken at random from the flock over a limited period to breed replacements. (b) pedigreed—care being taken that within the limits of the design each member of the flock contributes equally to the next generation. The following conclusions are drawn: 1. Artificial insemination is almost obligatory to maximize both genetic and operational efficiency in both types of flock. In type (a) flock matings are most inefficient,

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days apart) approximately 7 daughters per dam could be obtained. Since Robertson (1958) has shown the optimum number of daughters per sire for the estimation of intraclass correlations to be in the vicinity of 20 for traits with a heritability of 0.20, three dams mated to each sire could provide this number of progeny. This would provide full-sib families of about 7 daughters (or 14 sibs if broiler traits were of importance) which would be close to what Robertson has shown to be the optimum for estimating dam components, when both dam and sire components are to be considered for the same population. The optimum structure for the maintenance of the control flock with minimum numbers has been shown in the earlier discussion to be equal numbers of male and female parents, rearing two daughters and two sons per pair of parents. It is important to point out here that for that trait such as egg production, two daughters per dam would be extremely inefficient in estimating dam components of variance for a given total test space (Robertson, 1958). A ratio of one male to three females would give a substantially higher effective number of parents (see Table 1) than the ratio of one male to 5 females suggested by King (1958) and would be a better ratio for estimating genetic parameters, particularly for traits with medium to low heritabilities.

DESIGN OF POULTRY CONTROL

REFERENCES Crow, J. F., 1954. Breeding Structure of Populations. II. Effective Population Number in Statistics and Mathematics in Biology. Ed. by O. Kempthorne, T. A. Boncroft, J. W. Gowen and J. L. Lush. Iowa State College Press, p. 543-556. Crow, J. F., and N. E. Morton, 1955. Measurement

of gene frequency drift in small populations. Evolution, 9: 202-214. Goodwin, K., G. E. Dickerson and W. F. Lamoreux, 1955. A technique for measuring genetic progress in poultry breeding experiments. Poultry Sci. 34: 1197. Gowe, R. S., and A. S. Johnson, 1956. The performance of a control strain of S. C. White Leghorn stock over four generations on test at several locations. Poultry Sci. 35: 1146. Gowe, R. S., A. S. Johnson, J . H. Downs, R. Gibson, W. F. Mountain, J. H. Strain and B. F. Tinney, 1959. Environment and poultry breeding problems. 4. The value of a random-bred control strain in a selection study. Poultry Sci. 38: 443-462. Guhl, A. M., 1953. Social behaviour in the domestic fowl. Kansas Agr. Expt. Sta. Tech. Bui. 73. King, S. C , 1958. Personal communication. Latter, B. D. H., 1959. Genetic sampling in a random mating control population of constant size and sex-ratio. Australian J. Biol. Sci. (in press). Marble, D. R., 1957. Third Western New York Random Sample Poultry Test. Final Report. Mimeo. Cornell Univ., Ithaca, N.Y. Robertson, A., 1958. Experimental design in the evaluation of genetic parameters. Biometrics, (in press). Skaller, F., 1956. The Hagedoorn "Nucleus-system" of breeding—a critical evaluation based on an experiment with poultry. Proc. Australian Soc. Animal Prod. 1: 165-176.

Heritability of Laying-House Viability in a White Wyandotte Flock R.

Ministry

of Agriculture,

Northern

W.

HALE

Ireland, and The Queen's University

of

Belfast

(Received for publication August 18, 1958)

O I N C E the original work by Lush, ^ Lamoreux and Hazel (1948) and by Robertson and Lerner (1949), there have been few attempts to estimate the heritability of viability amongst laying hens. It is, however, only through the accumulation of estimates that generalisations can be made regarding heritability of particular characters. Hence the records of a White

Wyandotte flock, kept by the Agricultural Research Institute of Northern Ireland at Hillsborough, have been analysed for two years during which there was very high mortality, to estimate heritabilities of viability and of resistance to leucosis. The foundation stock for this flock was bought in 1950 from a number of farms on which there were no known cases of leucosis.

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except perhaps with very .large flocks requiring extensive facilities. 2. Environmental conditions should be optimal in brooding, rearing and housing to reduce any effect of natural selection. If possible the replacement stock should be reared and housed in isolation and hatching eggs taken at a season and age when the pullets had maximum reproductive fitness. 3. Flocks with equal numbers of both sexes are generally the most efficient unless the estimation of genetic parameters is of some importance, when three females should be used for each male. 4. In almost all circumstances, the pedigreed type of flocks has sufficient advantages to justify the extra labor of pedigree mating and wing banding.

471

STRAINS