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THE EVOLUTION OF ANIMAL BREEDING PRACTICES --

COMMERCIAL AND EXPERIMENTAL

Dr. A. W. NordskogProf. of Poultry Breeding

Iowa State UniversityAmes, Iowa

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THE EVOLUTION OF ANIMAL BREEDING PRACTICES --COF_iERCIALAND EXPERIMENTAL

I wish to describe what I think are some of the major changes in our

approach to animal breeding and with special emphasis on poultry breeding since

about 1900. I shall extrapolate from recent work---including some applied and

some theoretical--bearing on the question of what is in store for us, as

breeders, in the near future. I am aware that there are some hazards in doing

this. Yet any business enterprise should think about the future. At the

same time, we can predict the future only by projecting the past. Now, I

realize that not all of you will agree with what I say. But when two men agree

on everything one of them is not needed. So I'll be expecting to hear from

those of you who feel motivated, stimulated or perhaps irritated.

In a real sense, animal breeding is man-made evolution or animal breeding

is micro-evolution. Modern animal breeding is focused on rates of improvement;

we wish to speed up the rate of micro-evoluti0n" As aids to this objective

we employ as many of the tools of Science as we know how. In contrast, the

old animal breeders, such as Robert Bakewell, the Collings brothers and others

like them, relied on practices handed down from father to son or from successful

practitioners or to their disciples who aspired to success. Thus, the methods

and practices of animal breeding itself have undergone evolutionary change and•

undoubtedly will continue to do so_

For purposes of our discussion, I shall categorize Modern Animal Breeding

according to three more or less distinct but rather arbitrary schools which

are time oriented:

)

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i

I) Old School 1900-1930

2) New School 1930-1950

3) Contemporary School 1950-

The dominant feature of the Old School has be_n progeny testing. Also,

pedigree information is considered important, but the methods are empirical.

Rediscovery of Mendel's laws of inheritance, and its application to poultry

(and to a lesser extent to other livestock) was a notable feature of this era.

Most attempts to "mendelize" quantitative traits were unsuccessful. Perhaps

sex-linked feathering studied by Serebrovsky and D. C. Warren should be

considered exceptional although polygenes undoubtedly influence this trait also.

We can't say that the old school is a thing of the past because it is very

much with us today. The commercial production of pleasure and race horses still

relies mainly on the principles of the Old School. In dairy cattle breeding

progeny testing is important because of the wide acceptance and use of

artificial insemination. Perhaps we who pose as scientists need to recognize

that breeding practices under the Old School have been successful at least as

a business enterprise, and that maximizing genetic gains may not be the most

important criterion of sound and practical animal breeding.

The dominant feature of the New School has been family selection. Families

are usually thought of as a group of full-sibs or of half-sibs having a sire

in common. The current practices of commercial poultry breeding have been

strongly influenced by the New School. The first attempt to quantify animal

breeding was made under the New School. Concepts of heritability, genetic

correlations, selection differentials and the elementary prediction equation for

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genetic progress based on estimates of these genetic parameters were developed.

Prof. Lush showed in his 1947 papers in the American Naturalist the power of

family selection in increasing genetic progress when the heritability of a

trait is low. Also, Lush and his students demonstrated that under many cir-

cumstances progeny testing is inefficient because of the time required to complete

a progeny test and as a consequence, because the generation interval is

lengthened, the genetic gain per unit of time is less. Poultry breeders, then,

turned more and more to computing family averages and discarding the procedures

of progeny testing learned under the Old School.

The key to maximizing genetic progress seemed to be family testing and

high intensity of selection.

Certain techniques have enhanced the effectiveness of family selection--in

particular, the Smith-Hazel Selection Index. Through such an index, the breeder

is provided a method which permits him to place just the right amount of

attention on each of the several traits in which he is interested, provided he

has at his disposal reasonably reliable estimates of the heritabilities and

the genetic correlations of the traits selected as well as their economic values.

A further refinement of the Smith-Hazel Index considered first by Dr. Lush_-

and later by Robert Osborne as applied to poultry, pertains to optimum atten,

tion on individual hen records, full-sib family records and sire family records.

C6upTed with these-significant-advances-in-quantitative-methods-o£...............

breeding has been the rise of electronic computors, making the task-:of calculating

family averages, estimating genetic parameters and computing a selection index

for each bird in the flock, no matter how large, relatively simple.

) A parallel development of the New School, in the case of poultry, has been

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the use of inbreeding and hybridization to produce commercial hybrids more or

less patterned s/'terthe maize inbred hybrids. This, however, is a large

story in itself and is tangential to the more conventional trend to the

science of animal breeding I am attempting to focus attention on at this time.

The dominant feature of the Contemporary School, as I see it, is the idea

of an optimum selection intensity, as suggested by Lerner (1962) and others:

that maximum genetic progress in the long run is not necessarily obtained by

applying maximum selection intensity. •The Contemporary School is a logical

outgrowth of the New School. Under the Contemporary School, our outlook on

scientific breeding is becoming re-oriented to account for new developments

in theory coupled with results from selection experiments using laboratory

organisms--in particular Drosophila, Tribolium,.and mice as pilot organisms.

Theoretical inquiry has centered on the important subject of random gene

drift and inbreeding as a function of effective population number and what

effects these have on ultimate selection limits. Credit for most of the

recently published work in this area goes to Dr. Alan Robertson and his group

at _dinbohrgh and also to Dr. Crow of Wisconsin and to his frequent

collaborator, Dr. Kimura, of the National Institute of Genetics in Japan.

(Crow and Morton, 19_5; Kimura and Crow, 1963; Robertson, 1961, 1962). One

rather surprising implication that poultry breeders need to think about shows

that family selection is effective in promoting genetic increase early in a

breeding program but only at the expense of later gains when• one thinks in

terms of a selection limit (Robertson, 1960). And cozollary to this is that

the selection limit is least depressed when the selection intensity is no

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)

greater than one-half. Natural selection operates slowly. Haldane (1957)

showed on logical mathematical grounds that a species cannot afford to com-

pletely replace one allele for another more often than once in 300 generations.

A number of well-conducted selection experiments carried out over the

past 12 or 15 years gives us a better idea of what selection for particular

mating systems should be expected to do in commercial populations. Experiments

with Drosophila and Tribolium by Dr. Bell and his associates from Purdue, b_

Dr. Forbes Robertson and others at Edinburgh including Dr. Falconer, Dr. Alan

Robertson, Dr. Clayton and Dr. Latter, lead to the general conclusion that the

elementary prediction equation of genetic gains in terms of heritability and

selection intensity follows according to theory reasonably well in the earlier

stages of a selection program but that for selection carried out over many

generations the prediction equation has almost no value. (Clayton and

Robertson, 1957) •

Adding to the newer outlook of the Contemporary School I would place the

recent theoretical development of Effective Population Number (Ne) and the

concept of an Ideal population, the latter of which I think originated from

Crowbut which is nicely described in Falconer.s (1960) book, which is important

because this lays the groundwork for designing good control populations.

Clearly, in order to assess genetic gains from any system of breeding, we need-

to have populations which will be genetically stable from year to year.. As

applied to poultry Gowe, Robertson and Latter (1959) Showed that the most

stable control population would have equalnumbers of sires and dams and that

each sire and each dam should have two progeny. They showed that a population

)

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of 50 males and 250 females randombred would have an effective population

number of 167, but if the birds are pedigreed and if artificial insemination

is employed to insure, as far as possible, that every parent leaves sufficient

progeny such that every hen leaves one daughter and every sire leaves one son

and 5 daughters, that the effective nu._ibercomes up to 250° This increase

in the effective population number reduces the inbreeding per generation by

one-third that in a random selected population°

Importance of Natural Selection

I think the recognition of the forcG of natural selection in counteracting

the forces of artificial selection is an important concept recognize@ under the

Contemporary School. In this Connection, Dro Lerner's book, "Genetic Homeostasis",

has made a strong impact on our thinking. An important question of concern to

practical breeders is, for example, how fast can I select for some trait

without seriously reducing reproductive fitness° It appears that in the case

of turkeys that the breeding accomplishments in developing fast growing broad-

breasted birds have left the breeder with a good market bird but with breeding

populations which are seriously lacking in reproductive qualities.

In the case of egg laying chickens, the most important question seems to

be whether or not we have reached a plateau and, if so, what can we do about it.

In part of the study I reported to you this morning_ selection for rate of egg

production has been essentially ineffective for a 7-year period in spite of the

fact that genetic variation; as indicated by substantial heritability estimates

of 10-25% has not been exhausted° Along this line, Dr. John Morris in

Werribee, Australia, found that selection for egg production was not very effective,

which led him to conclude that breeders should pay more attention to correlated

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traits with egg production rather than to egg production itself (Morris, 1963).

Finally, rather extensive studies by Dr. Dickerson at Kimber Farms in the U.S.A.

has demonstrated beyond reasonable doubt that closed flock selection for index

egg production has not been highly effective in the populations he worked with.

(Dickerson, 1962). All of this, to my way of thinking, makes the question

of Natural Selection a very important one for the Contemporary animal breeder

to face up to.

The concept of Genetic Load and Natural Selection might be usefully extended

to the field of Experimental Poultry Breeding (Crow and Kimura, 1965). The

genetic load is a measure of the success of Natural Selection in sorting out

the best combination of genes and keeping the genotypes in optimum proportions_

Operationally, the genetic load concept is closely related to the phenomenon of

inbreeding depression. Those of you who attended the Roundtable last year will

recall that Dr. J. F. Crow discussed this subject. An interesting application

to chickens was reported by Pisani and Kerr (1961) in Brazil. He estimated that

the mutational load as influencing hatchability, was h or 5 times as great in

Barred Plymouth Rocks as compared to Leghorns. We know that Leghorns compared

with heavy breeds, are easier to form inbred lines from. This is assumed to

result from their carrying a smaller genetic load. Why can,t we learn to

synthesize populations of chickens--say from breeds other than Leghorns--which

will have lower genetic loads and therefore commercially more useful? Mild

inbreeding in sub-populations of a rather large parent population might be one

approach. In effect, the old idea of "purging" a population of undesirable

recessive genes by inbreeding still may have merit. Such populations would be

expected to have only short-term or transitory value on an evolutionary scale,

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but in terms of commercially used populations, this could be a long time.....

This may seem to create a paradox because it conflicts with the idea that

to make evolutionary or genetic progress we need a store of genetic variation..

Indeed, one of the things we worry about as breeders is the possibility that

genetic variance may become exhausted from selection. +In fact, experiments

on the use of irradiation •suchas those of Scossiroli (1954) in Drosophila, by _

Abplanalp, Lowry, Lerner, and Dempster (1960) in chickens and a swine radiation

experiment supported by the Atomic Energy Commission at lowa State were designed

to test the usefulness of radiation to increase genetic variance. Except for

Scoseroli,s experiment, ! am not aware of any real evidence that X-radiation has

effectively increased genetic variation which has selective value. I think that

most geneticists agree that mutations arising from X-radiation, with few exceptions,

tend to be detrimental. --_Bruce Wallace working with lethal genes in Drosophila

has presented some evid_ce that homozygous lethals have a selective advantage

in heterozygotes (Wallace, 1962, 1963). On the other h_d_ work bX Dr. Crow

and his associates at _isconsin _+arenot at all in accord with Wallace's work and

rather have presented evidence that lethals which are invariably recessive, at

least those which we are able to recognize_ have a selective disadvantage

(Hiraizumi and Crow, 1960; Crow _d Temin, 1964)_ In the long runImutant genes

with only a slight disadvantage contribute most to the "mutation load,,of a

population compared to mutations with a major selective dis_v_tage (Kimura,

Maruyama and Crow, 1963). This leads to another paradox, that slightly bad

genes are very bad while very bad genes are not so bad after all. This is

because the population can rid itself of the very bad genes with ease but the

) slightly bad genes with much greater difficulty.

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The upshot of all this is that although radiation may increase genetic

variations it doesn't seem to hold much promise as a means of providing the

practical breeder with useful genetic variation in the short run. On the other

hand_ it would seem that radiation experiments to increase useful genetic

variance needs to be planned on a long-time basis--certainly decades rather

than years, Also, we should be prepared to count on many more failures than

successes. For this reason, I dontt hold too much optimism for this with

chickens. One possible exception would be to use radiation on chickens in an

altered or stressed environment. I_tations are expected to be harmful under a

normal environment because the prevailing gene pool is expected to be optimal

for the normal environment. Hence, mutations obtained under, say a stress

environment, might be beneficial to animals subjected to the stress. Yet, it

would seem that life is too short to expect very much from radiating chickens.

Clearly, this is a situation where a pilot organism, such as Drosophila or

Tribolium with a rapid generation turn over, should have priority in demonstrating

success for the method_

I think this is about as far as I dare to push the idea of a Contemporary

School of Animal Breeding. Yet there are several additional new developments

which deserve at least some comment.

Mating Systems

In the case of chickens, practically all of our co_ercial chickens in the

U, S,_today are hybrids of one form or another--either strain crosses or breed

crosses or crosses between inbred lines. I think that, more on the basis of

experience than real experimental demonstration, commercial breeders have turned

to hybridization in its various forms in place of pure lines. I rather suppose

) that random sample tests have hastened this trend

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The most interesting development in systems of mating since inbred hybrids

has been the idea of breeding for cross-line performance using the system of

recurrent reciprocal selection. Studies comparing recurrent selection schemes

with inbreeding and hybridization and with conventional family selection have

been carried out with laboratory organisms by Dr. Bell and his associates of

Purdue and by Dr. Robinson, Dr. Kojima and others at North Carolina with both

Drosophila and maize. The results are not conclusive, but those who have had

experience with reciprocal recurrent selection are cautiously optimistic of the

method. Dr. Robinson has suggested the possibility that the greatest value of

reciprocal r_current selection would be to produce foundation stocks for the

production of inbred lines and subsequent hybrids from them (see Robinson's

talk at Tenth Roundtable in 1961).

I think, notable is the fact that those who have foll_w@d the inbreeding-

hybridization scheme, have continued to do so with considerable success even

though a clearcut and consistent advantage of the system over strain crossing

or recurrent selection has never been demonstrated.

Maybe we are ready to try some new selection schemes such as stabilizing

selection and disruptive selection. Some interesting experiments using these

selection technics have been carried out in England by Thoday (1965) and his

associates at Sheffield University who show that these procedures can change

genetic variability in Drosophila.

Stabilized selection reduces genetic variance and is really selection

for intermediates. Perhaps this might be used for traits which have plateaued

in a population. This would permit greater selection pressures to be applied

i) to traits which are less refractive to selection. Disruptive selection _s

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selection for two optimal values or for example, for both extremes in the same

population. Perhaps also this would be worth trying as a means of improving

egg production. Thoday has demonstrated that populations of Drosophila subjected

to disruptive selection for bristle number over several generations develop

greater "genetic flexibility"--i.e, the genetic variance for bristle number is

increased. When such a population is subsequently subjected to directional

selection for either high or low bristle number, greater genetic gains were _

obtained compared to comparably selected lines but not previously subjected to

disruptive selection.

SpecialUse of Index Selection

Index selection of the Smith-Hazel type is typically designed to give

maximum genetic gains for over-all merit. Index selection may also be used to

improve a single trait. For example, Abplanalp, Asmundson and Lerner (1960)

compared the use of an index to increase breast width in broilers with simple

selection for breast width. The index took account of body weight, keel length _

and shank length in addition to breast width. As might be expected, greater

gains were made with the selection index so that this line had broader breasts

than the line selected solely for breast width. However, it was some surprise

to find that egg production, hatchability, and fertility were greater in the

index line. One interpretation is that selecting for a single trait increases _._._

the genetic load more than selecting on an index. It seems possible, therefore; L

in the case of turkeys, that selection for breast width, using some index of

physical attributes, might be less harmful to reproduction than simple selection_

for breast width.

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Blood Groups and Related Techniques

Over the past several years commercial poultry breeders have shown an

intense interest in blood groups along with cattle breeders. I think the

results to date demonstrate that blood typing is not a simple and quick way

for poultry improvement. Likewise, in the case of dairy cattle, the possibilities

for quick improvement in milk yield or butter fat percentage doesn't look too

promising, This conclusion is quite aside from the possibility of the use of

blood typing as a tool for obtaining greater insight concerning genetic

mechanisms.

The determination of blood groups has unquestionable value in the identi-

fication of different kinds of breeding stock and can be and is being used by

some commercial breeders to control or to insure that the stocks held by

franchise operators are legitimate.

I don't think the evidence that the blood group technique is an important

tool for animal improvement is sufficiently demonstrated as yet. Many poultry

breeders who took up the method in the U. S. a few years ago have now dropped it.

Yet we need to bear in mind that it is rather significant that blood group alleles

appear to be segregating even in populations with high inbreeding coefficients.

Bearing on this point is a recent paper by Kimura end Crow (1964) who showed

that multiple alleles theoretically cannot exist as isoalleles (ie. with

nothing to do but exist). At the same time, overdominance effects of genes on

fitness favors the maintenance of multiple alleles in a population. Thus, the

observation by Briles, Allen and Millin (1957) that in 71 of 73 populations

tested segregation occurred at the B locus, suggests that the B blood group is

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related to fitness. Also, that the _B locus is strongly histocompatible may

be of practical significance if this has something to do with disease resistance.

Much yet needs to be learned. Recent studies on the inheritance of serum

proteins, egg white proteins, and other biochemical constituents should be

closely watched by the commercial breeder for possible application.

Heanwhile, I think what is most needed in the area of blood groups is

some carefully designed experiments which will adequately assess the practical

applications of the method.

Random Sample Tests

In my opinion, random sample tests have been instrumental in providing the

American poultryman with a better quality_ higher performing stock. This has

mainly resulted in the elimination from the market of chicks from Small and

relatively low performing breeders. Thus, the consequence of random sample tests

has favored the growth of breeders who are already large and successful at the

expense of these smaller breeders. In the short run, this seems to have been

beneficial to the poultry industry, but the question is whether this will be

good in the long run. The elimination of small breeders to the advantage of large

breeders may have some bearing on limiting genetic diversity and from this point

of view this may tend to limit the ultimate amount of improvement possible.

Summary

Finally, in lie_ of a conventional summary, I would like to conclude this

talk by asking the question, "What are the basic or essential ingredients of a

successful commercial breeding enterprise?,, I won't pretend that I can completely

answer that question, because I _on't think the science of Animal Breeding has

progressed sufficiently far to know precisely which elements usually making up a

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breeding o_eration are essential_ But perhaps there are some essential in-

gredients we can agree on. Also, we might ask: in the case of poultry and

chickens in particular, "What ingredients usually found in a successful breeding

enterprise might a breeder consider changing in light of available knowledge

today?"

To begin with, I think we might agree that a successful breeding operation

must be of adequate size in terms of number of breeding pens and in terms of

the test population from which selection for the breeding pens will be made.

On the next ingredient there may he divergent opinion: how much selection

pressure to apply? In our own experiments I have generally thought in terms of

selecting the best 20 to 25 percent of the females and lO to 15 percent of the

bedt males. I now wonder whether a successful breeding operation needs or should

select too intensely. In the case of a successful breeder who has "arrived,,so

to speak--might not a selection intensity nearer 1/2 be closer to the optimum?

Perhaps a successful breeder is not as far removed from a plateau as he would like

to believe especially as regards rate of egg production. At th_ same time, we

recognize that a commercial breeder who already has superior performing stock,

faces the practical problem of maintaining his competitive superiority. This

may be very difficult, if not impossible, if his strain has plateaued. In the

short run, the important practical question is where the breeder stands in

i relation to his competitors I believe private testing programs should include@

competitors, commercial chickens as a guide to performance. In fact, some kind

of a field test which involves cooperating farmers should be an important tool

for the breeder to determine his own competitive index. Whatabout control

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populations for the commercial man? I don't believe that control populations

as earlier discussed are critically essential in a strictly commercial breeding

program. The commercial breeder doesn't need to know whether he is making

genetic progress nearly as much as he needs to know how he stands in realtion to

his competitors° Hence, competitors, stocks compared each year with his own

experimental test groups is required as an essential ingredient.

I think the breeders of egg strains need to re-evaluate the importance

they attach to selection for specific traits. If the evidence which _ have

accumulated thus far is correct, then it seems to me that many breeders, in all

probability, are placing too much emphasis on rate of egg production if this

trait is plateaued. As Dr. John Morris of Australia has suggested_ perhaps

breeders should pay more attention to traits which are correlated with egg

production. Two of these would be body size and egg size. Also, if populations

have plateaued then breeders need to think more in terms of selection between

populations.

Finally, there is the question of data utilization in any breeding project.

The use of some definite selection index would insure a consistent plan of

selection from generation to generation. A small breeder who doesn,t have

facilities for deriving his own heritability and correlation estimates might

seem to be at some disadvantage. On the other hands the small breeder might

devise an effective selection index from information on heritabilities and

correlations already reported in the literature. These might even be better than

single estimates taken from even moderately large populations because _ know that

these estimates are subject to wide sampling fluctuations.

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Literature Cited

Abplanalp, H., V. S. Asmundson and I. M. Lerner. 1960. Experimental test

of a selection index. Poul. Scio 39: 151-160.

Briles, W. E., C. P. Allen and T. W. Millen_ 1957. The B blood group system

of chickens. I. Heterozygosity in closed populations. Genetics h2: 631-6h8.

Clayton, G. A. and A. Robertson. 1957 o An experimental check on quantitative

genetical theory. II. The long-term effects of selection. J. Genetics 55:

152-170.

Crow, J. F. and N. E. Morton. 1955. Measurement of gene frequency drift in

small populations. Evolution 9: 202-21h.

Crow, J. F. and R. G. Temin. 196h. Evidence for the partial dominance of

recessive lethal genes in natural populations of Drosophila. American

Naturalist XCVIII : 21-33.

Crow, J. F. and M. Kimura. 1965. The theory of genetic loads. Proc. XI Inter°

Cong. Genetics, The Hague, The Netherlands (1963): 3: h96-506.

Dickers0n, G.E. 1962. Experimental evaluation of selection theory in poultry.

Proc. XlI World,s Poul Cong. (Symposia), Sydney_ Australia, pp. 19-25.

Falconer, D. S. 1960. Introduction to Quantitative Genetics, p. h9. The

Ronald Press Company, New York, N. Y.

Gowe, R. S., A. Robertson and B. D_ H. Latter. 1959. Environment and poultry

breeding problems. V. The design of poultry control strains. Poul. Sci.

38: 462-h71_

Haldane, J. B. S. 1957. The cost of natural selection. J. Genetics 55: 511-52ho

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") Hirmizumi, Y. and J. F. Crow. 1960. Heterozygous effects on viability,

fertility, rate of development and longevity of Drosophila chromosomes

that are lethal when homozygous. Genetics 25: 1071-1082.

Kimura, M. and J. F. Crow. 1963. The measurement of effective population

number. Evolution 17 : 279-288.

Kimura, M. and J. F. Crow. 1964. The number of alleles that can be maintained

in a finite population. Genetics 49: 725-738.

Kimura, M., T. Maruyama, and J. F. Crow. 1963. The mutation load in small

populations. Genetics 48: 1303-1312.

Lerner, I.M. 1962. Perspectives in poultry genetics. Proc. XII World's

Foul. Cong., (Symposia) pp, 9-16. Sydney, Australia.

Lush, J.L. 19_7. Family merit and individual merit as bases for selection.

Am. Nat. 80: 318-342.

Morris, J.A. 1963. Continuous selection for egg production using short-term

records. Aust. J. of Agric. Res. 14: 909-925.

Pisani, J. F. and W. E_ Kerr. 1961. Lethal equivalents in domestic animals.

Genetics 46; 773-786.

Robertson, A. 1960. A theory of limits in artificial selection. Proc. of the

Royal Soc. 153(B): 23_-249.

Robertson,:Alan. 1961. Inbreeding in artificial selection programmes.

Genetical Res$ Camb. 2: 189-192.

Robertson, Alan. 1964. The effect of non-random mating within inbred Sines on

the rate of inbreeding. Gsnetical Res., Camb. 5: 164-167.

Scossiroli, R.E. 1952. _ffectiveness of artificial selection under

irradiation of plateaued populations of Drosophila melanogaster. I.U.B.S.

) Symposium on Genetics of Population Structure, Series B, No. 15, Pavia,

Italy, August 20-23, 1953.

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Thoday, J. M. 1965. Effects of selection for genetic diversity. Prec. XI

Inter. Cong. Genetics, The Hague, The Netherlands, (1963) 3: 533-540.

Wallace, B. 1962. Temporal changes in the roles of lethal and semi-lethal

chromosomes within populations of Drosophila melano_aster. Am. Naturali.%

96: 247-2.95.

Wallace, Bruce. 1963. Further data on the over-dominance of induced mutations_

Genetics 48: 633-651.

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_ DR. A. W. NORDSKOO: '_HE EVOLUTION OF ANIMAL BREWING PRACTICE8 -...... bom, cL , ' -

DR. JOHN QUISE_HERRY: With increasing genetic-environmental interactions

being described_ what potential does interaction of the geneticist and the

nutritionist offer?

NORDJKOG: I would think_ Dr. Quisenberry, that there certainly would be

opportunity for considerably more cooperation between ger_ticists and

nutritionists, i M_ybe you have some ideas on this you would llke to presente

AMP HICKS: You described disruptive selection in which the extremes were

preferred. Pure disruptive selection is associated with assortative mating,

There is a high - low selection scheme which has been used with some success In

plants

_RDSKOG: Perhaps this umy be a matter of definition, My interpretation of

Dr. Thoday,s disruptive selection is that he has two points or optima which are

selected rather than one optimum point of selection. The two Optima may be any

place you want to put them. This is called simple disruptive selection when you

put them out in the extreme_ ie. you select the extreme groups. But this is not

assortative mating. However, matings are at random within and between the high

and low selected groups. Stabilizing selection is seledting towards the intermediate.

Nowj these are the definitions as I understand them from Thoday but if you want

to define them differently perhaps this can be done.

VERNE LOGAN: Should the commercial breeder attempt to breed chickens suited

to a particular diet or ration? If you think s% would you suggest that the breeder

do this on a suboptimum or optimum level of the particular diet or ration?

)

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_ORDSKOG: I would think that logically a breeder should be doing his

breeding and selecting on the basis of the type of diets his customers would use.

He simply is trying to develop chickens which will be adaptable to his customers

and this would be for some sort of an average condition. The possibility of

developing chickens; let's say breeding chickens with suboptimum diets, is

something that could be approached experimentally. This might be worth while if

some poultrymen use rations which are deficient but if we recommend use of

complete rations then this is what we ought to base our breeding on. Hc_ever,

experimentally, one might try something else_

GRADY MARTIN: Is disruptive selection a sufficiently wide concept to cover

a split index placing negatively correlated traits in the two prongs and applying

one prong to males and the other to females? Might the increased r_eombination

opportunity reduce the 300 generations required for complete gene substitution?

NOP_DSKOG: I would say there is a man with some imagination. I think you

have an interesting idea and this would be a kind of disruptive selection except

for sex-linked genes°

JACK HILL: How does a breeder lessen the "genetic load,,in a population of

chickens?

NORDSKOG: Frankly, I am not sure how. I am just tossing this out and

maybe you can think of a way to do it. However, populations of breeds or lines

do differ in the genetic load. Therefore, this is genetic and if nature can make

different populations, why can't man do something along this line? Perhaps

inbreeding would be one approach. _vbe the idea of "cleansing,,a population of

certain undesirable lethal _nes by inbreeding is still a good concept. If you

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can take a strain of Barred Rocks that don't do very well but if you finally,

by enough inbreeding, get rid of enough lethal genes, let's say crossing them

with another strain of inbreds, maybe you would have a population that would hams

a reduced genetic load.

DR. J. L. LUSH: I believe that as Dr_ Crow uses the word genetic load,

you have got genetic load whenever you have got selection because it is nothing

in the world but a differential reproductive rate somewhere in the population.

NORDSKOG: Except in the cese of selection for metric traits which are not

related to fitness. Also if fitness is considered the trait you are selecting,

then you would be reducing the genetic load_

DR. J. L. LUSH: If all individuals have the same number of offspring then

you have no load, even if the number is small or large. It is the absence of

competition.

HISH SAADEH: What is your explanation of the wide divergence in genetic

load between the Barred Rocks and the other breed?

NORDSKOG: I would say that Barred Rocks have more undesirable recessive

genes. Mendelian segregation then would cause higher embryonic mortality.

G. E. DICKERSON: Shouldn't the expectation of "maximum selection limit

when selection intensity is approximately one-half" be qualifie_dby population

size, and hence by degree of inbreeding involved?

NORDSKOG: This is strictly a theoretical formulation, as I think Dr.(Alan

Robertson) put it. With any given plan one can get the inbreeding effect either

early or late but in the lo:_grun you get it. Dr. Wright's maximum avoidance of

inbreeding schemes may have some bearing on this.

DR. J. L. LUSH: We say you can cut it in two by maximum avoidance.)

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NORDSKOG: But in the long run you get to the same point of inbreeding.

DR@ G. E. DICKERSON: I was referring to the fact that "selection limits"

are determined really by degree of inbreeding, which can be kept lower with the

same selection intensity when effective population size is larger, (Ne).

DR. JIM ART_R: Has any breeder ever demonstrated genetic response to

selection for yearly egg production within a strain as compared to an unselected

control population?

NORDSKOG: What do you mean by yearly egg production, Dr. Arthur? Do you

mean total production? I think we do have some evidence that other components of

the total record are more highly heritable end we don't have the plateau

situation such as with the rate of egg production as I was trying to emphasize.

For example, one can genetically change early maturity and this certainly is

part of the total egg record° What one can do with the persistency, I am not

quite sure. Certainly one can remove the broody factor, which may or may not

have a bearing on the total egg record. _at I _m talking about is the case

where the total number of eggs is divided by the total number of days which

is called rate of egg production° This measure seems to be quite refractive

to selection in our experience

BOB SHRODE: Was it just an oversight or did you for some reason deliberately

avoid mentioning visual evaluation as a basis for selection by breeders of the

"old" school? I personally believe it was more prominent than progeny testing,

and it is still with us° _ch valuable time and attention is still wasted by

devoting it to poultry and livestock judging.

NORDSKOG: Certainly, Bob, I am in full accord with that. This would be

important in the case of livestock but not very important in the case of chickens.

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Certainly this is part of the "old" school but I would say, is more a part of

the "old, old" school and goes back to early breeders.

MEL HOGSETT: Do you think that any "enlightened" breeding firm practicing

selection for "multiple objectives,,ever exceeds the selection intensity of 1/2

for any one trait?

NORDSKOG: I think that is a pretty good question and since Msl Hogsett is

a Ph.d. from Iowa State University, I would be glad to let him answer his own

question because I think he knows. Clearly, if one has several traits of equal

importance the selection intensity for any one trait is only 1 times the selection

possible for one trait.

J. ROBERT SMYTH: How do you intend to control crossing-over in your marked

chromosomes at the present time?

NORDSKOG: I don't think that I should take the time to explain this

right now because it would take a little time, but I shall be happy to show

you what we do have in mind. In a nutshell, we'll have two lines where we have

chromosome "A", to produce the males, and chromosome "B", to produce the

females, but then we will take the males which are "AA" and cross with the

females, which will be B-, and that kind of a female will have the "A"

chromosome and we likewise mate the reciprocals having the other chromosome.

Hence, there would be no crossing over at the bar.