IC>YWa-! n (c z b i VOL. V. AUGUST. 1915 NUMBER 8 Mendelism and Sheep Breeding By DR. J. A. DETLEFSEN, College of Agriculture, University of Illinois NATIONAL WOOL GROWER I N the very earliest days of animal breeding, there was only one thought uppermost, in the breeder’s mind: to increase the numbers in his herd or flock. In the due course of time, however, it became apparent to the observing breeder that it was not wise or provident to breed only for numbers. Newer types and better adapted animals were desired. The progressive breeder always kept his mind open and alert to learn any new devices by which he could accomplish this end. Such methods or systems of breeding as line breeding, inbreed¬ ing, grading, and above all, rigid se¬ lection, were devised and practiced to meet particular or special ends. They were, and still are, the time-honored methods in the art of breeding. New methods and new systems will arise in the future, just as they did in the past, and they will likewise be accepted or rejected by the breeder according to their merits. We have frequently heard of Men¬ del’s Law of Heredity in the last few years, and it is only natural that breed¬ ers should inquire as to its essence and application to their problems. As is generally true with all new ideas, much confusion and wild speculation has been attached to this subject. At the outset, it may be well to state that Mendel’s Law is, in no sense, a sys¬ tem of breeding, but rather a scientific explanation of the transmission of characters from parents to offspring in any system of breeding. One of the most essential attributes to clear think¬ ing on the subject of breeding is to dis¬ tinguish carefully between a system of breeding and an explanation for the results of any given system. From time immemorial, we have been re¬ galed with one explanation after the other; and it is perfectly safe to say that most of them, when critically ana¬ lyzed, were found to be of little or no value. The primary prerequisite of any explanation for results seen in breeding is that it shall show how, when, and why characters are trans¬ mitted from parents to offspring. The most recent and withal the most suc¬ cessful theory to account for the trans¬ mission of parental characters ijs Men¬ del’s Law of Heredity. Whenever new discoveries, such as Mendel’s dis¬ covery, are made, the breeder inquires quite legitimately “Has it any imme¬ diate bearing on my plans and prob¬ lems in breeding on the farm or range?” Erroneous impressions are like weeds, easily produced and widely spread. This is particularly triie when scientific abstractions are involved as is the case with Mendel’s Law? It is not at all uncommon to find extrava¬ gant promises rashly made that Men¬ delism will solve all of our breeding problems. In fact, much discredit has been already attached to Miendel's Law by those who confuse it with systems of breeding, and attempt to use it without sufficient knowledge of the many scientific complexities and ramifications involved. There are, to be sure, many dangers of misleading the sheep breeder when one attempts to explain Mendelism in simple terms, primarily because the operation of Mendel’s Law is based upon biological and mathematical facts more complex than simple terms will explain. I am therefore well aware that I am in dan¬ ger of confusing the breeder and lay¬ ing myself open to criticism, when I try to point out any relationship be¬ tween Mendelism and some sheep¬ breeding problems, for, I must keep my thesis iri simple terms and avoid the technicalities of the classroom. With these precautions in mind, let us inquire what bearing Mendel’s Law has to sheep breeding. Gregor Mjendel, the Augustinian monk, prelate of the monastary at Birunn, Austria, worked on the prob¬ lem of hybridization using chiefly the edible or garden-pea. About 1865, he published two brief papers in the journal of the local scientific society stating the results of eight years’ work. His work was not well received, in fact was neglected and all but forgot¬ ten. The facts of the case are that Mendel was years ahead of his time and not until years after his death did the truth dawn upon the scientific world at large. In 1900 his work was rediscovered and corroborated. Men¬ del showed that an individual plant or animal is a sum total oF separate in¬ dependent characters, some large, oth¬ ers small, some quite apparent and others quite hidden. When these char¬ acters are transmitted they are trans¬ mitted as units or as a whole, and they may be recombined with one another in such ratios as we expect accord¬ ing to certain mathematical expecta¬ tions. To be concrete, let us illustrate how Mendel’s Law works by crossing a pure white guinea-pig male with a pure black female such as is shown in figs. 1 and 2. Here we are dealing with a simple pair of characters, and if we follow this simple pair without regard to any other differences what¬ soever, we shall find that all crossbred children will be black, as shown in fig. 3. We say then that blackness in guinea-pigs is dominant to whiteness, and we call the white or albino variety recessive because it recedes ^from our view in the crossbreds. Now although all these crossbred children are black in appearance, they obtained black from only one parent, while they 10 • THE NATIONAL WOOL GROWER August, 1915. obtained the hidden or recessive white condition from the other. In other ■ words, they are dual in nature, hav¬ ing obtained the visible black from one parent and the hidden w'hite from the other. We can easily prove this by mating black crossbred children to¬ gether to produce grandchildren, and we find that the crossbreds do not breed true but give among the grand¬ children blacks and whites again. Fur¬ thermore, these black grandchildren occur three times as frequently as the white ones in the long run. Mendel made clear to us why this result takes place. The white grandsire produces sperm all of which carry something (we shall not state what) to produce whiteness, while the pure black grand- dam produces eggs carrying a sub¬ stance giving the black color. Any of his sperm might fertilize any of her eggs, but the crossbred children so produced carry both the blackness and whiteness, although they are black in appearance because the black color covers over the white or is dominant to it. These crossbred children are, _oLc£u i] --<■ lw.tli males and females and. as we said, dual in nature. Being dual in nature, the males now produce sperm half of which carry black and. half of which carry white; and the fe¬ males produce eggs in similar manner. In other words, black and white sepa¬ rate from each other when the germ cells are formed, so that a germ cell contains either one or the other of this pair of characters but never both. Such being the case, we can now see why Ave obtain among the grandchil¬ dren a ratio of 3 black to 1 white in the long run. It is simply because when the crossbred children are mat¬ ed together the combination of germ cells which come together in fertiliza¬ tion to produce the grandchildren can only be as follows: 1. Sperm carrying black can ferti¬ lize eggs carrying black and give pure black young. 2. Sperm carrying black can ferti¬ lize eggs carrying white and give crossbred black young because black, is dominant. 3. Sperm carrying white can ferti¬ lize fggs carrying black and give crossbred black young because black is dominant. 4. ISperm carrying white can ferti¬ lize eggs carrying white and give pure whites* young, no black being ipresent. We see that there are three times as many, combinations giving black as white, hence our ratio. Furthermore, it will, be noted that in the fitst com¬ bination the black is derived frqm both parents, but in the second and third combinations black is derived from one parents and white from the other. Thus it happens that one-third of the black grandchildren are pure for black and breed true, while two-thirds are crossbred for black and breed as the crossbred children did. Extensive breeding experiments have prdven this completely, for we could never tell from external appearance whether the grandchildren are pure for black or not, since black is dominant. It will be noted that the white male in fig. 1 has a rough coat with swdrls, but the dam in fig. 2 has a <. smooth coat. ^ If we follow roughness and smoothness in a cross just as we did black ajnd white, we find that the cross¬ bred children are rough-coated as is shown in fig 3, because roughness is dominant to smoothness. ^Breeding the crossbred children together will give three times as many rough grand¬ children as smooth ones in the long run. The reason is obviously the same ajs that given for the black-white cross. [ The case becomes more complex if we attempt to follow black, white, roughness, and smoothness simultane¬ ously in a cross. If we cross a pure white, rough male (fig. 1) to a pure black, smooth female (fig. 2), the rough character and black character are dom¬ inant as we might expect from the pre¬ vious illustrations, and so the cross¬ breds ire black, rough individuals (fig. 3). If we now breed the crossbred children together, we shall obtain in the long run the following classes: 9 Black Rough (fig. 4). 3 Black Smooth (fijg. 5) 3 , White Rough (fig. 6) 1 White Smooth (fig. 7 1 This “splitting up” or segregation and recombination of characters in the hybrids is the essence of Mendelism. It will be noted that we began with a rough, white male and a smooth, black female, and now we have in the grand¬ children two new combinations, black- rough and white-smooth. In general, we may say that the greater the num¬ ber of differences in the parents, the greater the variability in the progeny. Let us suppose, for the sake of illustra¬ tion alone, that the breeder wished such a combination of characters as the black-rough combination. How would he get it to breed true? We saw that when the crossbred black, rough children were bred together, they gave new forms or variations. We should also find that, among the grand¬ children, not all black-rough indivi¬ duals are equally pure in breeding. Some would be pure for either black¬ ness or roughness, but very few for both. We could find by experimental breeding which individuals are pure for both blackness and roughness and thus start a pure breeding race of black-rough giMtvea-pigs immediatellfc There is another method by which we could get a pure black-rough-coated race. We could mate individuals of the desired type together and keep eli¬ minating the culls, as well as those in¬ dividuals which produce the largest number of culls. Eventually we should establish a pure race of black-rough in¬ dividuals. In our domestic animals there is no real fundamental difference in the transmission of characters from that of our illustration, except that we deal with very many characters, most of which are too complex and subtle for such simple elementary analysis. For those who desire a more detailed con¬ sideration of Mendelism, such books are to be recommended as Professor James Wilson’s “Principles of Breed- ing” (published by Vinton & Co.) or that excellent little book by Professor Castle called “Heredity” (published by D. Appleton & Co.). Enough has been given in our illus¬ tration of the working of Mendel’s law to show what we must expect in some August, 1915. THE NATIONAL WOOL I GROWER 11 of our sheep breeding problems. Some breeders are trying to combine such complex characters as density fineness and weight of fleece, hardiness, the flocking instinct, and the like from the Merino blood, with length of wool, mutton conformation and the like from the Lincoln, Leicester, Oxford, Rom¬ ney Marsh, or similar blood. This is really an experiment in hybridization, very much like the rough-white guinea- pig crossed to the smooth black, and the analogy is instructive to say the least. The breeder sometimes obtains a very desirable type by crossing these two widely different varieties of sheep, particularly if he uses pure-bred stock to begin with. Then, having obtained a desirable cross, he wishes to hold the type secured. Here is where the dis¬ appointments are bound to arise. One hears breeders say they would give much if they could hold the remark¬ able blend of characters which they have obtained in a cross. The cross, however, is like any cross in which the parents differ in many respects, and the progeny are bound to show increas¬ ed variability because so many charac¬ ters combine and recombine in all the various possibilities. In other words, when a breeder crosses a Merino type with a mutton type, to secure the good characters of both, he must expect to get much variability in subsequent gen¬ erations. It is the same case as the white-rough guinea-pig crossed to the black-smooth one, only our sheep cross is more complex and deals with a larger number of characters. The de¬ sirable blend can only be held at the expense of much selection and culling. The undesirable types should be eliminated, and the desirable combinations continually bred to¬ gether. Eventually we may hope to fix, hold, and breed pure the combination of Merino and mutton qualities which we seek in such a cross, but it takes much effort, time, and se¬ lection. Cross breeding induces vari¬ ability and Mendel showed us the rea¬ son for this by showing how charac¬ ters are rearranged in the offspring in all sorts of possible combinations. Any combination obtained is not necessari¬ ly pure, for it may have dominant characters covering up the hidden or recessive ones. To purifytjthe type, inbreeding and selection are necessary, and the risk of rejecting many culls must be faced. Such has been the experience of inos with some mutton type such as Lincoln rams. The expected happened, for this wide cross gave much varia¬ tion in early years and required much culling. To my mind, the variation was due to the recombination of those many characters, in which the original FROM CASTLES HEREDITY breeders when any new type has been founded by crossing. The writer was much interested in the history and method of building up Aire-Carriedale/ breed of sheep. This mew breed rep¬ resents the efforts of crossing Mer¬ parent stocks differed. The culling represents the elimination of the un¬ desirable combinations. The desirable combination has been purified, at the expense of rigid selection. If any at¬ tempt is to be made in our country to Fl^'s. 4.5 6 . 7 - Tyjoes op Cj ramd cVi