THOSE "THEORISTS"!

Evening Post, Volume CXXI, Issue 115, 16 May 1936, Page 22

THOSE "THEORISTS"!

THOROUGHBRED BREEDING

THE MARCH QF SCIENCE

.So little is, yet known about the (Science of genetics by the majority of • persons >vho take a practical or' general interest in the breeding of horses and other domestic animals that it is not surprising that these people should scoff at the so-called "theorists" and 'declare that there is no such thing as a true science of breeding. These people quote innumerable examples,' all to illustrate their own contention, satisfied, to believe that heredity. has -no applicable laws because they can- . not see them operating around them ..in everyday' life. Another generation, and the viewpoint may be .different, as it is already'tending to become in the breeding of such .domestic animals as cattle, sheep, and '■ pedigree dogs, as well as of poultry. Let it "be understood at-the outset, ' however, that the laws of heredity, as they have been developed particularly by Mr. T. H. Morgan, an American geneticist, in experiments with- the Urosop'hila (vinegar) fly during the ' last. twenty-five, years, do : not teach : one .how to breed outstanding individuals, -as "• at present, of course, is the aim of the thoroughbred horse-breeder. All that these laws do is to state at best a probability, sometimes not more than a possibility; for the combination of hereditary factors cannot yet. be controlled in the laboratory, let alone in the ordinary practice of breeding. But an assiduous study of'these laws','as they : are now known in such ac- ; curate detail as to be generally, ac- ' cepted as' scientific fact, could lead to . the establishment,of pure lines in any stock, subject to the continual .appearance of mutations, which, unless i the mutation were a possible improvement' on the established line;, would crdinafily be culled out. .: ;'• SURPRISING DOGMATISM. ■In view of the already-known facts of heredity it is therefore surprising to read in such an authoritative journal as the "Blood Stock Review.: "The theory that the inheritance of colour is linked with inheritance of the manifold characteristics that go to make up racing ability is about -the most groundless theory ever accepted so ' generally -.in this business of- breeding thoroughbreds. Coat colour is inherited in' accordance with the Mendelian, : /laws.'. There is no evidence-in the science of genetics to indicate any such limitation of inheritance in accordance ; with colour as is visioned.by the colour theorists. As a . matter of fact, 'accepted scientific''theory holds- exactly •the opposite." ,". ". ..■:■.• l ■ <.-. ' '. The writer in the; "Blood Stock Re- , view" has apparently hedged himself in-with a bias against; .any of heredity without going to the num- : erous scientific brochures on the subject that; have-been published during the last ten years. If he had-read any ■of these works he would certainly have refrained from so dogmatic a remark ■ .that "there, is no evidence-, in the sciericeof genetics to indicate any such limitation of inheritance in accordance with colour as is visioned by the colour, theorists. As a matter of fact, accept- > ed; scientific theory holds exactly the 'opposite.''; " ■'■",. ": One wonders where his /'accepted : .scientific theory", rwas : gleaned. It must have been from,books published ' over twenty : years ago. rScientific research more thah4en years "ago was able to postulate as convincing fact, •so far, as inductive methods will establish facts, that one of the first extensions ..- to the scientific demonstration j and explanation of Mendel's two laws ■■■> (segregation "of the hereditary factor's, and free assortment of the hereditary , factors, the factors now being better : known as genes) was the phenomenon •of-;""linkage" of the genes'. Linkage is '..now a technical word explaining this ■phenomenon, but the "Blood Stock Re-(view"-writer apparently does not realise that it is more than a descriptive .\word. . • ~.'■: ... "■' '■";, :' •'- .;• ■-■■. A-.. HEREDITARY FACTORS. '.'■' Briefly the phenomenon may be exi'lplained: The nucleus, of eyery .cell in a living ;body—it is the germ, .cell that :is particularly important. here^trans--s;forms at certain times', e.g;, when it is dividing, into a great' number of fine that can be viewed under the ' microscope. As the. - preparation for division proceeds these threads shorten ,'ihto thick rods lying side by side in ' the nucleus. When an ordinary cell ( divides the'rods (called chromosomes) split in halves longitudinally, and each of the two new cells takes a full com'plement.bf the chromosomes, which become merged. in, the. . cytoplasmie nucleus when the new cells are again iii the, resting stage. When a germ •cell divides, however, the chromosomes do hot split, but one half gonestc one '; ofvthe new cells andi one half (or^in the case of a" male; cell one' less than qne' half) goes to the, other cell. , ' v'/iThese chromosomes can'be^seen im-. ,der the microscope, and it is now.acdepted on other almost irrefutable scientific- proof that the chromosomes , carry the hereditary factors along their length,:.these factors being known as' ithe genes, or ultimate .hereditary ele-ments.-The-'cell of a horse, for in-: stance, -may' contain 48 chromosomes, ►and each-chromosome carries a great rfurnber of genes, whose position;along their proper chromosome can^be scientifically mapped,1' though, as'far as is known, this mapping has not yet been attempted'-with ..the 'geries- of the thoroughbreds ; ■■: ■v .: ■.'.'•. . ■'' -■ ■ ■■ ; iSJNKAGE""PHENOMENON. ,:j ■ The phenomenon of "linkage" is ■' a corollary of the ■ established phenomenon ,of the chromosome' anrd the unvjitnessed but almost inevitably -to-be-assumed phenomenon of the genie. The chromosome are individualistic,' that is,' the 'separated chromosomes can be easily distinguished by an experienced worker through the .microscope; .and each chromosome carries its particular set of genes at deßnite points along, its ■jjength. factors (genes) are simply, those'that exist on the :same chromosome. "'.- .■ ■• ■Waen a mating takes place the offl spring, technically known as the. soma, obtains its full complement of chromosomes in the fertilised egg cell as a fr.esult of receiving half from the un- ;\ fertilised ovum, and half (or one less than half, as. the case may be, the /reason" for which is not pertinent at present) from the spermatazoon. It is not the genes that segregate and pass into the soma, but the 'full chromosomes, and generally it may be said that every chromosome (half from the sire's outfit" and half from the dam's outfit) that goes over carries its own complete set of genes or actual hereditary factors, which are therefore said to be "linked." In .other words, if a gene for chestnut colour, a gene for well 7 developed' heart, and a gene for depth" of chest;-as a general but, of course, not , scientific-example,™-lay- on -the.'.: same, chromosomerthey wpuld, under the law of linkage, all pass over if'that >vas one of the chromosomes 'Stp "•.■go";dhto the genetic cdnstitvition^;ofthey,;iiew'fertilised 'ovum."*::?v;"'*'.",';':-i-' •■v-.'l-' ■}:•:■ ••'" .'•■■■ -■; ■'■■ The phenomenon;oty'linkage".is interruptedi. inii'lmanyj''Jalready.-lcnown "ways:: :For, example; there vis sthe phenomenon i.of . -~' f\' byeri",, '•; under which' chromosomes,.; under ;4he influence1, of- each;'other,;.as :aby'jJtwisting. round each, other, break once-or several times, and their parts join up-in various combinations before;: Ihe divi-. sion- of the, cell, so that thevehfom'o-?-soines that go over into th'e/soma may consist part .of one origirial chromo-: some and part of another, ,or others Then there are certain genes on other chromosomes that-may -act:'as^modj.ners 'to the ; expected. characteristic' from a certain gene,1 so that, for instance, what would otherwise have been a bright chestnut becomes a' liver chestnut. Even more important1 than these is the radical distinction -between "dominant" and "recessive" genes, the latter" often not being manifest in the developed individual, though the factor is still i&tently carried in the genetic constitu-

tion and may reappear in later genera- | tions.: ' " : ■ ."■■ ■" :. • ' These are only some .of the,known laws of heredity, and they are stated in general terms to show how far astray the modern writer may go if'he discusses breeding as a /science, without further knowledge of-modern research than the "Blood, Stock' Review" critic apparently possesses. . The science may be still in its infancy so far as practical results go, but dt has already established half a dozen or more demonstrable laws, and, one of these laws.that may not be disputed'is that "linkage" is a definite phenomenon, so that, tor example, coat colour will be always inherited along with other characteristics, good and bad. There may of course be no linkage with all "the manifold characteristics that go to make up racing ability," but there I may be linkage with sufficient of them to assure, for instance, that a chestnut from a certain mating will be a good horse if the animal does not sulI fer subsequent to its birth from environmental circumstances.