4 UNrvERsiTY OF New H.-vMrsHiRE [Sta. Bull. 331 



The explanation for such seeming inconsistency has been supplied 

 by the science of genetics as originated l^y Mendel, ^^■hich has definite- 

 ly established the fact that the heritable traits of an animal are trans- 

 mitted by specific unit substances ("genes'') and that each trait is 

 always represented in the germ plasm by a definite number or a def- 

 inite combination of such genes. The fact that only half of these 

 genes from each parent can actually be used in the formation of any 

 trait of the new individual, and that the physiological process of elim- 

 inating the excess is governed by pure chance in selection of those 

 retained, explains why some characteristics are not always trans- 

 mitted as expected. A gene representing a desirable trait in one pa- 

 rent may by chance be eliminated and its place taken by a gene from 

 the less desirable ancestral line. The different genes retained from 

 unlike parentage do not fuse nor form a composite. They merely mix 

 and are present like any group of individuals and the trait whose de- 

 velopment they determine retains the identity of the parent from 

 which it came. Thus genes are handed down intact from generation 

 to generation, but the numerical proportion of genes may be changed. 

 By inbreeding, the type will be intensified because similar genes are 

 available from both parents and the chance of reappearance is in- 

 creased ; by outcrossing, the character of genes in the offspring be- 

 comes more mixed and the chance of transmitting certain ancestral 

 characteristics by this generation decreases accordingly. 



It is in this critical diiTerence between a mixture which assumes 

 a fusion and a mixture which represents an aggregate of separable 

 individuals that the pedigree system which assumes a fusion of 

 germinal sul)stance diifers from the modern method based on a more 

 specific foundation. 



The stiKh" of applied genetics then l)ec(>mes a stud}- of heritable 

 traits or characteristics of the individual. 



The practical l>reeder. of course, regards the individual as a heri- 

 table composite A\hich assumes a correlation of the difi^erent body 

 characteristics that form a type. By crossing contrasting types the 

 geneticist. hoAvever. has found that from the viewpoint of type trans- 

 mission the animal is really composed of an innumerable number of 

 independently heritable traits or characteristics, each dominated by 

 its own genes. 



Thus while leg length for example is commonly assvmied to rep- 

 resent correlated inheritance of parts, our experiments have shown 

 that the lengths of the different leg bones (humerus, radius, meta- 

 carpal, etc.) are dominated by independent gene complexes. These 

 experiments have further shown that in a cross with sheep of unlike 

 size, the mean body measurements of the first and of the second 

 (inbred) offspring generations Avere both between the mean sizes of 

 the parentage, but in their proportions of conformation each differed 

 from the parent breeds, and they even differed from each other. 

 That is. the combination of genes that were transmitted consisted 

 of a chance mixture some of which were, of course, not desirable. 



Originally, it was assumed that each body trait, or what may be 

 considered as an independently heritable characteristic, is dominated 

 bv single determiners or genes as happened to be the case in the clas- 



