Blood-Lines 

The word "blood-line " is often used by breeders and is found in many 
advertisements and current animal breeding writings. It is rare, however, 
in textbooks on animal breeding and still rarer in textbooks on genetics. 
In general, blood-line is synonymous with pedigree but is not so 
definite. Sometimes it is used more nearly in one of the senses of family, 
as, for example, when a man suggests performance testing of many ani- 
mals in a breed to find out "which blood-lines are the most productive and valuable," or wants to learn "which are the most prominent blood-lines of the breed." 

Sometimes blood-line is used to convey the idea of relationship, as 
when a man says that two animals "have nearly the same blood-lines," or 
that some animal "has valuable blood-lines/' In the first case he implies 
that the two animals are closely related, and in the second case he implies that this animal is closely related to some ancestors whose descendants are highly valued. As a measure of relationship blood-line is an indefinite and sometimes misleading substitute for the probability of likeness 
which is expressed accurately in the coefficient of relationship. Often it 
makes the relationship seem much higher than it really is. Blood-line is a 
convenient term, however, because almost everyone understands it in a 
general way. 

Sometimes blood-line is used to describe a linebreeding or an in- 
breeding program, as when a breeder says he "believes in mating together animals of similar but not identical blood-lines." He thus conveys a vague idea of what would be more precise but probably not so readily understood is he used the inbreeding coefficient and the relationship coefficient to state how intensely he planned to inbreed and how closely he was trying to keep his herd related to some noted ancestor. 

Sometimes blood-line is used to infer that a whole complex of in- 
heritance is transmitted as a unit unchanged from parent to offspring, 
generation after generation. This idea comes from studying pedigrees 
backward. The present famous animal is traced through his sire to a 
grandsire and through it to a great grandsire, all of which were outstand- 
ing individuals of their breed. Looking back to what happened, we some- 
times see an unbroken succession of outstanding merit. If we could turn 
the pedigree around and look forward from the first famous animal in the 
line, we might see what really happened. The outstanding individual which was the first in this blood-line was used in one of the leading herds of the breed. He had many sons and daughters; and, as far as the breeder could pick them out, only the best of his sons were saved for tentative use in leading herds, where they were mated to better-than-average females. 
That son whose offspring proved him to be the best became the leading 
sire of his generation and his supposedly best sons were eagerly sought 
and in turn were tried out in the leading herds of their time. This may 
have lasted several generations, or at least as long as even one outstanding son of the outstanding sire in each generation could be found. In a breed where one herd or a small group of herds which get their sires from each other maintain a leading position over many years, it will sometimes happen that from grandsire to sire and to son there was an unbroken succession of outstanding breed leaders. This will become familiar to everyone who studies pedigrees of that breed, and people will soon be referring to this as a "very valuable blood-line." Really what happens in such cases is nothing more fundamental than an intense selection among the sons in each generation. 

Because of its vagueness, blood-line is in bad repute as a scientific 
word. Its claim to retention in the animal breeder's vocabulary is that it 
is widely used now and that everyone understands at least in a general 
way what is meant by it. The relationship coefficient and inbreeding 
coefficient are not yet widely used and understood. They would often 
require a long translation or explanation. There is no way to make 
blood-line quantitative, but it is often useful where only a qualitative 
meaning is necessary. 

It is more nearly correct biologically to think of the individual as one 
knot in an enormous network of descent, rather than as belonging to 
some blood-line. The network is irregular in practically all respects ex- 
cept that each individual has two and only two parents. Figure 44 shows 
a network which corresponds in a small way 1 to the irregular and inter- 
locking lines of descent which constitute the pedigree structure of a breed. 
Each small circle represents an individual, and the short straight lines 
connect parent and offspring. Each individual's ancestry widens out rap- 
idly until, not many generations into the past, its pedigree includes nearly 
the same animals as the pedigrees of its contemporaries do, but with some 
ancestors repeated rarely and others repeated many times. Some indi- 
viduals leave many sons and daughters, others few, and still others leave none. The breed is continuous in time and space and changes but slowly. 
Except that the figure shows more inbreeding and hence more separation into distinct families than is usual
The individuals are discontinuous, and each is different from all the 
others. Each individual is related to all the others but in widely varying 
degrees. One blood-line can no more be lifted out by itself than one 
strand of a fishing net could be picked up without picking up all the others.

FIG. 44. The pedigree of a population, showing that it is a network of descent and is not composed of "blood lines" which are separate. 
Those nearest would be affected soonest and most strongly. The fishing 
net, however, is much more regular than the pedigree structure of a breed. 

SUMMARY 
"Blood-line" is an elastic term used sometimes as synonymous with 
family, sometimes as a substitute for relationship, and sometimes to de- 
scribe vaguely a breeding system. 

Because of its vagueness, blood-line is in bad repute as a scientific 
term. But, because it is so widely understood by breeders, blood-line will 
sometimes be found useful in conveying a general qualitative idea about 
breeding topics where the speaker does not wish to call attention to the 
quantitative aspect of that idea. 

REFERENCES 
Malin, D. F. 1923. The evolution of breeds. Des Moines: "Wallace Publishing Com- 
pany. This book contains abundant references to blood and the word "strain," 
or family. These show how one can speak more definitely on the subject and 
yet avoid the use of "blood-line." 

Whitney, Leon F. 1933. The basis of breeding. New Haven: E. C. Fowler. (Pre- 
sents many arguments against any use at all of "blood" to mean inheritance.)