A " line " is a "strain" or
family continuation; the individuals composing it are usually much more closely
related to each other than is usual for the general population (random-bred).
Inbreeding and linebreeding is almost the same. Inbreeding is any mating of relatives, while linebreeding is a mating of an ancestor with a decendant in a breeding plan which may or may not be carried on for generations: so, a mating of father x daughter, son x mother, or grandfather x grand-daughter is linebreeding but is also inbreeding.
A strain is a group of individuals all descended from common ancestors and related closely enough to be similar in their genetic factors. In a strain, all members carry the same general attributes, good and bad, although in somewhat lesser and greater degree in individual birds. Their expectancies can be largely forcasted. When families are continually outcrossed, there is no inbreeding, but each mating is a step in the dark as to its progeny.
Given a bird of outstanding qualities, ie. frequency, depth, tight roll, speed, and proper balance; desiring to perpetuate these qualities in a number of offsprings, inbreeding of some nature must be resorted to or these qualities will soon be reduced to a small percentage even if careful selection is employed.
Example: An outstanding cock: A. In the first mating to an unrelated bird, the F1 are 1/2 A; in the second unrelated mating, 1/4 A; in the third, 1/8 A; in the fourth, 1/16 A; and in the fifth, 1/32 A, possessing just a little over 3% of A's blood.
Linebreeding , the result is quite different: a mating of A to his daughter gives F1 with 3/4 A's blood lines; a granddaughter backcrossed to A gives progeny with 7/8 A's blood lines. So, in just two matings, young are produced with over 87% of A's blood lines.
Reference: Levi 1969 (606)
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