382 ABSTRACTS
Genetics and range sheep improvement. JULLIUS E. NORDBY (1940). The
Scientific Monthly, October, pp. 310—320.
T HE student of animal genetics has found it necessary to make use of a very large num-
ber of the special fields of science. He hears of gene organisers and morphogenetic
substances ; he must study endocrinology ; he must understand polarity and physical
gradients ; he may even venture into the field of biological philosophy. To fulfil the
responsibility not only of the improvement or development of farm animals but the
efficient production of animal products, such as wool and meat, the animal geneticist
has sought new frontiers for help.
Analytical genetics or genotypic analysis, so far as the number and kind of genes
involved and their interactions and linkage relations are concerned, has made creative
genetics possible. The genetic formulae for a number of characters have been written
or postulated. These have all been relatively simple. Those characters which have
particular economic value are, however, complex. ' The opinion seems fairly unani-
mous that the utility characters such as size, vigor, physiological performance, etc. are
affected by many genes, and that, very likely, some of these are interacting in a very
complex manner.
Moreover, there is an interaction between genes and environment
which obviously contributes to the difficulty of making a satisfactory genetic analysis of
complex quantitative characters. '
If the gene pattern of every character were known the problem of combining the
desirable genes in such a way as to give sheep that were better than what we now have
would still be tremendous. It would require years of work and thousands of sheep.
If as few as ten pairs of genes were involved in a given character, nearly 60,000 genetically
different combinations would be possible. '
' Seed ' and not ' soil ' must bring about
improvement in sheep, unless the environment or soil can be permanently improved.
For a genetic analysis of complex characters, homozygosity would seem essential.
Rigorous inbreeding would make this possible. Phenotypically desirable progeny, how-
ever, do not indicate a high degree of homozygosity in either parent. The progeny test
is suggested as a method of measuring genotype if adequate recognition is made of the
matings involved. Hybrids are often superior to their parents. This is likely not due
to homozygosity but rather to the action of complementary genes from both parents.
Heterosis operates similarly although to a less degree in the progeny of a cross of two
strains or lines of the same breed. Hybrid vigour is governed to a considerable extent
by the degree of homozygosity for dominant desirable genes in each parent.
Within a line highly inbred animals which have a high degree of homozygosity do
not usually give vigorous offspring. The maximum utility of inbred lines will probably
be obtained by outcrossing and crossbreeding. _ If such is the case, inbred lines should be
developed to their maximum reliability which will be indicated by the ability, on crossing
with another equally inbred line, to repeat desirable performance.
Outcrossing tends to conceal undesirable genes, while inbreeding tends to expose
them and fix both these and the desirable genes. Large numbers of sheep may be neces-
sary for inbreeding, therefore, because of severe culling. Furthermore, intense inbreed-
ing of heterozygous lines may cause such a large number of undesirable segregates that
an otherwise successful line may be eliminated by culling. Less intense inbreeding would
overcome this.
Inheritance cannot be measured before it has had an opportunity to express itself.
Progeny testing, therefore, offers the best means of measuring the inheritance of both
sires and dams. Success will likely vary with the crossing of different lines since no two
lines will carry the same complementary genes. Any line may also be improved by in-
troducing genes from another. Hence many desirable genes may be brought together
by the discovery of complementary qualities, through progeny tests of outcrosses, and
their reintroduction into the line by convergent crosses. The use then of rams from lines
of known performance on ewes of commercial flocks would seem to be the most efficient
use to which such rams might be put.
Unrelated lines may arise from inbreeding as well: lines for long staple, good mutton,
open face or smooth skin. These may be used for corrective breeding.