Sheep - AgRIS

elationships. Certain coat colors are the trademark of the some breeds of livestock. This is
probably because this can be easily recognized. It is thus important that the breeder must
conform to the breed requirements for this trait otherwise he will not be in the purebred
business for long.
Type and conformation have been used as the basis of selection for many years throughout
the world. Type may be defined as the ideal of body construction that makes an individual best
suited for a particular purpose. This basis of selection has merit in some instances. The
conformation of a draft horse is such that he is better suited to pulling heavy Ioads than he is to
racing. On the other hand, the reverse is true of the thoroughbred.
The performance of individuals has also been given some attention in the development of
some of our breeds of livestock. For many years thoroughbred horses have been selected for
breeding purposes for their speed. Dairy cows have been selected for their ability to give large
amounts of milk and butter fat. In beef cattle and swine, however, less attention has been paid
to selection for performance and carcass quality until recently.
Increased emphasis is now being placed on selection for performance and carcass
quality, because breeders realize that the type or conformation of an individual is not
the best indicator of its potential performance or its carcass quality. Appropriate
measures of these traits must be applied before progress can be made in selection for
them.
The correlation between type and carcass quality is greater in some instances than is the
correlation between type and performance. The meatness of hogs by a visual inspection, can be
assessed but this is not reliable. Better methods are baskfat probes on live animals, actual
weighings, and measuring of lean meat in the carcass.
The fact that type and performance are not usually closely related indicates the importance
of selecting separately for the important traits in livestock production. If the correlation
between type and other traits is low, it means that they are inherited independently and that they
can be improved only if selection is practiced for each of them.
Individuality for certain traits should always be given some consideration in a selection
program. However, it is more important in some instances than in others. It is most important as
the basis of selection. when the heritability of a trait is high, showing that the trait is greatly
affected by additive gene action. High heritability estimates also suggest that the phenotype
strongly reflects the genotype and that the individuals that are superior for a particular trait
should also possess the desirable genes for that trait and should transmit them to their offspring.
The greatest disadvantage of selection on the basis of individuality is that environmental
and genettic effects are sometimes difficult to distinguish. Much of the confusion rnay be
avoided by growing or fattening the offspring being compared for possible selection purposes
under a standard environment. Even then, it is still possible to mistake some genetic effects for
environmental effects. This is less likely to happen, however, in the outstanding individuals
than in those that have a mediocre record. For instance, a bull calf placed on a performance test
may make a poor record because of an injury or because of sickness while on test. But if he
makes an outstanding record, it is certain that he possessed the proper genes and in the right
combination as well as the proper environment to make the good record. It cannot always be
certain, however, whether an individual with a mediocre record would have done better even if
adverse environmental factors had not interfered. We can be certain that his record is poor, and
by culling on this basis, elimination of the genetically poor individuals is possible. This chance
is worth taking, even though we may discard some genetically superior individuals
occasionally.
Studies of selection on the basis of individuality within inbred lines of swine have shown
that selection favoured the less inbred litters. This is another way of saying that selection
probably favoured the more heterozygous individuals, and this may be true also in many cases
where inbreeding is not involved to a great extent. Chance combinations of genes may make an
individual outstanding, but his offspring may be inferior, because he cannot transmit his
heterozygosity to his offspring. The breeder should avoid keeping superior individuals from
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