214 GENETIC CORRELATIONS IN AUSTRALIAN MERINO SHEEP [Vol. XXVIII, Pt. IV,
DISCUSSION
Where the correlations between various traits are calculated on related indi-
viduals, the sampling errors decrease as the relationship between them increases.
In this study the correlations between various characters measured on the same
individual have been obtained, and these values involve relatively more sampling
errors which are at present unknown. Nevertheless, the general picture presented
does give some indication of the pattern of the relationship among the characters
studied.
Accepting the computed estimates of Table II, it seems that selection for one
of the body measurements will produce correlated responses in the same direction as
other body measurements, body weight and mutton type; while it will decrease
the skin fold score. Similarly, selection for body weight will improve other correlated
characters. Selection for long cannon bones will result in sheep which are longer,
wider and slightly deeper in size, with a slight increase in mutton type and body
weight; at the same time the fold score will decrease.
The principal genetic antagonism found is that of fold score with all other
characters studied. Fold score has a negative correlation with five months' body
weight and no significant correlation with 11 months' body weight.
The importance of genetic correlations among various traits has been long
recognised, as evidenced by Darwin's statement: "Hence, if man goes on selecting,
and thus augmenting any peculiarity, he will almost certainly modify uninten-
tionally other parts of the structure owing to the mysterious laws of correlations".
Probably the most important basic cause is the multiple effect of a gene or block of
genes. A gene affecting one character may also affect other characters at the same
time. Lush [1948] suggested that as far as truly pleiotropic effects are concerned
it does not matter whether they are due to entirely different primary effects of each
gene or whether the gene has a single primary effect which starts a chain of conse-
quences that eventually affects two or more traits, perhaps in opposite directions.
Various causes of genetic correlations have been suggested from time to time by
various workers. Lush [1948] has suggested that if past selection has been effective
it is likely to have caused a negative correlation. The frequencies of genes which
have favourable effects on both characters, and of genes which affect one character
favourably, but have no effect on the other, would have been raised to a high level so
that they contribute little to the additive genetic variance. Genes with a beneficial
effect on one character and deleterious effect on the other, would have had their
frequencies brought to an intermediate level by continued selection for both
characters. This class of genes will contribute more to the additive genetic
variance so that characters will tend to be negatively correlated.
Linkage between genes has also been suggested as a cause of genetic correlation.
However, it will usually have only a minor effect because crossing over in a freely
interbreeding population leads to 'coupling' and 'repulsion', heterozygotes
becoming equally frequent. The effect of linkage causing genetic correlation can be
detected in the first two generations after a cross between previously separate strains
or varieties.