Íslenskar landbúnaðarrannsóknir - 01.03.1970, Page 78

76 ÍSLENZKAR LANDBÚNAÐARRANNSOKNIR
on number of lambs per mating in two
year olcl ewes.
This effect seems to be connected with
the presence or absence of the A^-allele
only, because no difference is found be-
tween homozygous and heterozygous white
sires. As this effect is only found in two
year old ewes and not in 3 and 4 year old
ewes, it could be argued that this is only
a chance deviation which could be expect-
ed to turn up when several tests are carried
out. The results obtained by Edgar (1962),
on the other hand could possibly be taken
as a support for the view that the ranr
effect found was a genuine one and would
be expected to manifest itself more clearly
in two year old ewes than in older ewes.
Edgar found that the time taken for the
ova to reach the uterus was shorter in two
year old ewes than in older ewes and he
suggested that this early passage into the
uterus might endanger the life of the ov-
um.
Tables 34—37 sliow that the effect of the
genotype of the ewe on nurnber of lambs
per rnating can be attributed wholly to the
presence or absence of the Aj-allele. There
is no detectable difference between the
white ewes of unknown heterozygosity and
the known white heterozygotes, so the ef-
fect does not seem to increase with horno-
zygosity for Ax.
The effect of the A^-allele is highly
significant for the tlnee age groups exa-
mined, and in no case can any interaction
between colour genotype and farm-years be
demonstrated, for number of lambs per
mating. This indicates tliat the interaction
found for number of lambs per ewe lamb-
ing in tables 33 and 35 has mainly been
a scale effect. In no case has an interaction
between genotype of ewe and genotype of
ranr been significant.
The results obtained in tables 32—37 are
in good agreement with the fact that the
proportion of nonwhite lambs in table 28
was founcl to be higher among twins than
singles. The fertility has been shown to be
appreciably higher arnong nonwhite ewes
than among white ewes. This results in a
higher proportion of twins among non-
white ewes, and as nonwhite ewes also
have a higher proportion of nonwhite
lambs the result will obviously be a higher
proportion of nonwhite lambs among twins
than singles. This does not necessarily
mean that no other factors are behind the
differences found in table 28.
E. EFFECT OF COLOUR GENOTYPE
OF PARENTS ON LAMB SURVIVAL
Information was available on lamb sur-
vival up to the age of 6—8 weeks for 3047
lambs out of known genotypes with re-
spect to the A-locus. The dead lambs were
classified as having either been born dead
or died at birth, died between birth and
shearing, or survived until shearing time.
The number of lambs in each of these
three categories are shown in table 38,
where they are also classified according to
the genotype of the parents with respect
to white colour.
A comparison among the individual mat-
ing groups under the assumption of the
same distribution on survival groups gives
a x26 = 4-729 (0.70 > P > 0.50). There is
thus no indication of any effect of the At-
allele on the survival rate of lambs up to
6—8 weeks of age.
F. SEX AND COLOUR RATIOS
AMONG SINGLES AND TWINS
IN KNOWN MATINGS
As was shown in table 28 there was a
significantly higher proportion of white
lambs among males than females. At the
same time it was shown in table 26 that
there was a significant difference between
segregation ratios in the reciprocal matings