Íslenskar landbúnaðarrannsóknir - 01.03.1970, Side 101

COLOUR INHERITANCE IN ICELANDIC SHEEP 99
inheritance, one can see from table 58 that
blaze markings can by no means be re-
garded a constant feature among the sheep
which show white markings. Different types
of blaze markings occurred among the par-
ents, and if these were based on a multi-
factorial inheritance, one would expect
some combinations of genotypes with
white blaze to give progeny without white
markings. This has not happened, however,
except eventually in the unexpected case
shown in tables 57 and 58, but that case
can easily be explained as extreme lack of
expression of white markings due to genetic
and environmental modification.
The white collar is frequently found in
the Icelandic sheep, but it has never been
found on its own, as sheep with white
collar have always shown some white on
head and feet as well. The white collar in
the present study seerns to be in every re-
spect comparable to the white collar found
by Vasin (1928). Yet the inheritance of
white collar in the present study is quite
clearly recessive to no white markings. This
is shown by the extensive use of 4 rams
with white collar in the present study. In
matings to ewes without wliite markings
the white collar behaved as a completely
recessive character. The final proof that
the conditions hood and white collar are
recessive is the fact that the segregation
ratios in the matings SXS2 X S2S2 and
S2S, X Sxs2 are quite close to the expected
1:1 ratio. If the hood and white collar were
dominant, all the rams showing these mark-
ings would have been homozygous for white
markings, as they did not give any off-
spring without white markings in the mat-
ings S2S, X S,S2. They would therefore
have been expected to give considerable
excess of progeny with white markings in
the mating S,S2 X SiS2> but this is clearly
not the case, cf. table 57.
The conclusion must therefore be that
the three types of white markings, hood,
blaze markings and white collar are all re-
cessive in tlie Icelandic sheep and that they
are only different modifications of reces-
sive white markings caused by homozygosity
of the allele S,.
It should be remembered that so far all
white markings have heen assumed to be
caused by the same allele, S,. There might
eventually exist several alleles for white
markings at this locus, and some of the
variation in tlie extent of white markings
could then possibly be accounted for by
difference between alleles. So far separate
alleles have not been isolated in the present
study, and the amount of variation in the
white markings will clearly make it a diffi-
cult task to decide whether the extent of
the white markings is caused by segregation
of more than one allele at this locus or to
the action of modifying genes and intra-
uterine environment.
C. EFFECT OF LOCI B AND S
ON FERTILITY
The effect of loci B and S on number of
lambs per ewe lambing arnong 2 year old
ewes has been examined. The analysis gave
no indication of any effect of the geno-
types at these two loci on this aspect of
fertility. In table 59 are shown the least
squares estimates within farm-years of the
tlie black versus brown genotypes and of
genotypes without white markings versus
those with white markings. The results of
the analyses of variance are given in table
60.
D. LINKAGE BETWEEN COLOUR
LOCI
Very limited evidence is available on link-
age between colour loci in the present data.
With respect to linkage between loci A