Íslenskar landbúnaðarrannsóknir - 01.03.1970, Blaðsíða 41

COLOUR INHERITANCE IN ICELANDIC SHEEP 39
some matings than in others one may as-
sume that the observed anomalies are some-
what below the actual number of discrep-
ancies that could occur.
The exceptions are so few, however, that
the main hypothesis about the allelic
systern at the A-locus explains the results
reasonably well. Each exception, on the
other hand, could throw some new light
on the inheritance of the colours and each
case will therefore be treated separately in
the following.
1. Individual cases.
Case 1. Mating X A5A5.
Two nonwhite lambs.
One of the lambs was born on farm No.
4. Date of mating 5/1/1960, date of lamb-
ing 30/5/1960. Two lambs, white male and
grey male.
The sire No. 04008, genotype AjAj-----
---, was very well tested previously and
never gave any nonwhite lamb except this
one.
The dam, No. 04036, black, genotype
AgAgB^—SjS^, had previously given 5
white lambs when mated to homozygous
white sires, 2 black lambs, one of tliem
with white markings, when mated to a
black sire with white markings of geno-
type A5AgB^B^S^S^, and two black lambs
when mated to a black sire of genotype
AgAgB^B^S^- .
l’he jrrobability that the dam carried a
dominant gene which suppressed white
was therefore less than 0.05, and the prob-
ability that she carried any unmanifested
allele other than Ag was 0.0625 or less. The
ewe was therefore unusually well tested
with respect to both these possibilities.
Another ram used on the farm in the
same year, No. 04011, genotype A-^AoBjB^
S^Sj, could have been the sire of these
lambs.
In case of correct record the lamb can
only be explained by either a mutation of
Ax to A2 or by some kind of an inter-
action between the gene for white and
rare unknown genes presumably obtained
from both parents and therefore recessive.
As the lambs could be produced by an-
other sire on the same farm in the same
year that seems a more likely explanation.
Mismothering is also a possibility.
The second lamb was born on farm No.
5. Date of mating 12/1/1956, date of lamb-
ing unknown. One lamb, a brown female
with white markings.
The sire, No. 01509, genotype A^A^ —
----- —, was extremely well tested, and this
was the only exception among liis jiro-
geny.
The dam, No. 05116, brown, genotyjre
AgAgB^B^S^S^, had altogether 10 white
lambs out of other matings to homozvgous
white sires, and further one black lamb
when mated to a white sire of genotype
AjAgB^BjSjS^. The probability that she
carried a dominant gene which sujrpressed
white colour was therefore extremely low.
The brown ewe lamb born in 1956 was
kept for breeding. She had altogether 6
lambs wlien mated to homozygous white
sires, all of them white. When mated to
a white sire of genotype A^A^BjBo-------
she had twins, one white and the other
darkgrey.
Both dam and daughter therefore seem-
ed to give jrrogeny witli colours that
woukl be expected from ordinary brown
ewes.
Faulty jrarentage and mismothering can
most likely be excluded because no other
sire used on the farm in that year was
known to carry genes for both brown jiig-
ment and white markings.
This case seems therefore to have been
a mutation from Ax to Ag.
As sire No. 01509 was homozygous white
his genotype with respect to pigment and