Íslenskar landbúnaðarrannsóknir - 01.03.1970, Blaðsíða 117

COLOUR INHERITANCE IN ICELANDIC SHEEP 115
white markings is in close agreement with
the hypothesis that white markings in sheep
are partly due to delayed melanoblast
migration. Delayed migration would lead
to absence of meianocytes in the early form-
ed follicles, and only follicles forrned after
the melanoblasts had reached the affected
areas would be expected to produce pig-
rnent.
Some other process must also be assumed
to be at work simultaneously with the de-
layed migration. If delayed migration only
was responsible the animals developing the
dark spots in the undercoat in white areas
would be expected to have an evenly dark
undercoat in affected areas. It seems there-
fore likely that the number of melanoblasts
in affected areas is reduced and that the
late arriving, scattered melanoblasts are too
few to give a completely pigmented under-
coat, the result being dark spots scattered
through the undercoat in the white area.
It is noticeable that the dark spots in the
undercoat are usually denser in phenotypes
with small amount of white than in ani-
mals with extensive white areas.
SUMMARY
A short description is given of the origin
of the Icelandic breed of slieep. Sheep hus-
bandry practices are described with parti-
cular reference to the mating season and
the lambing tirne and identification of
individual sheep. Possible sources of errors
in the mating and lambing records are
discussed (Chapter I).
Three types of pigment are found in tlie
Icelandic sheep, i.e. tan (rust-red), black
and brown. White sheep are either pig-
mentfree or show tan pigment. Nonwhite
sheep show either black or brown pigment,
sometimes accompanied by some tan pig-
ment. Four colour patterns are known in
nonwhite sheep: grey, badgerface, mouflon
and grey mouílon. Nonwhite slieep with-
out pattern are either black or brown. The
patterns manifest themselves on back-
ground of either black or brown pigment.
Altogether 16 main colours are possible
in nonwhite sheep in addition to white, so
17 main colours can be expected in the
Icelandic sheep. Sixteen of these colours
have been observed, and a detailed de-
scrijrtion is given of each of these.
White markings in nonwliite sheejr mani-
fest themselves independently of jrigment
type and jrattern. There is great variation
in the extent of white markings. A 4 digit
numerical code for colour descrijition is
described, where the first two digits refer
to the rnain colour and the last two refer
to the extent of white markings. A scoring
system with scores 1—5 was used to describe
the amount of tan colour in white lambs
at birth, while scores 1—7 were given for
varying amount of dark fibres in grey lambs
at birth.
Reference is given to colour descriptions
in other studies where it is reasonably cer-
tain tliat tlie colours are the sarne as in the
present study (Chapter II).
Several experiments, aimed at revealing
the mode of inheritance of the colours are
described. The results from the initial ex-
periments formed tlie basis for the follow-
ing hyjrothesis about the inheritance.
Genes affecting colour are found at 3
loci A, B and S.
Two jrigment types are jrroduced at the
B-locus, black pigment by a dominant all-
ele, B,, and brown pigment by a recessive
allele, B2.
At the A-locus, the 6 following alleles
are found.
The dominance relationship between A2,
A3, A4 and A6 is such that inhibition of
pigrnent dominates over pigment produc-
tion.
Two alleles are found at the S-locus, S4,
dominant, for absence of white markings