Íslenskar landbúnaðarrannsóknir - 01.03.1970, Síða 113

COLOUR INHERITANCE IN ICELANDIC SHEEP 111
action of modil'iers, and wliite heacl spot
in animals heterozygous for white markings
by incomplete dominance of the dominant
allele for full pigmentation.
If the above assumptions were valid,
rnost of the discrepancies between earlier
investigations and the present study could
be explained.
CHAPTER IX
Gene action at colour loci in sheep
and their homology with colour loci in rodents
A. THE A-LOCUS
The patterns produced by the alleles at the
A-locus are a resuit of the inhibition of
production of black and brown pigment in
either certain parts of the body or in cer-
tain types of follicles or both.
The A-alleles manifest themselves in
presence of both black and brown pigment,
and the pigment type is easily detectable
in presence of all the A-allefes except Ax.
It will therefore be assumed that the black
and brown pigment types are eumelanin
pigments, similar to those found at the
B-Iocus in the tnouse.
The possibility that the brown (choco-
late) sheep colour reierred to in the present
work is in reality a dark modification of
phaeomelanin pigment and not eumelanin
must be rejected for the following reason.
A recessive gene, e, is well known at the
extension locus in guinea pigs and in the
black rat which results in production of
phaeomefanin onfy in ee-animals. This
gene, however, when homozygous, masks
completely the effect of the alleles at the
agouti locus, while the brown colour found
in the Icelandic sheep allows the expres-
sion of all the lower A-alfeles (for references
see Searle, f968).
It may be assumed with some degree of
confidence that the tan pigment found in
some white Iceiandic sheep is phaeomelan-
in. This assumption is partly based on the
phenotypic similarity between tan colour
in sheep and yellow in mice. Both colours
occur as a result of the action of the top
dominant allele at wliat must be regarded
as the same locus.
An important point in connection with
the relationship between white colour in
sheep and yellow colour in niice is the well
known lack of tan colour in most white
wool producing sheep breeds of the world.
In that connection it is worth noting that
a recessive fading gene, f, is known in
guinea pigs which in homozygotes results
in complete fading of all phaeomelanin,
while eumelanin is unaffected. The inter-
mecliate alleles in the albino series also
sometimes affect phaeomelanin pigment to
a greater extent than eumelanin (Searle,
1968). It is also likely that several minor
modifying genes affect the occurrence of
tan pigment in sheep. In wool producing
breeds selection against pigmented wool
has been carried out for a very fong time.
The lowered amount of tan pigment in
homozygous wliite animafs compared with
lieterozygotes, found in the present study,
would afso tend to lower the amount of
tan colour in white, wool producing sheep.
It is furthermore likely that the tan pig-
ment decreases with decreasing fibre dia-
meter of the wool. In a sefection experi-
ment for and againsl hairy birthcoat in