Íslenskar landbúnaðarrannsóknir - 01.03.1970, Page 116

114 ÍSLENZKAR LANDBUNAÐARRANNSÓKNIR
ibly or whether the effect is produced dur-
ing the formation of the follicle only.
l’he effect of the intermediate A-alleles
in sheep shows clearly that the inhibiting
effect within the dorsal or the ventral area
is limited to the period o£ follicle forma-
tion only and is not a permanent feature
of the tissue from which the follicles arise.
The Ag-allele thus affects only the primary
follicles on the upper part of the body,
with the result that they do not produce
black or brown pigment. Secondary follicles
in the same area, which are formed from
the same tissue, are able to produce black
or brown pigment. The same allele is with-
out effect on the belly, so that all follicles
there produce black or brown pigment. The
A6-allele has completely the reverse effect.
The primary follicles on the back and sides
are not affected by the inhibition of this
allele, while all follicles on the belly and
the secondaries on the upper part of the
body have been inhibited frorn production
of black and brown pigment.
A peculiarity in connection with the
action of the A-alleles in sheep is that all
the alleles Alt A2, A3, A4 and A6 seem to
have the same function. Tliey all inhibit
production of black and brown pigment to
some extent. The difference between the
alleles is not due to diíferent function, but
due to different sites o£ function and dif-
ferent tirnes of operation during foetal
development. The Aj-allele acts on the
whole of the animal and during the entire
period of follicle formation. Tlie A3-allele
acts early on the upper part of the body
and then becomes inactive there and is
completely inactive on the belly. The A2-
allele is inactive while the primary follicles
are being formed, but becomes active dur-
ing the formation of the secondary follicles
all over the body, while the A4-allele is
only active on the upper part of the body
during the formation of the banding of
the agouti coloured fibres, but fully active
on the belly, until eventually during the
very late stages of formation of secondary
follicles which may then produce black or
brown pigment.
B. THE B-LOCUS
The action of the alleles at the B-locus in
sheep seems to be closely similar to that
found at the B-locus in mice (Searle, 1968).
It can therefore be assumed with some con-
fidence that the black and lirown pigment
found in the present study is eumelanin.
The B-locus in sheep shows the same mode
of inheritance and similar phenotypic ef-
fect to that found at the B-locus in the
mouse. The locus therefore meets these two
criteria of genetic homology (Searle, 1968).
The action of the alleles at the A-locus on
the pigment produced by the B-locus has
been discussed in connection with the
action of the A-alleles.
C. THE S-LOCUS
This locus seems phenotypically compar-
able to the S-locus in mice, but even more
so to the S-locus in guinea pigs. In mice
the effect of the spotting gene in ss homo-
zygotes seems to be a complete absence of
melanocytes from spotted areas, while in
spotted guinea pigs melanocyte migration
seems to be delayed, so that they have arriv-
ed too late to enter the follicle but are
present in the skin (Searle, 1968). The
situation in sheep definitely indicates pre-
sence of some melanocytes in the skin in
white areas in spotted animals. This is
seen from the development of small, penny-
size, dark spots in the undercoat in wliite
areas in some spotted animals. If the areas,
which are white at birth, contained no
melanocytes, these dark spots would not be
able to develop.
The occurrence of these dark spots in the
undercoat in white areas of sheep showing