Íslenskar landbúnaðarrannsóknir - 01.03.1970, Side 114

112 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
slieep, Purser and Karam (1967) found a
markedly increased proportion of red
kempy fleeces as a result of selection for
kempy birthcoat in the Welsh Mountain
sheep.
Tan colour occurs frequently among
homozygous grey Icelandic sheep, and the
outer coat of the badgerface sheep often
shows an extensive amount of tan at birth.
This tan colour is the same as the tan
colour found in white sheep, and because
the genes in question are alleles and the
locus seemingly homologous to the agouti
locus in mice, the assumption that the tan
pigment is phaeomelanin seems reasonably
well founded.
It must at the same time be stressed
that as far as can be seen, no chemical
determination is so far available of the tan,
black and brown pigment in sheep. The
above assumptions about homology of pig-
ment types must therefore remain tentative
until the necessary chemical analyses have
been carried out.
The alleles at the agouti locus in the
mouse have several features in common
with those found in sheep, but in some re-
spects the sheep series shows features not
described in the mouse so far.
In both species the top dominant allele
sliows complete dominance over all the
others, and in botli species the top domin-
ant lias a pleiotropic effect.
The lower alleles show the same charact-
eristic dominance relationship in both
species. Inhibition of eumelanin production
dominates over eumelanin production
(Hollander and Gowen, 1956). In both
species alleles are found whicli show the
sum effect of two separate alleles. In sheep
the A6-allele has the sum effect of alleles
A2 and A4, while in the mouse the Aw-
allele has the sum effect of the alleles A
and a‘.
In spite of the many similarities between
the A-series in sheep and mice, the A-alleles
in sheep show some features which have
not been found in mice so far and some of
the features characteristic for the series in
mice are not found in the sheep. The A-
allele in mice has for example not been
found in the Icelandic sheep. The agouti
colouring found by Vasin (1928), which he
claimed to be due to a recessive gene and
which only seemed to be expressed in the
presence of the gene for white, would not
lit into the A-series found in the present
study, because a homozygous gene which is
expressed only when A4 is present cannot
belong to the same allelic series as Ax.
Löfvenberg and Johansson (1952) suggest-
ed that the gene for grey in their study
might be homologous to the agouti gene
in rodents. This does not seem likely in
light of the present study. There is no
visible effect of the A2-allele in heterozyg-
ous form which results in an apical or sub-
apical band in the birthcoat fibres. In
homozygous grey lambs the birthcoat fibres
may have a dark tip followed by an al-
most pigmentfree distal part of the fibre,
but this is a different effect from the band-
ing found in some lambs carrying allele
A4. In those cases of mouflon coloured
lambs where the agouti banding is seen in
the birthcoat, the tip of the fibre may be
dark, followed by a light-brownish band,
which again is followed by a dark colourecl
distal portion, or the tip may be light-
coloured ancl the remainder of the fibre
dark.
The remarkable difference between tlie
heterozygous and homozygous grey lambs
at birth at described earlier, p. 89, shows
also that the A2-allele behaves differently
from the A-allele in mice. If the A2-allele
occurred in mice it would most likely have
only a very slight effect in heterozygous
form.
A feature of the A-alleles in sheep not
found in tlie agouti series in mice so far
is the effect of the alleles on eumelanin