Íslenskar landbúnaðarrannsóknir - 01.03.1970, Síða 115

COLOUR INHERITANCE IN ICELANDIC SHEEP 1 13
TABLE 61
Effect of alleles A-^—Ag on pigment production by body regions and follicle types.
-)- = presence, — = absence, of black and brown pigment.
Allele Back Belly
Primaries Secondaries Primaries Secondaries
Ai _
Ae + — - —
A2 + — + —
A3 — + + +
A4 + + —
Ab + + + +
pigment inhibition in certain follicle types.
(For references in connection with follicle
types in sheep see Ryder and Stephenson,
1968). This difference between the species
may well be due to difference in follicle
morphology rather than to difference in
the action of the alleles. It is nevertlieless
of considerable interest, particularly in
view of the other main feature of some of
the A-alleles which is common to both
species, i.e. their differential effect on the
dorsal and the ventral part of the animal.
It is well established that the hair follicles
in yellow mice and in the yellow areas in
black and tan mice contain melanocytes
which are capable of producing eumelanin
in non-yellow areas. In the yellow areas the
eumelanin production of these melanocytes
is inhibited, however, so that they can only
produce phaeomelanin. (For references see
Foster, 1965 ancl Searle, 1968).
It seems logical to assume that the in-
hibition of production of black and brown
pigment in certain follicle types in sheep
is of the same nature as the inhibition of
formation of black and brown pigment in
certain body regions in slieep and mice.
The effects of the 6 alleles found at the
A-locus in sheep can then be describecl in
terms of their inhibiting effect as shown in
table 61, where -j- means presence and —
means absence of black and brown pig-
ment.
The picture that emerges from table 61
gives certain clues to the possible nature
of the action of tliese alleles, which are not
available from the rnouse series. In black
ancl tan mice the migrating melanoblasts
which originate in the neural crest and
later become the pigment producing me-
lanocytes of the follicles, find an environ-
ment which allows eumelanin production
on tlie dorsal part, but one which inhibits
eumelanin production on the ventral part
of the animal. Tlie inhibiting effect on the
belly is a function of the particular en-
vironment witliin the follicles and seems
to result from the action of the af-allele
either at the time of formation of tlie fol-
licle or its action on the tissue from which
the follicles are formed prior to their
formation. Because only one type of pig-
ment is produced within the area of eume-
lanin inhibition, the a^-allele does not tell
us whether the tissue from which the fol-
licles are formed has been affected irrevers-