Íslenskar landbúnaðarrannsóknir - 01.03.1970, Page 97

COLOUR INHERITANCE IN ICELANDIC SHEEP 95
fay their assumed breeding value for other
characteristics.
It must finally be mentioned that on
the assumption that selective fertilization
and selective mortality are operating as
indicated by table 44, the calculated gene
frequencies in table 53 are biassed.
CHAPTER VII
Analysis of segregation of loci B and S,
and linkage between colour loci
A. SEGREGATION AT THE B-LOCUS
Table 55 shows the results from matings
of both parents with known genotypes at
the B-locus classified by genotypes of par-
ents and sex of lambs, anci table 56 shows
a comparison of observecl with expected
frequencies in matings showing segrega-
tion.
As table 55 and 56 show the segregation
results are in excellent agreement with the
assumption that black and brown pigment
are produced by two allelic genes, Bx and
Bo, and Bx being dominant to B2. There
is no difference between reciprocal matings
and no difference between sexes of progeny.
It is also noticeable that no detectable ex-
ceptions from expectation have occurred.
The classification of the pigment types has
been done correctly as far as can be check-
ed.
B. SEGREGATION AT THE S-LOCUS
Table 57 shows the results from matings
with both genotypes of parents known at
locus S prior to mating. In the table tlie
progeny have been grouped into two classes
only, those showing no white markings and
those showing white markings.
The results of table 57 agree well with
the hypothesis that white markings are
caused by a single, recessive gene S2, in
homozygous condition. The lack of pro-
geny showing white markings in matings
with both parents lieterozygous for white
markings must be ascribed to chance fluc-
tuation, the probability of all 8 progeny
showing absence of white markings being
0.100. As several lambs with white mark-
ings were obtained from matings where
both parents were without white markings
and of unknown genotype previously, there
is no doubt that the character is recessive.
The lamb without white markings out
of parents showing white markings in table
57 was born on farm No. 1. Date of mating
25/12/1960, date oí lambing 18/5/1961.
Twins, a brown male witli white markings,
colour 0926, and a greybrown female,
colour 0600. Sire No. 01080, brown with
white markings, colour 0981 genotype
A5A5B2B2S2S2. Dam No. 01823, grey-
brown with white markings, colour 0662,
genotype A2 A5B2B2S2S2. Faulty parentage
in this case is excluded. The most likely
explanation is that the extent of the white
markings is affected by a number of niodi-
fying genes and by chance variation, and
that this effect has led to a complete ab-
sence of white markings in this particular
lamb. The sire showed white markings
only on liead and feet and gave several
progeny showing only a very limited
amount of white markings.
The mating SjS2 X S2S2 has altogether