Íslenskar landbúnaðarrannsóknir - 01.03.1970, Síða 65

COLOUR INHERITANCE IN ICELANDIC SHEEP 63
type of mating only gave progeny born in
1961, one might have assumed that the
accuracy of colour description had been in-
creasing gradually and had reached per-
fection in 1961. The results in table 23
indicate strongly that the cliscrimination
between grey ancl black has been equally
accurate over the whole period of data
collection.
Another possible explanation of the
deficiency of grey progeny would be that
certain rams had eitlier been mistakenly
classified as grey when they were actually
black, or that some of the grey rams carried
a deleterious gene closely linked with the
A2-allele which reduced the embryonic
survival of grey progeny.
Under both those hypotheses one would
expect a significant difference between
sires witli respect to tlie segregation ratio.
Table 24 gives the results from tlie A2A-
X A5A5 matings grouped according to
sires.
The two heterogeneity x2’s in table 24
are not significant. This is naturally no
proof that there is no real tlifference be-
tween sires witli respect to segregation
ratios, particularly because the more reli-
able heterogeneity ^2 is rather large. If on
tlie other hand tlie deficiency of grey lambs
was due to sire differences only, these
would have been expected to sliow up
more clearly in the above test.
A repeated assessment of the behaviour
of the A2-allele on independent data would
make possible a final conclusion with re-
spect to the significance of the deficiency
of the A2-allele in certain matings.
No independent data are available where
the genotypes of both parents are known
prior to the mating. But bearing in mind
that all the grey sires used before 1960
were darkgrey, and that all the homozyg-
ous grey sires usecl in 1960/1961 were light-
grey, the darkgrey rarns could all be assuni-
led to be heterozygous grey, of genotype
A2A5. Several of the darkgrey rams were
tested for heterozygosity in their first year
of use and they were all sliown to be of
genotype A2A5. The progeny from the test-
year of each sire is not included in table 9,
so the results from these test-year matings
can be used as an independent source of
segregation information with respect to the
A2 Ab-genotype among sires. l'he results of
the matings of these rams in their test-year
to ewes of known genotypes are shown in
table 25. Also included in the table are
the results from table 9 for the same mat-
ings. Matings with less than 10 progeny
have been excluded from the table, in
order to obtain x2’s of reasonable relia-
bility for comparison of the two data sets.
The comparisons shown in table 25 make
it clear that the data from the test-year are
in agreement with the data from the
known matings. None of the heterogeneity
X2’s between sets are significant and the
sum of the 4 heterogeneity ^‘-’s between
sets is not significant either (x24 = 5.853;
0.50 > P > 0.30). The A2A5 dams give a
higldy significant deficiency of grey lambs
in the test-year, while the results from the
other ewe groups in the test-year do not
differ significantly from expectation. The
sum of both sets of data for the 3 last ewe
groups in the table together shows, how-
ever, a significant deviation from expecta-
tion, and there is a good agreement be-
tween both sets with respect to the sum
of the 3 ewe groups.
The conclusion derived from tliis addi-
tional information must therefore be that
there is a significant deficiency of progeny
showing the A2-allele in matings where
this allele is segregating.
B. SUBNORMAL VARIATION
IN SEGREGATION RATIOS
It is clear from tables 18—25 that esti-
mates of the allele parameters a1( b^ and