Íslenskar landbúnaðarrannsóknir - 01.03.1970, Blaðsíða 90

88 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
TABLE 49
Observed frequencies of progeny with score for grey colour by mating groups
and score classes
Mating group Ratio of hom. : het. Score class
a 1 b c Sum
i 1 : 0 25 ii 2 38
2 1 : 1 22 ii 29 62
3 1 : 2 6 8 8 22
4 0 : 1 1 42 179 222
Sum | 54 72 218 344
grey lambs are very frequent in the lighter
coloured classes, 68.1 per cent of them
occurring in classes 1 and 2 and 26.7 per
cent in classes B, 4 and 5, while only 5.2
per cent are expected to occur in score
classes 6 and 7. The heterozygotes show a
completely different picture, with less than
1 per cent expected in score classes 1 and
2, 19.3 per cent in score classes B, 4 and 5
ancl 80.2 per cent expected in the dark-
grey score classes 6 and 7. The difference
between the proportion of homozygotes
and heterozygotes in score class a is highly
significant. It can also be seen froni table
51 that the probability that a heterozygous
grey lamb will be found with scores 1 or 2
for grey colour is very low. Lambs having
scores 3, 4 and 5 are about equally divided
arnong homozygotes and heterozygotes,
while lambs with scores 6 or 7 will very
rarely be found to be homozygous.
When the expected values in the score
and mating classes in table 50 are calculat-
ed by inserting the values of the parameters
in table 51, a comparison of the expected
values with those observed in table 49
yields a x24 = 4.889 (0.30 > P > 0.20). The
estimated parameters have thus improvecl
the agreement between observed and ex-
pected frequencies to such an extent that
tlie remaining variation between mating
classes is nonsignificant. The variation in
score distribution between mating groups
can thus wholly be attributed to the differ-
ence in distribution of homozygotes and
heterozygotes on score classes.
The fact that the heterozygous grey
lambs are predominantly darkgrey and the
homozygous lambs mainly lightgrey raises
an interesting point in connection with the
dominance of the A2-allele over the A5-
allele. It is conceivable that if an allele
with the same effect as A2 existed in a pre-
dominantly hairy animal this allele would
hardly have any detectable effect in its
heterozygous state due to lack of undercoat,
and would be regarded as recessive under
such conditions.
The increased lightening of the colour
in homozygotes woulcl make the effect de-
tectable, and the allele would be regarded
a recessive. The presence of the undercoat
in the mixed wool type sheep provides
material for the allele to act upon, so that
its presence in heterozygous form can be
detected and hence the allele must be re-
garded as dominant over Ag.
As shown earlier, p. 63, there was a