Íslenskar landbúnaðarrannsóknir - 01.03.1970, Side 69

COLOUR INHERITANCE IN ICELANDIC SHEEP 67
ing white parents has not been pointecl out
previously as far as can hy seen, and no
previous explanation of tliis phenomenon
is therefore available.
It was therefore decided to examine the
present data further in light of this find-
ing and see if an explanation could be
found.
C. EFFECT OF SEX AND TYPE
OF BIRTH OF LAMB
ON SEGREGATION RATIOS
The first step taken was to examine the
connection between sex of lamb and col-
our, irrespective of nrating. For this pur-
pose all single and twin lambs in the
whole material with known sex were
grouped according to whether these were
wliite or nonwhite. Table 28 shows the
number of white and nonwhite lambs for
each sex for singles and twins separately,
together with the proportion of white
lambs in each group. In table 29 are given
X2’s for comparisons of interest among the
proportions of white lambs.
Tables 28 and 29 sliow clearly that the
proportion of white lambs is different for
the two sexes, the males showing a signi-
ficantly liigher proportion of wliite larnbs
than females. This sex difference is similar
for both singles and twins. It is also clear
from the tables that the proportion of
white lambs is significantly higher among
singles than among twins.
Table 30 shows the proportion of males
among nonwhite lambs when they are
classified by colour pattern and sex, and
also when classified according to black and
brown pigment.
Table 31 shows the x2’s resulting from
comparison among the sex ratios in table
30.
Table 31 shows no indication of a signi-
licant difference in sex ratio among non-
white lambs when classified according to
colour pattern, pigment type and type of
birth.
It should be pointed out that the de-
ficiency in progeny carrying the A2-allele
is not related to sex or type of birth of
the progeny, cf. tables 30 and 31. The com-
parison of the sex ratios within lambs
carrying A2 with homozygous A5A5 lambs
shows no significance, neither within nor
between types of birth. The deficiency ol
progeny carrying allele A2 as shown in
tables 18, 19 and 25 is therefore independ-
ent of sex and type of birth of the pro-
geny.
D. EFFECT OF COLOUR
ON FFRTILITY
The high proportion of nonwhite lambs
among twins compared with singles as
shown in table 28 could arise from a variety
of environmental causes. The nonwhite
ewes in the data collection could for ex-
ample be found mainly in flocks with higli
standards of feeding or there might be a
different age distribution of white and
nonwhite ewes. Changes in feeding prac-
tices over the periotl of data collection
might also have been concomitant with
changes in the proportion of nonwhite
ewes in the material.
In order to examine the possible sources
of variation in fertility the characters
“nurnber of lambs per ewe lambing” ancl
“number of lambs per mating” were sub-
jected to an analysis of variance. Tlie
method of fitting constant (Goulden, 1952,
Harvey, 1960) was used with the modifica-
tion that the generalised inverse of the de-
sign matrix was calculated so that impos-
ing restrictions on the simultaneous equa-
tions before inversion of the rnatrix was
unnecessary (Searle, 1966). In order to re-
duce the size of the matrix when a large
number of equations was being solved the
technique of absorbing a set of equations