Íslenskar landbúnaðarrannsóknir - 01.03.1970, Síða 119

COLOUR INHERITANCE IN ICELANDIC SHEEP 117
colour of ewe on fertility, white ewes of 2,
3 and 4 years of age giving 0.132, 0.176 and
0.134 lambs less per mating, respectively,
than nonwhite ewes of the same age groups.
Nurnber of lambs per ewe lambing was
also highly significantly lower for white
than nonwhite ewes among 2 year old ewes.
No difference in fertility could be demon-
strated between heterozygous and homo-
zygous white ewes, and no such difference
was found between nonwhite ewes of dif-
ferent genotypes either. Grey ewes showed
comparable fertility to other nonwhite
ewes, which indicates that embryonic mor-
tality is not responsible for the deficiency
of the A2-allele.
A significant effect of colour of ram on
number of lambs per mating was found for
2 year old ewes (P < 0.05), white rams giv-
ing 0.080 lambs less per mating than non-
white rams. No such effect was found for
3 and 4 year old ewes. No interaction was
found between colour of ewe ancl colour
of ram. No effect was found of colour of
parents on survival of lambs up to 6—8
weeks of age.
When singles and twins frorn known mat-
ings were grouped by sex and colour (white
or nonwhite) a significant deviation from
expectation was found within several of the
matings. The deviations were found among
both white and nonwliite ewe groups.
Three parameters attributing the variation
to selective embryonic mortality were
estimated from matings involving genotypes
with all combinations of alleles A4 and Ag,
and were all found to affect the segregation
ratios significantly. The parameters were:
UT = advantage of sperm carrying allele At
over sperm carrying allele Ag (P<0.01); vT
= advantage of sperm carrying allele Ag
and tlie X-chromosome over other spernr
in matings of white ewes (P<0.01); and s
= excess mortality of white embryos in
white ewes (P < 0.05). The parameters uT
and vT were significant for twins only while
S was estimated as a common parameter
for both singles and twins.
Homozygous white lambs were shown to
have significantly less amount of tan colour
on the body than heterozygous white lambs
(P < 0.05).
Hogozygous grey lambs were shown to
be predominantly lightgrey with scores 1
and 2 while heterozygous grey lambs were
predominantly darkgrey with scores 6 and
7. No indication was found of a disturbed
ratio between homozygous and heterozyg-
ous grey lambs.
Frorn the matings A2A5 X A2A5,
A2A5 X A5 A5 and A5A5 X A2A5 two
parameters, p and q, for selective fertiliza-
tion were estimated; p, the probability of
fertilization by sperm carrying the allele
A2, was found to be 0.408 ±0.041, and q,
the probability of retention of allele A2 in
an A2A3 egg fertilized by sperm carrying
allele A5, was found to be 0.42 ± 0.041.
The two parameters accounted for a highly
significant amount of the variation in the
above matings (x22 = 9.999; P<0.01), the
remaining variation being nonsignificant
(X2! = 0.039; 0.90 > P > 0.80).
Estimates of gene frequencies at the A-
locus among 547 white ewes mated to
AgAg-rams were for alleles Ax, A2, A3, A4,
Ag ancl A6, respectively: 0.679 ± 0.020;
0.064 ± 0.010; 0.007 ± 0.004; 0.219 ± 0.018;
and 0.000 (Chapter VI).
Segregation ratios at the B-locus were
found to agree well with expectation, and
no exceptions from expected colours were
found.
Segregation ratios at the S-locus were also
found to agree well with expectation. One
lamb without white markings was obtained
where white markings were expected.
No effect was found of alleles at loci B
and S on fertility.
Data on linkage between loci A, B and
S were limited but showed no evidence of
linkage (Chapter VII).