Íslenskar landbúnaðarrannsóknir - 01.03.1977, Síða 82

80 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
TABLE 3. MAXIMUM LIKELIHOOD ESTIMATES 07 PROPORTIONS OF EWES WITH
2 (jj), 1 (fl.) and 0 (1-E-a) EGGS FERTILIZED, TOGETHER WITH
EXCESS MORTALITY RATE, s, OF EMBRYOS SIRED BY ABNORMAL RAMS.
Ram Parameters estimated ± s.e. X1
£ a (l-£-£) S
Grettir 215 0.53±0.04 0.40±0,04 0.07 ±0.05 0,5 0 ± 0.0 8 0.11
Draupnir 0.58 ±0.03 0.34±0.02 0.08±0.03 0.41±0.04 2.00
Durgur Q.51±0.04 0.4 2 ±0.04 0.07 ±0.04 0.53 ±0.08 3.17
respective comparison groups, both with
respect to twinning rate and return rate.
The estimates of p, q and s together with
their standard errors are shown in table 3.
The ram Bolli 236 son of Steinka 6600,
was mated to 5 ewes during the winter
1975/76. All the ewes conceived at the
first mating, and 4 gave birth to twins and
one gave birth to a single. Of the 9 lambs
born, 8 were males and 1 female. This sex
ratio differs significantly from a 1:1 ratio
(P=:0.02).
The sex ratio among the lambs sired by
Grettir 215, Draupnir and Durgur was close
to expectation.
The ram Loki 245, son of Steinka 6600,
born in 1976, was unable to mate during
the mating season 1976/77.
DISCUSSION.
The fertility depression described here
shows some remarkable features. The effect
of the abnormal rams on the fertility of
the ewes they serve is dramatic and consis-
tent from one year to the next. This is
therefore a completely different effect from
that described by Parker (1969) who found
evidence of a ram effect of a temporary
nature on ewe fertib'ty.
The ram effect in the present investigation
is reflected in increased return rate of ewes
on heat on one hand and lowered twinning
rate on the other hand. This phenomenon is
consistent with increased embryonic mortality,
but could conceivably álso be the result of
an unusually low fertilizing capacity of the
spermatozoon from the abnormal rams. The
fact that the abnormality occurs in both
Draupnir and his son Durgur, and the ewe
Steinka 6600 and three of its sons show
abnormal reproduction pattern indicate cle-
arly the heriditary nature of the trait.
The three estimates of embryonic mortality
are close to 50 per cent, implying that every
second embryo has been lost before or during
implantation. This in turn is consistent with
heterozygosity in the rams for a dominant
lethal. The only known type of an inherited
dominant lethal with full penetrance is a
balanced karyotype of a translocation hetero-
zygote.
Translocation heterozygotes in New Zea-
land have been reported to show normal
fertility in most cases (Buére, 1974a), but
in some cases a deficiency of female lambs
has been observed among the progeny of
rams which are heterozygous translocation
carriers (Chapman and Bruére, 1975).
The deficiency of female lambs among