Íslenskar landbúnaðarrannsóknir - 01.03.1970, Page 92

90 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
zygotes and heterozygotes in mating gronps
2 and 3 of table 49 differs from the expect-
ed mixture of 1:1 and 1:2 respectively.
This agreement with expectation indi-
cates strongly tliat the disturbed segrega-
tion of the A2-allele found earlier is not
due to differential mortality, but rather
due to selective fertilization.
J. SELECTIVE FERTILIZATION
INVOLVING ALLELES A2 AND A5
The discrepancy between observed and ex-
pectecl frequencies of the A2-allele as found
earlier, see p. 59—63, can not be ascribed
to incrcased mortality of embryos carrying
the A2-allele, because the fertility of the
ewes of genotype A2A5 in table 32 is not
different from that of other nonwhite ewes.
It has also been shown in connection with
the distribution of homozygous and hetero-
zygous grey lambs on colour scores that the
ratio between homozygous ancl hetero-
zygous grey lambs seems unaffected by the
deficiency of grey lambs. These findings
therefore indicate that the discrepancy is
due to selective fertilization rather than
selective mortality.
As pointed out earlier, no evidence was
found of any difference between the sex
ratio among grey lambs on one hand and
other nonwhite lambs on the other hand.
This indicates that the mechanism behind
the selective fertilization involving the A2-
allele is different from the one involving
alleles Aj and A5.
It is also noticeable that the matings
A2A5 X A5A5 and A5A- X A2A5 both
give a similar deficiency of grey progeny,
and that the mating A2A5 X A2A5 also
gives a marked deficiency of grey progeny,
which is a completely different picture
from that obtained from the AXA5 X A^Ag,
A^Ag X A5A5 and A5A5 X A^Ag-matings.
The three matings involving the A2A5
and A5A5 — genotypes indicate that the
selective fertilization is both active among
the sperm from A2A5-sires and also that
sperm carrying allele A5 favours the re-
tention of the A5 — carrying chromatid of
the fertilized egg, while the A2-allele is
expelled more frequently in the second
polar body at the seconcl maturation divi-
sion (see e.g. Hancock, 1962). Of the 62
lambs with score for grey colour in mating
group 2 of table 49, altogether 56 come
from A2A5-dams mated to sires of geno-
type AjA^, A2A2 and A2A4. As the grey
lambs in this mating group show no devia-
tion from expectation with respect to
distribution on colour s core classes, the
sperm carrying the A2-allele can be regarcl-
ed as nonselective with respect to alleles A2
and A5 in the egg from the A2A5-ewe.
This leads to the definition of the fol-
lowing probabilities:
p = probability of fertilization bv
sperm carrying allele A2
1 — p = probability of fertilization by
sperm carrying allele A5
q = probability of retention of allele
A2 in an A2A5 egg fertilized by
sperm carrying allele A5
1 — cj = probability of retention of allele
A5 in an A2A5 egg fertilized by
sperm carrying allele A5.
The above probabilities lead to the zygo-
tic probabilities shown in table 52.
From the observed segregation within
the above matings shown in table 9, the
maximum likelihood estimates of p and q
can be found by the method of maximum
likelihood scoring as described earlier.
This leads to the following estimates and
standard deviations:
p = 0.408* ± 0.041, ancl
q = 0.420* ± 0.041.
*) Signilicantly different from 0.5 (P < 0.05).