Íslenskar landbúnaðarrannsóknir - 01.03.1970, Blaðsíða 93

COLOUR INHERITANCE IN ICELANDIC SHEEP 91
TABLE 52
Zygotic probabilities in rnatings A2A5 X A2A5, A2A5 X A5A5 and
A5A5 X A2A5 as functions of p and q
Allele from j Mating
Dame Sire A2A5 x A2A5 A2A5 x A5A5 A5A5 x A^As
Ao A2 0.5p
A5 0.5p p
a5 A2 (i-p)q q
A5 A5 (i-p)O-q) i-p 1-q
Genotype of prog.: A2A2 and A2A5 p + q - pq p q
A5 ^5 1 - p - q + pq i-p l-q
The three matings above all gave liigh
X2 values when the observed segregation
was compared with expectation as shown
on p. 60.
These three x2’s added together give
X23 = 10.038 (0.02 > P > 0.01). '
A comparison of the observed segregation
ratios with those expected from the prob-
abilities in the bottom part of table 52 and
using the estimated maximum likelihood
values for p and q, leads to x2i = 0.039
(0.90 > P > 0.80). The two parameters, p
and q tlius account for all the abnormal
variation in those three matings, and the
X22 = 10.038-0.039 = 9.999 (P<0.01)
shows that the combined effect of the two
parameters is highly signilicant.
The conclusion from the above estima-
tion must lie that when sperm containing
alleles A2 and A5 compete for fertilization
of an ovum, the sperm carrying the A5-
allele will íertilize a significantly higher
proportion of the ova than the sperm carry-
ing the A2-allele, ancl further that the A5-
carrying sperm will retain the chromatid
carrying the A5-allele of the ovum signi-
ficantly more frequently than that carrying
the A2-allele. This conclusion is only part-
ly comparable to that reached by Bateman
(1960), who found selective fertilization
due to differential sperm transport operat-
ing in connection witli the T-locus in mice,
while his data showed selective penetration
of eggs of different genotype, and not
selective retention of different chromatids.
Selective retention of chromatids can
only occur if the locus in question is at
some distance from the centromere so that
crossing over has occurred, resulting in a
heterozygous ovum at the second matura-
tion division.
It is unknown by what kind of mechan-
ism selective retention of a chromatid carry-
ing a particular allele in the ovum could
be produced by a given allele introduced
by the spermatozoon. The phenomenon,
however, rnight possibly be viewed in the
light of somatic segregation Somatic segre-
gation seems at least partly to be a
function of the particular genes carried
by the autosomes involved (Ohno, 1966).
Two chromatids carrying a certain allele