Íslenskar landbúnaðarrannsóknir - 01.03.1970, Blaðsíða 94

92 ÍSLENZKAR LANDBÚNAÐARRANNSOKNIR
at a given locus might thus show a
specific affinity to each other and re-
pulsion towards the two chromatids carry-
ing a different allele at the same iocus.
This might lead to active double nondis-
junction which would result in somatic
segregation without tetraploidy. The active
retention of the A5-allele of an A2A5-ovum
by A5-carrying sperm could be assumed to
operate in a comparable way, the A5-carry-
ing chromosome of the sperm attracting
the A5-carrying chromatid of the egg and
showing repulsion towards the A2-carrying
clnomatid.
It is of interest to note in this connec-
tion that nonwhite spots in white sheep
are relatively frequent. If the nonwhite
spots were to be regarded as arising from
mutations, one has to assume an unusually
liigh mutation frequency in order to ac-
count for the nonwhite spots according to
Brooker and Dolling (1965). The sanre
authors found higher frequency of black
spot aninrals arnong known white hetero-
zygotes than among the remainder of the
population.
More recent results by the same authors
(Brooker and Dolltng, 1969 b) on Piebald
Merinos show that nrany of their Piebald
ranrs have been honrozygous wlrite. They
conclude that heterozygosity for the gene
for white does not affect the occurrence of
black spots. On the other hand, one Pie-
bald ram known to be heterozygous for the
gene for white nrated to heterozygous white
ewes gave significantly nrore Piebald off-
spring than the Piebald ranrs lronrozygous
for the gene for white, when they were
nrated to nonwhite ewes.
The lack of distinction between tlre dif-
ferent recessive A-alleles carried by the wlrite
lreterozygotes in their study does not ex-
clude tlre possibility that a relation exists
between black spots and the A^Aj-geno-
type.
Somatic segregation in the follicle fornr-
ing tissue of AxA5-genotype would be ex-
pected to result in black spots. Thus it
seems possible that a connection might
exist between selective retention of the A5-
allele in the A2A5-egg and the frequent
black spots in white A^A^-sheep, botlr
phenonrena being related to a particular
effect of the A5-allele. It nrust be stressed,
however, that tlris type of connection is
highly speculative and the correct explana-
tion of the mechanism leading to selective
retention of a chromatid may be of a conr-
pletely different nature.
K. GENE FREOUENCIES
AT THE A-LOCUS
The data in the present study have not
been collected as a randonr sanrple of colour
data fronr the Icelandic sheep population.
Calculation of gene frequencies fronr the
data on the whole would therefore not give
unbiassed estinrates of the true frequencies
of the different genes in question. Matings
in the present matérial are also known to
deviate widely fronr randonrness with re-
spect to colour, so any nrethod of estinra-
tion which is based on the assumption that
the nratings are randonr could not be appli-
ed to this material.
It was felt, however, tlrat one might ob-
tain a picture of one aspect of gene frequ-
encies in the present material which would
not be appreciably biassed. This consists
of counting tlre progeny of each colour
obtained from white ewes when matecl to
A5A5-sires. The results of this count are
shown in table 53, where progeny showing
both black and brown pigment have been
added for each allele.
As table 53 shows, 67.9 per cent of the
lanrbs out of the mating in question have
been white and the remaining 32.1 per cent
nonwlrite. Under assumption of random
segregation, the heterozygous white ewes