Íslenskar landbúnaðarrannsóknir - 01.03.1970, Side 78

Íslenskar landbúnaðarrannsóknir - 01.03.1970, Side 78
76 ÍSLENZKAR LANDBÚNAÐARRANNSOKNIR on number of lambs per mating in two year olcl ewes. This effect seems to be connected with the presence or absence of the A^-allele only, because no difference is found be- tween homozygous and heterozygous white sires. As this effect is only found in two year old ewes and not in 3 and 4 year old ewes, it could be argued that this is only a chance deviation which could be expect- ed to turn up when several tests are carried out. The results obtained by Edgar (1962), on the other hand could possibly be taken as a support for the view that the ranr effect found was a genuine one and would be expected to manifest itself more clearly in two year old ewes than in older ewes. Edgar found that the time taken for the ova to reach the uterus was shorter in two year old ewes than in older ewes and he suggested that this early passage into the uterus might endanger the life of the ov- um. Tables 34—37 sliow that the effect of the genotype of the ewe on nurnber of lambs per rnating can be attributed wholly to the presence or absence of the Aj-allele. There is no detectable difference between the white ewes of unknown heterozygosity and the known white heterozygotes, so the ef- fect does not seem to increase with horno- zygosity for Ax. The effect of the A^-allele is highly significant for the tlnee age groups exa- mined, and in no case can any interaction between colour genotype and farm-years be demonstrated, for number of lambs per mating. This indicates tliat the interaction found for number of lambs per ewe lamb- ing in tables 33 and 35 has mainly been a scale effect. In no case has an interaction between genotype of ewe and genotype of ranr been significant. The results obtained in tables 32—37 are in good agreement with the fact that the proportion of nonwhite lambs in table 28 was founcl to be higher among twins than singles. The fertility has been shown to be appreciably higher arnong nonwhite ewes than among white ewes. This results in a higher proportion of twins among non- white ewes, and as nonwhite ewes also have a higher proportion of nonwhite lambs the result will obviously be a higher proportion of nonwhite lambs among twins than singles. This does not necessarily mean that no other factors are behind the differences found in table 28. E. EFFECT OF COLOUR GENOTYPE OF PARENTS ON LAMB SURVIVAL Information was available on lamb sur- vival up to the age of 6—8 weeks for 3047 lambs out of known genotypes with re- spect to the A-locus. The dead lambs were classified as having either been born dead or died at birth, died between birth and shearing, or survived until shearing time. The number of lambs in each of these three categories are shown in table 38, where they are also classified according to the genotype of the parents with respect to white colour. A comparison among the individual mat- ing groups under the assumption of the same distribution on survival groups gives a x26 = 4-729 (0.70 > P > 0.50). There is thus no indication of any effect of the At- allele on the survival rate of lambs up to 6—8 weeks of age. F. SEX AND COLOUR RATIOS AMONG SINGLES AND TWINS IN KNOWN MATINGS As was shown in table 28 there was a significantly higher proportion of white lambs among males than females. At the same time it was shown in table 26 that there was a significant difference between segregation ratios in the reciprocal matings
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