a1 Agricultural Research Council's Poultry Research Centre, West Mains Road, Edinburgh, 9
Two mosaic birds are described, from a sex-linked F1 cross (Rhode Island Red ♂ × Light Sussex ♀). They are sexually normal males and full brothers. Each has the following abnormalities on one side of the body: absence of the maternal dominants S (silver, sex-linked), W (white skin) and Co (Columbian restriction of black), and reduction in size. In one case all the melanocytes appear to be derived from the smaller side, so that the plumage pattern is identical on the two sides; in the other there is a minor deviation (modified expression of Co) from normal F1 plumage pattern on the larger side. Mated to Brown Leghorn hens, both mosaics bred as normal F1 males, with no evidence of germinal mosaicism. Over thirty full sibs were all found to be normal.
A purely paternal origin of the smaller side appears to be excluded in both cases by the lack of striping in the down, and by minor deviations from normal Brown Leghorn phenotype in adult plumage pattern. Origin by some kind of maldistribution of chromosomes at first cleavage division would require the X-chromosome and possibly as many as three autosomes to be affected simultaneously.
Two further unrelated crossbred mosaic males are described. Each has yellow skin (w) on one side of the body. In one case this is accompanied by a slight difference in plumage pattern, reduction in size and abnormal proportions in the limb bones; in the other merely by a slight reduction in size. Loss or non-disjunction of a single chromosome at first cleavage division is a possible explanation of these.
(Received October 28 1959)