Pigmentation of sheep. III. Piebald pattern in Merinos

Piebald Merino sheep have one or more rounded areas of light grey to black fibres in an otherwise white coat. Spotting was found on all body regions except the tail and there was considerable variation between animals in the size and number of spots. It was usually possible to distinguish between Black and Piebald phenotypically. The pigmented and white areas of a Black animal were usually distributed over the body according to a set pattern while the location of the spots on Piebald sheep did not conform to any recognizable pattern and invariably showed positional asymmetry. One animal which was both Black and Piebald is described. Piebald spotting is not determined by factors at the Black locus; the only exception is a Piebald phenotype produced by a somatic mutation involving this locus. It is most probable that the Piebald pattern in Merinos is controlled by a recessive gene which does not show full penetrance. A possible explanation for this incomplete penetrance is discussed.