1. A half-and-half mosaic pullet (case N) from the cross Rhode Island Red ? x Light Sussex ? is described. The right side of the bird shows all the characters to be expected in a pullet from this cross. The left side is abnormal in the following ways: (i) The shank is yellow instead of white. (ii) The plumage is a darker shade of reddish brown. (iii) The feathers, especially the flight feathers of the wing, have a frayed appearance, due to imperfectly formed barbules. (iv) The whole of the left side of the body is very much smaller than the right, and the limb bones, in particular, are abnormally proportioned. 2. Two females are described which are asymmetrical only in respect of the length of the legs. One of these had originally been regarded as a mosaic, but it now appears that in both cases a copper ring accidentally left too long on the right shank of the young chick hss been the cause of hypertrophy of the right tarsometatarsus and tibiotarsus. 3. The twenty-three known half-and-half mosaic fowls are discussed in relation to the theory of Crew & Munro, which postulates that all such cases are due either to elimination, or to non-disjunction, of a single autosome at first cleavage. The following objections to this theory are pointed out: (i) The data available on the proportions of the limb bones in several cases are inconsistent with the requirements of the theory. (ii) At least five cases, and probably six, are mosaic in respect of a sex-linked character. (iii) Three cases are mosaic in respect of more than one autosomally inherited character (apart from size). In no case do linkage data support the idea that the genes concerned are all situated on the same chromosome, and it is unlikely that this could be true in all cases. (iv) The theory requires unsupported assumptions about the dosage relations between alleles at two loci. (v) The theory assumes that an extra autosome above the normal diploid number will tend to swing the sex in a male direction; this is the reverse of what is to be expected on the sex-chromosome: autosome balance theory of sex determination. 4. An attempt is made to find plausible and consistent explanations for as many as possible of the known cases. These are clearly due to a diversity of causes, and fall into two main types: (i) Those in which somatic segregation has occurred at first cleavage in an otherwise normal zygote. In some, the somatic segregation has taken the form of maldistribution (elimination or non-disjunction) of a single pair of autosomes, or possibly of somatic crossing-over. Simple somatic gene-mutation may account for a few cases. In others (gynandromorphs) elimination of an X-chromosome has occurred. In yet others, multiple irregularities in the distribution of the autosomes seem to have occurred; to this cause are assigned only those cases (of which N is one) which cannot be satisfactorily explained in any other way. (ii) Those originating from a compound zygote; more specifically, in which one side of the body is of purely paternal origin (and diploid), the other side being of normal bisexual origin. This may, or may not, lead to gynandromorphism. 5. The explanations suggested for individual mosaics are all open to some degree of doubt. Certain other difficulties remain, notably the absence of any gynandromorphs genetically female on the right side. There is also some difficulty in reconciling the irregular type of mosaic found in some species of bird (pigeon, turkey) with the bilateral half-and-half characteristic of other species (fowl, finches).
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- 1 - Life Sciences ; 2 - Biochemistry, Genetics and Molecular Biology ; 3 - Genetics