Alan Cock

Half-and-half mosaics in the Fowl

Summary1.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).

The interpretation of autosexing

Summary1.Autosexing breeds of fowl may be divided into those which carry the sex-linked gene for barring (B), and those which do not. In the former group, the autosexing properties are due to the imperfect dominance of B.2.A table (Table 2) is presented showing the possible dominance relations between a sex-linked gene which is a hypomorph, hypermorph, antimorph, amorph or neomorph, and its wild-type allele, both without dosage compensation, and in conditions of partial and full dosage compensation.3.A study of the ‘ticks’occurring on birds having various constitutions for the two sex-linked loci B and S silver) shows that the ticks are due to somatic elimination of part of an X-chromosome. It shows further that B is a neomorph (b having no action on pigmentation) without dosage compensation, or possibly a hypomorph (b having a very weak action in the same direction as B) with, at most, a slight degree of dosage compensation. This in itself is sufficient to explain the autosexing properties of barred breeds, without resorting to the improbable assumption that females have a Y-chromosome which carries b.4.Anomalies are pointed out in the dominance relations at several sex-linked loci (w, A, B, Hw) ofDrosophila melanogaster, which are difficult to explain on Muller’s theory of dosage compensation.5.There is no reason to attribute any autosexing effect to either allele at the silver locus of the fowl. There is some evidence to suggest that the sex hormones, rather than a specific sex-linked gene with a dosage effect similar to that of B, are responsible for autosexing in non-barred breeds. On whether the testicular or the ovarian hormone is responsible, the available evidence is conflicting.

A case of incomplete twinning in the rabbit.

Summary1.A monstrous Dutch rabbit with a single head and a double thorax and abdomen is described, and its probable mode of origin discussed. It is concluded that while it is primarily a result of incomplete twinning, a considerable degree of secondary fusion has taken place.2.The two members of the monster have different pigmentation patterns; it is suggested that this is due to the same (unknown) causes as the asymmetrical markings of ‘mismarked’ Dutch rabbits.3.An attempt has been made to obtain statistical evidence that normal monozygotic twins occur in rabbits. The results were inconclusive; while not giving any evidence in favour of their occurrence, they provide no reason for supposing that they are particularly rare.