THE GENETIC CONTROL OF PATTERN 



199 



that this is really a phenomenon of gene-autarchy, and that the pattern 

 of a bird with, say, black wings and a light body, is really a pattern of 

 something which causes a colour gene to mutate to a black-determining 

 allelomorph in the wing areas. It is known that mutable genes may 

 mutate with particular frequency in some tissues, but Wright's 

 hypothesis is not yet proved and would in fact be difficult to test. It 

 is clear that some irregular spottings are produced in this way (p. 372). 



Fig. 93. The Formation of Bars in Feathers of Fowls. — The curves give the 

 concentration of hormone (e.g. thyroxin) during its absorbtion and excretion 

 after a single large dose. Different parts of the feather have different thresholds 

 below which they show no reaction in pigmentation; the threshold is lowest in 

 the central, axial, region (Threshold 3). Regions of low threshold have, however, 

 a longer latent period before they begin reacting. If the dose is excreted as in 

 the upper curve, pigment is formed in all parts of the feather (as shown by the 

 shaded areas) and gives a transverse bar; but excretion as in the lower curve 

 gives no deposition of pigment in the intermediate regions, and causes the 

 formation an axial and a marginal spot. 



(Modified from Lillie.) 



Transplantation work^ has recently shown that the determination of 

 feather colour in different breeds of fowls takes place even earlier than 

 was previously realized; pieces of skin behave autarchically as regards 

 feather-colour when transplanted between chick embryos of three 

 days incubation. At this time the type of feather (neck, body, hackle, 

 etc.) has not yet been determined, and the feathers appearing in the 

 region of the graft are determined by the region of the host's body 

 and not by the region from which the graft was taken. Probably, how- 

 ever, the actual feathers are not formed by the graft, which seems 

 to sink underneath the ectoderm and give rise only to the pigment. 



Another example of a colour pattern which is dependent on an 

 * Willier, Rawles, and Hadom 1937. 



