THE GENETIC CONTROL OF PATTERN 



205 



pattern. Well-known examples are the genes controlling the shape of 

 cocks' combs. ^ The recessive form is the single comb; one dominant 

 factor R converts this to the rose comb, while another P gives the pea 

 comb. The combination RP produces the walnut comb. Each comb 

 shape is a definite and distinct pattern. We have not yet any full em- 

 bryological data on this case. Many other genes with clear-cut effects 



Fig. 97. Hereditary Homoosis. — A Proboscis of normal Drosophila, frontal 

 view, extended position; 6 and C the same for proboscipedia, showing the tarsus- 

 like characteristics, f.o, filter apparatus, f fulcrum, g.p. gustatory papillae, h haustel- 

 lum, hp hypopharynx, 0./. oral lobes (labellae), lb labrum, m.p. maxillary palp, 

 mx maxilla, o.p. oral pit, pt pseudotracheae, r rostrum. 



(From Bridges and Dobzhansky.) 



on the underlying patterns will probably be found among genes 

 determining colour varieties, such as the different types of banding in 

 snails. 



The fact that a gene changes an underlying pattern and does not 

 merely modify its expression is perhaps most obvious for genes which 

 alter the number of elements in an organ. All genes determining 

 meristic segmentation are of this kind. Good examples are the factors 

 four- jointed and dachs which cause the formation of four instead of 

 five tarsal joints in the legs of Drosophila. 



Rather similar are the factors which cause hereditary homoosis, that 

 ^ Punnett 1923 



