Developmental Genetics 



263 



FIGURE 30-1. Normal ( riy,lu i and Creeper {left) roosters. {Courtesy ofL. C. Dunn; 

 reprinted by permission of McGraw-Hill Book Co., Inc., from Study Guide and 

 Workbook for Genetics by I. H. Herskowitz. Copyright, 1960.) 



appeared in the domestic fowl in which the 

 legs were shortened so as to give the impres- 

 sion that the bird was creeping. This ab- 

 normal "Creeper" phenotype and the normal 

 phenotype^ can be seen in the roosters at the 

 left and right, respectively, of Figure 30-1. 

 The genetic study of this phenotype gave 

 the following results. Reciprocal crosses 

 of Creeper by normal gave a 1 : 1 ratio of 

 Creeper : normal chicks. Creepers crossed 

 to Creepers gave, in the adult stage, 775 : 388 

 as Creeper : normal, a result which can be 

 considered an excellent fit to a 2 : 1 ratio. 

 It is reasonable to suppose, therefore, that 

 Creeper is heterozygous for a single pair of 

 segregating genes, in which the Creeper gene, 

 Cp, is dominant to its normal allele, +. The 

 2 : 1 ratio is taken to indicate that the mutant 

 homozygote Cp Cp is lethal (as is the case, 

 see p. 65, for the mutant homozygote when 

 yellow mice are crossed together). The view 

 that Cp acts as a recessive lethal received 

 support from a comparison of the survival 

 rate of embryos from normal parents with 

 that of embryos both of whose parents were 

 Creeper. It was found that about 25% of 



^ Studied by W. Landauer, by V. Hamburger, by 

 D. Rudnick, and by L. C. Dunn. 



the embryos which would have survived in 

 the former case died on or before the third 

 day of incubation in the latter case. 



What is the developmental, phenogenetic 

 connection between Cp Cp which acts as 

 a recessive lethal, Cp + which produces 

 Creeper, and + + which produces normal? 

 Although Cp Cp usually dies at about the 

 third day of incubatiorf, on rare occasions it 

 may survive 19 days, or about until the time 

 of hatching from the shell. Such a rare 

 Creeper homozygote is shown at the left of 

 Figure 30-2 (the comparable normal indi- 

 vidual is at the right) and possesses the follow- 

 ing syndrome of malformations: The eyes 

 are split, are smaller than normal, and have 

 no eyelids. The head is misshapen and the 

 body is smaller. The skeleton is not ossified 

 and, as seen on top of the black paper used 

 as background in the Figure, only the digits 

 of the limbs are well formed. 



A study of Cp + development shows, at 

 seven days of incubation, that the leg buds are 

 shorter than in normal embryos. This mor- 

 phological manifestation of Cp action must 

 be based upon events occurring earlier in 

 development, for at 48 hours of incubation 

 (Figure 30-3), a Cp + embryo (left) is smaller, 



