Phenotypic Effects of Gene Action 



79 



figure 6-8. Variation in preferred tempo. 

 (After C. Stem.) 



ence in score, there is clearly a genotypic 

 contribution to this personality trait. 



Studies of twins for the mental disease 

 schizophrenia show concordance of 86% for 

 identicals and 14% for nonidenticals. How- 

 ever, it is likely that the environment is not 

 the same for both types of twins, differences 

 in social environment causing more discord- 

 ance in the case of nonidenticals than in the 

 case of identicals. Nevertheless, in support 

 of the view that not all the concordance for 

 identicals is attributable to their similar en- 

 vironment and that some genotypic basis 

 exists for concordance are two cases of iden- 

 tical twins who were separated, raised in dif- 

 ferent environments, yet were concordant at 

 about the same age. 



Different people, of course, score differ- 

 ently on I.Q. examinations. The differences 

 in ability to answer questions on these exam- 

 inations can be used to measure what may 

 be called test intelligence. Although the 

 scores of nonsiblings vary widely above and 

 below 100, the difference between the scores 

 of twins reared together is only 3.1 for iden- 

 ticals but 7.5 for nonidenticals. Clearly, 

 identity in genotype makes for greater simi- 

 larity in score. Identicals reared apart have 

 scores that differ by 6. In this case the 



greater difference in environment makes for 

 a greater difference in performance of iden- 

 ticals, but this is still not so great a difference 

 as is obtained between nonidenticals reared 

 together. Therefore, both genotypic and en- 

 vironmental factors affect the trait, test in- 

 telligence. 



In the case of ABO blood group we have 

 already discussed the nature of the genetic 

 factors involved. Although the twin and 

 other methods used in this section tell 

 whether genotypic differences are associated 

 with the occurrence of the other phenotypes 

 considered, they provide no information re- 

 garding the location, number, or recombina- 

 tional properties of the genes involved. 



Developmental Effects 



Many of the mutants present at fertiliza- 

 tion in multicellular plant and animal forms 

 are detected by some visible change they 

 produce in morphology. This phenotypic 

 change is usually macroscopic and is first 

 noted a considerable time after the organism 

 starts its development. How does the mu- 

 tant change normal development in order to 

 produce the new morphological result? The 

 answer to this question involves the manner 

 in which phenotypes (of any type) come 

 into being via gene action, and is the subject 

 of phenogenetics, one aspect of which is 

 developmental genetics. 



Consider the genetic and phenogenetic in- 

 formation obtained from studying one par- 

 ticular case. In the chicken a novel type 

 occurs whose legs are so short that they give 

 the impression that the bird is creeping. 

 This abnormal "Creeper" phenotype and 

 the normal phenotype 3 can be seen in the 

 roosters in Figure 6-9. 



The genetic study of this phenotype gives 

 the following results: reciprocal crosses of 

 Creeper by normal produce a 1 : 1 ratio of 



3 Studied by W. Landauer. V. Hamburger, D. Rud- 

 nick, and L. C. Dunn. 



