Phenotypic Effects of Gene Action 



85 



notochord from young T T embryos; under 

 similar conditions, however, mesoderm from 

 T T embryos does not form cartilage or 

 vertebrae when it surrounds presumptive 

 notochord from normal embryos. We must 

 conclude — contrary to expectation — that the 

 normal inductive relationship is disturbed in 

 T T because its mesoderm is incompetent to 

 respond to the normal inductive stimuli of 

 presumptive notochord tissue. 



The preceding discussion shows that ge- 

 netic changes can influence or control de- 

 velopment in multicellular organisms by 

 modifying ( 1 ) the relative growth rates of 

 parts (as in Creeper) and (2) the over-all 

 growth rate (as in pituitary dwarfism) with- 

 out affecting the competence of some or all 

 of the tissues affected. Genetic changes can 



also affect differentiation by changing tissue 

 competence (homozygous Creeper limbs). 

 When adjacent tissues interact by induction, 

 differentiation can be modified by gene- 

 caused changes in competence to respond to 

 inducing agents (nonresponsiveness of T T 

 mesoderm to presumptive notochord) and, 

 presumably, also by changes in inducing 

 ability. It should be realized, however, that 

 although differentiation and development in 

 higher multicellular organisms involve inter- 

 cellular interactions of all the types men- 

 tioned, some cellular traits are produced 

 solely through the intracellular action of the 

 genotype. Such behavior occurs, for exam- 

 ple, in mutants induced during embryogeny 

 which have detrimental or lethal effects in 

 the cells containing them. 



SUMMARY AND CONCLUSIONS 



Different alleles may produce detectable effects upon viability at any stage in the life 

 history of an individual and may modify the expected phenotypic ratio so that certain 

 classes of offspring are in excess, in reduced frequency, or are absent. The last effect 

 is produced by dominant as well as recessive lethal genes. 



A gene usually produces effects upon a wide variety of morphological and biochemical 

 traits. These pleiotropic effects are the consequence of a pedigree of causes traceable, 

 in some cases, to a single action on the part of the gene. It is hypothesized that most, 

 if not all, genes have only one, probably biochemical, phenotypic action. 



Penetrance and expressivity depend upon both the genotype and the environment. 

 The traits most useful for the study of transmission genetics are those whose penetrance 

 is 100% and whose expressivity is uniform when subjected to the normal variations 

 of environment. In human beings the occurrence of essentially duplicate parts within 

 an individual, and of identical and nonidentical twins, offers the opportunity to test 

 the effect of environment and of genotype upon the penetrance and expressivity of a 

 given phenotypic alternative. 



It has been shown that a considerable number of physical and mental traits are 

 determined by the joint action of genotype and environment, sometimes one and some- 

 times the other having the greater influence. 



The twin methods described do not study the transmissive characteristics of the 

 genotypes involved. They do not, therefore, reveal anything regarding the location, 

 number, or recombinational properties of genes. 



Phenogenetics, the study of how genetically determined phenotypes come into being. 



