Chapter 30 

 DEVELOPMENTAL GENETICS 



WHEN we are dealing with mu- 

 tations other than point 

 mutations, the genetic alter- 

 natives can often be recognized cytologically 

 by modifications produced in chromosomal 

 appearance. In the case of point mutation, 

 however, we are restricted to a study of the 

 phenotypic consequences of the change in 

 genotype. In fact, we may usually note the 

 presence of a mutant (of any type) by its 

 phenotypic effects, that is, by its effects on the 

 characteristics of an individual. Since the 

 characteristics determined by gene action are 

 themselves not inherited, the question is, how 

 are these traits produced? What happens 

 between the time the zygote is formed and 

 the time the trait appears? Development. 

 We are interested, therefore, in the ways in 

 which development is influenced by different 

 genotypes, or in what we may call develop- 

 mental genetics. 



You realize, of course, that regardless of 

 the importance of a trait produced by a gene, 

 we cannot learn anything about the role of 

 the gene in the production of the trait, unless 

 there is some genetic alternative which pro- 

 duces a change phenotypically. We would 

 never learn of the existence or of the role in 

 development of a gene, if its presence, ab- 

 sence, or alternative forms produced no 

 difference in phenotype. Whenever two dif- 

 ferent genetic constitutions produce two 

 different effects on the phenotype, it should 

 also be realized that what is recognized is not 

 the total phenotypic effect of one genotype 

 and how this effect has been changed by 



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another genotype; what is seen is only the 

 change in development which has been made 

 by the change in genotype. 



Let us invent an example that will illustrate 

 both these points. Suppose a particular gene 

 actually is responsible for the production of 

 an entire protein composed of a chain of a 

 hundred amino acids. So long as no genetic 

 alternative is known which produces a change 

 in this protein, there is no way of knowing 

 whether this protein is the result of a specific 

 gene's activity, or is entirely the result of the 

 action of the total genotype together with 

 the environment. But, suppose a mutant 

 occurs which substitutes one amino acid in 

 this protein for another, and that this pheno- 

 typic change is detectable. From this result 

 we could conclude only that the normal gene 

 places one amino acid and the mutant gene 

 another amino acid in this protein molecule. 

 Note again that we would have learned not 

 the total effect of the gene, but just the differ- 

 ence between the phenotypic results of the 

 two genetic alternatives. 



Under ordinary circumstances, when mu- 

 tants present at fertilization in multicellular 

 plant and animal forms are detected, it is 

 because they produce some visible change in 

 morphology. This is usually a macroscopic 

 phenotypic change, identified a considerable 

 time after the organism starts its development. 

 Two questions arise in this connection. What 

 is the genetic basis for the mutant involved? 

 The answer to this can be obtained by utiliz- 

 ing the principles and methods already dis- 

 cussed in previous Chapters. How does the 

 mutant change normal development to pro- 

 duce the new morphological result? The 

 answer to the latter question deals with learn- 

 ing how phenotypes (of any type) come into 

 being via gene action, and is the subject of 

 phenogenetics, a study which is of broader 

 scope than developmental genetics. 



Let us see what genetic and phenogenetic 

 information can be obtained from studying 

 one particular case. A novel phenotype 



