74 



CHAPTER 6 



individuals with essentially identical geno- 

 types. 



We are already familiar with the effect ol 

 genotypic variations upon penetrance under 

 essentially constant environmental condi- 

 tions. The penetrance of an allele may de- 

 pend upon the nature of its partner allele 

 in cases of complete or partial dominance, 

 and the penetrance of one or a pair of alleles 

 may be modified by its epistatic- hypostatic 

 relations to nonallelic genes (Chapter 4). 

 Similarly, it can be shown that variable ex- 

 pressivity may be the consequence of dif- 

 ferences in either or both the environment 

 and the genotype. 



One should be careful to differentiate be- 

 tween penetrance and expressivity on one 

 hand and dominance and epistasis on the 

 other. Suppose, with respect to height, TT 

 always produces tall, TT always short, and 

 TT always medium. Although the hybrid 

 shows incomplete, partial, or no dominance, 

 there is 100% penetrance for each of the 

 three genotypes. If among the mediums 

 there was some variability in size (due to 

 variations in environment or the rest of the 

 genotype), we would have different expres- 

 sivities for the 100% penetrant TT geno- 

 type. If, however, for the same reasons. 

 TT sometimes produced a short individual. 

 this would be a case of nonpenetrance of 

 T in TT. 



The terms penetrance and expressivity 

 were used to compare the phenotypic events 

 that occur in different individuals genetically 

 identical in one particular respect. That is, 

 once any phenotypic expression occurred 

 within an individual, the genotype was said 

 to be penetrant, and all other phenotypic 

 comparisons between penetrant individuals 

 were considered matters of expressivity. In 

 fact, however, one can also correctly speak 

 about penetrance within an individual for 

 those cases in which the particular genotype 

 has two or more occasions to express itself. 

 Thus, for example, the gene for Polydactyly 



has two apparently equal chances to be pene- 

 trant in the case o\ the hands, and two ap- 

 parently equal chances to be penetrant in 

 the case o\ the feet. The genotype may be 

 penetrant in one hand (six fingers) and not 

 in the other (five lingers), it may be pene- 

 trant in the feet (each foot having six toes 

 — represented as 6.6) and not in the hands 

 (5.5). When differences in penetrance (or 

 expressivity) are shown by essentially dupli- 

 cate parts of the same individual (one hand 

 having seven and the other six digits, or one 

 hand having a large and the other a small 

 extra digit), one can be reasonably certain 

 that these differences have an environmental 

 and not a mutational basis. However, when 

 different individuals are compared with re- 

 spect to penetrance or expressivity, it is often 

 impossible to attribute, with assurance, simi- 

 larities or differences among them to geno- 

 type or to environment, if both of these 

 factors can vary in uncontrolled ways (as 

 already implied on p. 12). 



Studies of Human Twins 



In organisms other than man experimental 

 conditions can be controlled so that a stand- 

 ard genotype exposed to different environ- 

 ments shows to what extent environment is 

 responsible for phenotypic variability, where- 

 as a standard environment to which different 

 genotypes are exposed reveals to what ex- 

 tent these genotypes produce different 

 phenotypes. Since neither the environment 

 nor the genotypes of human beings are sub- 

 ject to experimental control, how can we 

 determine to what extent a particular human 

 trait is controlled by genotype (nature) and 

 by environment (nurture)? Fortunately, 

 this nature— nurture problem can be studied 

 using the results of certain naturally occur- 

 ring phenomena. 



An individual contains many different 

 parts which presumably have the identical 

 genotype. Accordingly, as mentioned, one 

 can attribute to nurture any phenotypic dif- 



