( II \l' I I K 1 



FIGURE 1-2. Male Siamese cat, grown under 

 temperate conditions, showing the same pig- 

 mentation pattern as the Himalayan rabbit. 

 {After C. E. Keeler and V. Cobb.) 



Phenotypic similarity — two rabbits from 

 a certain pure line (genetically black 

 rabbits) both have black coats even 

 though one individual grew at high 

 temperatures and the other grew at 

 low temperatures. 

 4. Different genotypes 



in different environments 



Phenotypic difference — a rabbit from a 

 genetically black line, grown in a cold 

 climate, has black fur, while a rabbit 

 from a Himalayan line, grown under 

 temperate conditions, is Himalayan, 

 i.e., white except for the extremities 

 (paws, tail, snout, and ears), which are 

 black (see Figure 1-2). 



Phenotypic similarity — a rabbit from a 

 genetically black line grown at a mod- 

 erate temperature and a rabbit from a 

 genetically Himalayan line grown at a 

 cold temperature both have black fur. 



The final example illustrates that genotypi- 

 cally different individuals which are pheno- 

 typically different in one environment may 

 become phenotypically similar when placed 

 in different environments. The all-black 

 Himalayan rabbit is termed a phenocopy of 

 the genetically black rabbit. Persons who 



arc genetically diabetic and take insulin arc 

 phenocopies of genetically normal persons 

 who do not take insulin. Genetically normal 

 embryos whose mothers are exposed to the 

 drug thalidomide develop into phenocopies 

 o\' genetically abnormal, phocomelic persons 

 lacking most or all of the lour limbs. So 

 both normal and abnormal phenotypes can 

 be phenocopied. 



The case of coat color in rabbits is instruc- 

 tive in another respect. The rabbit that is 

 genetically black will always produce a black 

 coat no matter what the temperature is, pro- 

 vided the temperature is not lethal. In the 

 case of this genotype there seems to be no 

 range of phenotypic expression with respect 

 to temperature variations. In the Himalayan 

 strain, however, the situation is different, as 

 already described in part. If grown at very 

 high temperatures such rabbits have entirely 

 white coats. In this case the phenotypic 

 range of reaction, or norm of reaction, of 

 the genotype is relatively great, varying with 

 increasing temperature from completely 

 black through the Himalayan pattern to 

 completely white. 



We are now in a position to answer the 

 question concerned with the basis of simi- 

 larities and differences among offspring or 

 between them and their parents. Extending 

 the principles just described for beans and 

 rabbits to all other kinds of organisms, in- 

 cluding man, it is concluded that not only is 

 the genetic material different in different 

 species of organisms, but that it can also 

 differ from one organism to another in the 

 same species. Phenotypic similarities be- 

 tween individuals may occur when they are 

 carrying the same or different genotypes, and 

 phenotypic differences between individuals 

 may or may not be accompanied by geno- 

 typic differences. 



Having agreed that genetic variation 

 exists within as well as between species, 

 one may now ask: How does genetic varia- 



