Chapter *1 



GENETIC MATERIAL 



S' 



INCE human beings are curious, 

 you surely have already noticed 

 certain things about yourself. 

 In the first place, you recognize yourself as 

 being the same kind of creature as your par- 

 ents. Your parents gave rise to you. another 

 human — not to a cereal, a fish, or a bird. 

 This is so even though the raw materials from 

 which you were initially constructed and 

 from which you subsequently grew were 

 originally nonhuman. Let us, therefore, 

 start by assuming the existence of some in- 

 trinsic factor which determines that humans 

 shall beget humans. We can call this inborn 

 factor for the genesis of like from like the 

 genetic factor. Since each kind or species of 

 living thing, be it plant or animal, produces 

 offspring of its own kind we can generalize 

 and hypothesize that each species of organ- 

 ism has such a built-in genetic factor. But 

 we must now also admit that the genetic fac- 

 tors for dog, for apple tree, and for man must 

 all differ in some way in order to produce such 

 different organisms as end products. 



You must have also noticed that, in respect 

 to certain details, you are similar to and dif- 

 ferent from your parents. What is the basis 

 for this? You have already observed from 

 common experience that the environment in 

 which parents and children live can some- 

 times be the cause of similarities and dif- 

 ferences between them. Thus, if the caloric 

 content of the diets of parents and children 

 is similar they will weigh more nearly alike 

 than if their caloric intake is different. Are 

 all similarities and differences among human 

 1 



beings produced by environment? Or, does 

 the intrinsic genetic factor we have invented 

 to be responsible for like begetting like play 

 a role in the production of the detailed simi- 

 larities and differences which we see upon 

 comparing children with parents? 



This question may be answered after con- 

 sidering the results of studying certain bean 

 plants.' The particular kind of bean plant 

 referred to reproduces as we do, sexually, 

 a difference being that a single plant performs 

 the functions both of male and of female 

 parent. Assume, for the present, that the 

 genetic factor is transmitted from the parent 

 to the offspring, and that the transmitted 

 factor must be the same type as that of the 

 parent. Let us also assume that the genetic 

 factor has a natural rather than a super- 

 natural or spiritual basis. If the genetic factor 

 has a natural basis it ought to have a material 

 basis and have chemical and/or physical 

 properties as have other material things. 

 We are led, therefore, to postulate the exist- 

 tence of genetic material. Let us now con- 

 sider a particular bean seed. When the 

 plant grown from this seed produces offspring 

 bean seeds (Figure 1-1 A), we find that the 

 offspring bean seeds vary from each other 

 in size, some being very small, some small, 

 and some medium. On our hypotheses these 

 seeds must all have the same type of genetic 

 material, genetic constitution, or genotype. 

 The simplest explanation we can offer for 

 the size differences between them is that this 

 variation was caused by environmental dif- 

 ferences which occurred during seed forma- 

 tion. This idea can be tested by growing 

 each of these seeds and scoring the size of 

 seeds that they produce. When this is done 

 it is found that each seed also produces off- 

 spring bean seeds of very small, small, and 

 medium sizes, regardless of the size of the 

 parent seed itself. And this test can be made 

 generation after generation with the same 

 result. We can term such a line of descent, 

 ^ Based upon W. Johannsen's experiments. 



