Chapter 1 



GENETIC MATERIAL 

 AND MITOSIS 



S 



urely each of us has observed 

 that we are the same kind of 

 creatures as our parents. They 

 gave rise to us, other humans — not to a 

 plant, or a fish, or a bird. Let us start, 

 therefore, by assuming the existence of some 

 intrinsic factor which determines that hu- 

 mans shall beget humans, and let us call this 

 inborn factor for the genesis of like from 

 like the genetic factor. Since each plant and 

 animal produces offspring of its own kind, 

 or species, we can generalize and hypothesize 

 that every species of organism has such a 

 built-in genetic factor. But it must also be 

 admitted that the genetic factors for dog, 

 for apple tree, and for man all differ in some 

 way in order to produce such different or- 

 ganisms as end products. 



In addition to basic likenesses within the 

 species, each person is similar to and differ- 

 ent from his parents in respect to certain 

 details. What is the basis for this? If par- 

 ents and offspring have similar caloric in- 

 takes, all will weigh more nearly alike at a 

 comparable age than if their caloric intakes 

 were different. Apparently, then, environ- 

 ment in which parents and children live can 

 sometimes be the cause of their similarities 

 and differences. But are all similarities and 

 differences among human beings produced 

 by environment, or does the genetic factor 

 presumed responsible for like begetting like 

 play a role in their production? 

 1 



In trying to formulate the answer to this 

 question, it may be helpful to consider the 

 results of certain studies with bean plants. 1 

 The particular kind of bean plant concerned 

 reproduces sexually, a single plant perform- 

 ing the functions both of male and female 

 parent. For the present, assume that the 

 genetic ) actor is transmitted from the parent 

 to the offspring, and that the transmitted 

 factor must be the same as that of the par- 

 ent. Assume also that the genetic factor 

 has a natural rather than a supernatural or 

 spiritual basis. If the genetic factor has a 

 natural basis then it ought to have a material 

 basis and have chemical and or physical 

 properties, as have other material things. 

 One is led, therefore, to postulate the exist- 

 ence of genetic material. 



Genetic Material 



Consider a particular bean. When the plant 

 grown from this seed produces offspring 

 beans (Figure 1-1 A), these are found to 

 vary in size, some being very small, some 

 small, and some medium. According to the 

 assumptions made, these beans all have the 

 same type of genetic material or genetic con- 

 stitution — or genotype. The simplest ex- 

 planation one can offer for the size differ- 

 ences is that they were caused by environ- 

 mental differences occurring during seed for- 

 mation. This view can be tested by growing 

 each of the beans and scoring the size of 

 seeds each produces. When this is done, 

 each bean is also found to produce offspring 

 beans of very small, small, and medium 

 sizes, regardless of the size of the parent 

 seed itself. This test can be repeated gen- 

 eration after generation with the same result. 

 Such a line of descent, whose members carry 

 the same genotype, can be called a pure line. 

 The manifestation of the genotype in traits 

 or characteristics (size, in our example) is 



1 Based upon W. Johannsen's experiments. See 

 reference on p. 12. 



