Genetic Material 



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 between children or 

 between them and their parents. Extending 

 the principles just described for beans and 

 rabbits to all other kinds of organisms, in- 

 cluding man, we conclude that not only is 

 the genetic material in different species of 

 organisms different, but it can differ from one 

 organism to another in the same species. 

 Phenotypic similarities between individuals 

 may occur when they are carrying the same 

 or different genotypes, and phenotypic dif- 

 ferences between individuals may or may not 

 be accompanied by genotypic differences. 



Having agreed that genetic variation exists 

 within as well as between species, we may 

 ask: how does genetic variation arise? If 

 you breed a pure line of large beans for many 

 generations you will find on rare occasions 

 a very small bean which will give rise to 

 offspring beans ranging from tiny to small, 

 and which clearly make up a new, different, 

 pure line (Figure 1-lC). What apparently 

 has happened is that the genetic material in 

 the pure line of large beans has somehow 

 changed to another transmissible form which 

 henceforth causes the production on the av- 

 erage of very small beans. Such a change in 

 the genotype that is transmitted to progeny 

 may be said to be produced by the process 

 of mutation, while the new type of individual 

 may be called a mutant. 



Just as it is easy to ascribe differences be- 

 tween dogs and cats to genetic differences, 

 so it is often simple to tell that certain dif- 

 ferences between lines of the same species 

 have a genetic basis. There are many strains 

 or breeds of pigeons, dogs, cattle, and of 

 other domesticated animals each of which 

 differs from the other in phenotype. That 

 many of these differences are due to genetic 



differences has been established by the re- 

 tention of these phenotypic differences even 

 after the different breeds are grown genera- 

 tion after generation in essentially identical 

 environments. Revealed in this way, the 

 genotypes within a species are of immense 

 variety. We should keep this already present 

 genetic variation in mind in seeking to learn 

 something more about the nature of the 

 genetic material. 



In order to learn more about the genetic 

 material we should examine more closely the 

 material things comprising organisms, par- 

 ticularly those materials which are trans- 

 mitted from parent to offspring. Most types 

 of organisms are composed of usually micro- 

 scopic building blocks called cells plus sub- 

 stances which have been manufactured by 

 cells. These organisms start life either as a 

 single cell, or by the fusion of two cells into 

 one, or as a group of nonfusing cells de- 

 rived from their parents. In those cases 

 where the new individual starts life as one 

 or a group of nonfusing cells derived from 

 a single parent, reproduction is said to be 

 asexual, while in cases where two parents 

 contribute cells, reproduction is said to be 

 sexual. In sexual reproduction two cells, 

 called gametes, fuse in the process of fertiliza- 

 tion into one cell, called the zygote, which 

 is the start of a new individual. In higher 

 animals these gametes are called egg (female) 

 and sperm (male), and the zygote the ferti- 

 lized egg. In the bean plant, male and 

 female gametes are produced in the same in- 

 dividual which, as already mentioned, serves 

 as both parents, and self-fertilization nor- 

 mally occurs; in human beings the two kinds 

 of gametes are produced in separate indi- 

 viduals of different sex, so that cross-fertiliza- 

 tion always occurs. 



When is this hypothetical inborn genetic 

 material transferred? Is it transferred simul- 

 taneously with the very inception of the new 

 individual or is it transferred sometime after? 

 Is the transfer accomplished only once, sev- 



