Chapter *2 



GENE SEGREGATION 



WE HAVE been led to postulate 

 the existence of genetic ma- 

 terial from observations re- 

 garding the similarities and the differences in 

 phenotypes which occur among offspring and 

 between them and their parents. Hypo- 

 thetically, this material is transmitted from 

 generation to generation, and we can hope 

 to learn more about it by studying the oc- 

 currence of traits in lines of descent. Perhaps 

 this procedure will reveal additional trans- 

 mission properties of the genetic material. 

 We shall, therefore, continue to study what we 

 may call transmission genetics. 



There is a choice to be made at this point. 

 We could investigate the genetic material 

 either in lines reproducing asexually or in 

 lines, like the beans already discussed, which 

 reproduce sexually by self-fertilization. In 

 both cases we would be dealing with pure 

 lines. However, instead of taking either of 

 those paths of investigation let us turn our 

 attention to the study of the genetic material 

 in organisms reproducing sexually by cross- 

 fertilization. In the experimental work de- 

 scribed henceforth, it can be assumed, unless 

 stated to the contrary, that appropriate pre- 

 caution has been taken to assure that the 

 phenotypic similarities and differences de- 

 scribed are genotypic in origin and are not 

 the result of varying environmental condi- 

 tions. 



It is possible to obtain different strains of 

 a cross-fertilizing animal or plant which show 

 phenotypic differences with respect to a given 

 trait. For example, for the trait height one 



line might be short, the other tall; or for 

 the trait color one line might be red and the 

 other white. The question raised now is what 

 will happen phenotypically in the offspring 

 if two lines showing different alternatives for 

 the same trait are crossed? Can we, by 

 studying the results of the first and subse- 

 quent generations, learn anything regarding 

 the genetic material? 



Let us consider some specific experiments 

 like this which can be performed with the 

 garden pea,^ first with respect to what should 

 be done and why it should be done. Then 

 we can examine the results obtained and 

 discuss what they reveal regarding the genetic 

 material. 



The garden pea plant is a favorable or- 

 ganism for these studies because it is simple 

 and inexpensive to raise and the length of a 

 generation is short enough to permit the 

 study of a number of generations in succes- 

 sion. Although garden peas are normally 

 self-fertilizing, it is possible also to cross- 

 fertilize them; in fact the experimenter can 

 control all mating by simple and appropriate 

 techniques. Moreover there exist a num- 

 ber of strains differing phenotypically with 

 regard to different traits. It is necessary, of 

 course, to breed these strains by self-fertiliza- 

 tion for several generations and to observe 

 the phenotypes, to make sure that pure lines 

 have indeed been obtained. 



Which pure lines should we cross together? 

 Since we do not know what phenotype to 

 expect in the offspring it would be unwise to 

 use as parents two lines whose flowers differ, 

 say, in shades of pink or whose seeds differ 

 only in average size. For such traits might 

 be subject to variation due to the action of 

 the environment which could cause the phe- 

 notypes in the two parental strains to overlap. 

 This would confuse our deciding from the 

 phenotypes what genotypes were present. 

 So we should first select for use only strains 

 which show a sharp, nonoverlapping, easily 

 ^ Based upon G. Mendel's experiments. 



