( 'hapter 3 



SEGREGATION OF ALLELES 



Ti 



|he similarities and the differ- 

 ences in phenotypes, both 

 among offspring and between 

 them and their parents, have led us to postu- 

 late the existence of genetic material. Since 

 this material is supposedly transmitted from 

 generation to generation, we may be able 

 to learn more about the transmissive prop- 

 erties of the genetic material by studying the 

 traits that recur in lines of descent. This 

 area may be called "transmission genetics." 

 We could investigate the genetic material 

 either in lines reproducing asexually or in 

 lines (like the beans already discussed) that 

 reproduce sexually by self-fertilization. How- 

 ever, instead of taking either of these paths 

 of investigation, both of which deal with 

 pure lines, let us study the transmission ge- 

 netics of organisms reproducing sexually by 

 cross-fertilization. In the experimental work 

 described henceforth, it can be assumed, 

 unless stated to the contrary, that appro- 

 priate precautions have been taken to assure 

 that the phenotypic similarities and differ- 

 ences described are genotypic in origin and 

 are not due to variations in environmental 

 conditions. 



Different strains of a cross-fertilizing ani- 

 mal or plant often show phenotypic differ- 

 ences with respect to a given trait. For 

 example, with respect to height, one line 

 might be short and the other, tall; or with 

 respect to color, one line might be red and 

 the other, white. The question to be raised 

 now is what will happen phenotypically in 

 the offspring when two lines showing dif- 

 31 



ferent alternatives for the same trait are 

 crossed? Will such results tell us anything 

 about the genetic material? 



Consider some specific experiments that 

 can be performed with the garden pea, 1 first 

 with respect to what should be done and 

 why it should be done. Then we can exam- 

 ine the results obtained and discuss what 

 they reveal regarding the genetic material. 



The garden pea is well suited for this work 

 because it is simple and inexpensive to raise 

 and has a generation length short enough to 

 permit the study of a number of successive 

 generations. Although garden peas are nor- 

 mally self-fertilizing, they can also be cross- 

 fertilized; in fact, the experimenter can con- 

 trol all mating by simple and appropriate 

 techniques. Moreover, there are numerous 

 strains which differ phenotypically with re- 

 gard to different traits. It is first necessary, 

 of course, to self-fertilize each strain for 

 several generations and check the pheno- 

 types, to be sure that pure lines are being 

 used. 



Which pure lines should one cross to- 

 gether? Since we do not know what to ex- 

 pect in the offspring, we should avoid using 

 lines whose traits, for environmental reasons, 

 are so variable that a phenotype in one line 

 also occurs in the other (which was the sit- 

 uation in the bean strains studied in Chapter 

 1 ). Such phenotypic overlaps could prevent 

 us from deciding from the phenotypes what 

 genotypes were present. Consequently, we 

 should select for study only those strains 

 showing a sharp, nonoverlapping, easily de- 

 tected difference. For simplicity, we should 

 use only strains having a single major dif- 

 ference. We should study only lines that 

 can be successfully cross-fertilized in both 

 directions; that is, the male gamete should 

 be furnished sometimes by the one line and 

 other times by the other line. Such re- 



1 Based upon the experiments of G. Mendel (see 

 p. s-9). 



