16 



CHAPTER A 



GAMETES 



Va r y 



1 r y 



GENOTYPES 



Va rr yy 



PHENOTYPES 



'A Wrinkled Green 



figure 4-6. Test cross of the F 3 dihybrid ( Rr Yy) with the double recessive indi- 

 vidual ( rr yy). 



of segregation by the members of a single 

 pair of genes and of independent segrega- 

 tion by different pairs of genes. 



Whenever one is dealing with complete 

 dominance, a cross to an individual reces- 

 sive for the pairs of genes involved will al- 

 ways serve to identify the genotype of the 

 other parent, since the phenotypic types and 

 frequencies of the offspring will correspond 

 to the genotypic types and frequencies oc- 

 curring in the gametes of the latter. This 

 kind of cross is, therefore, called a test cross, 

 or a backcross when one of the parents in 

 the series of crosses is homozygous recessive 

 for the genes under study. 



We are now in a position to return to a 

 consideration of the material basis for genes. 

 If one gene pair is to be associated physically 

 with the corresponding short regions in a 

 pair of homologous chromosomes, within 

 which an exchange leading to a chiasma 

 cannot occur, the question is, where, in rela- 

 tion to one pair of genes, is a second pair 

 located? Two possibilities occur — either 

 both pairs are on the same chromosome pair 

 or they are on different, nonhomologous 

 chromosome pairs. Consider the latter as- 

 sumption — that different pairs of genes are 

 located on different pairs of chromosomes. 

 If this is true, then there are several differ- 



ent arrangements that the parts of different 

 pairs of chromosomes may take relative to 

 each other at metaphase I of meiosis (Fig- 

 ure 4-7). 



It has been established that different pairs 

 of chromosomes arrive at metaphase I in- 

 dependently of each other. Moreover, it is 

 entirely reasonable that the orientation to- 

 ward the poles, of the centromeres in tetrads 

 at metaphase I and in dyads at metaphase 

 II, is not influenced by the presence or ab- 

 sence of chiasmata or exchanges. If, as in 

 Case A (Figure 4-7), no exchange occurs — 

 and, hence, no chiasma is formed — between 

 the centromere and gene pair A a or between 

 the centromere and gene pair Bb, alignments 

 I and II, being equally frequent, will result 

 in four different, equally frequent types of 

 gametes at the end of meiosis. The same 

 result is also obtained either when there is 

 a chiasma between the centromere and the 

 gene in question in one tetrad but not the 

 other (Case B), or when a chiasma occurs 

 in each of the tetrads (Case C). Both in 

 Case CI and CII the dyads can orient to 

 the poles at metaphase II in four equally 

 likely arrangements, with the same net re- 

 sult, four equally frequent types of gametes. 

 Therefore, independent segregation of dif- 

 ferent pairs of chromosomes can serve as 



