44 



CHAPTER 6 



GAMETES 



9 



Try 

 1 ry 

 1 ry 

 Iry 



GENOTYPES 



'A Rr Yy 



k 74 Rr yy k 



T V4 rr Yy T 



V4 rr yy 



PHENOTYPES 



V4 Round Yellow 

 V4 Round Green 

 V4 Wrinkled Yellow 

 Va Wrinkled Green 



FIGURE 6-6. Test cross or backer oss of the F, 

 di hybrid (Rr Yy) with the double recessive indi- 

 vidual (rr yy). 



We are now in a position to return to a 

 consideration of the material basis for genes. 

 If one gene pair is to be physically associated 

 with the corresponding short regions on a 

 pair of homologous chromosomes within 

 which a chiasma cannot occur, the question 

 is, where in relation 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. Let us 

 examine the assumption that different pairs of 

 genes are located on different pairs of chro- 



mosomes. If this is true, then there are 

 several different arrangements that the parts 

 of different pairs of chromosomes may take 

 relative to each other at metaphase I of 

 meiosis (Figure 6-7). 



It has been established that different pairs 

 of chromosomes line up at metaphase I 

 independently of each other. Moreover, it 

 is entirely reasonable that the way cen- 

 tromeres in a tetrad orient toward the poles 

 at metaphase I would be uninfluenced by 

 the presence or absence of chiasmata in that 

 tetrad. If, then, as in Case A, there is no 

 chiasma between the centromere and gene 

 pair Aa nor between the centromere and 

 gene pair Bb, alignments I and II, being 

 equally frequent, will result in four different, 



