42 



CHAPTER 6 



TYPES AND RELATIVE 

 FREQUENCY OF GAMETES 



? cT 



T y 1/4 RY 



Ry 

 rY 



V4 rY 



V4 ry 







RY 



Ry 



rY 



ry 



/4 RY 







U Ry r\ 

 /4 rY ly 



'A ry 



A RY 



'/4 Ry r\ 

 '/4 rY ly 



'/4 ry 



Genotypes 



/16 RRYY 



/16 RRYy 



/1 6 RrYY 



/1 6 RrYy 



/I 6 RRYy 



/16 RRyy 



/1 6 RrYy 



/16 Rryy 



/1 6 RrYY 



/1 6 RrYy 



/1 6 rrYY 



/1 6 rrYy 



/16 RrYy 



/16 Rryy 



/1 6 rrYy 



/16 rryy 



OFFSPRING 



Genotypic 

 Frequency 



X 1/16 



RRYY 



RRYy 



RrYY 



RrYy 



RRyy 



Rryy 



rrYY 



rrYy 



rryy 



Phenotypic 

 Frequency 



X 1/16 



9 Round 

 Yellow 



3 Round 

 Green 



3 Wrinkled 

 Yellow 



1 Wrinkled 

 Green 



FIGURE 6-4. Independent segre- 

 gation and random fertilization in 

 a cross between identical dihy- 

 brids. 



You know that crossing together two mono- 

 hybrids (heterozygotes for one pair of genes) 

 yields a 3 : 1 phenotypic ratio of domi- 

 nant : recessive trait. If the recombinational 

 activity of one pair of genes is independent 

 of the same behavior by another pair of 

 genes, both being heterozygous and showing 

 dominance, then the two independent 3 : 1 

 ratios may be combined in the progeny as in 

 Figure 6-5. That chart may be read : among 

 the offspring, the X which are round (because 

 of segregation, random fertiHzation, and the 

 dominance of R in the cross Rr X Rr) will 

 be % of the time also yellow and V^ of the 

 time will be also green (because of segrega- 



tion, random fertilization, and the dominance 

 of Y in the cross Yy X Yy)\ so, of all 

 progeny %f, will be round yellow and Ke 

 round green, etc. 



You see, then, that independent segrega- 

 tion by two different pairs of genes results in 

 the formation of gametes whose gene combi- 

 nations, new and old, are in equal frequency. 

 In the present case, the dihybrid (hetero- 

 zygote for two pairs of genes) received both 

 R and Y from one parent and both r and y 

 from the other, its gametic recombinations 

 being Ry and rY. Had the dihybrid received 

 Ry from one parent and rY from the other, 

 the gametic recombinations would have been 



