Independent leles 





The branching track in Figure 4—4 can 

 be read from the top: 1 4 of female gametes 

 are /?y and are fertilized : 4 of the time by 

 /?y mal; (producing %$ of all off- 



spring a Ul? > > | of the time fertiliza- 

 tion is by .R > male gamete 

 all offspring are RR > :rom this origin), 

 etc. Summing up like classes, the kinds 

 and relative numbers of gene n i of 



phenotypes are obtained as shown in the 

 figure. The observed ratio (Figure 4—1 



. client agreement with the expected one. 



The branching track may be used to ob- 

 tain the 9:3:3:1 pbenotypic ratio more 

 simp. . know that crossing together 



two identical monohybrids (beterozygotes 

 for only one of all the gene pairs under con- 

 sideration ) yields a 3:1 phenotypic ratio of 

 dominant : rec . . trait If the recombina- 

 tional activity of two pairs of genes is inde- 

 pendent- both pairs being heterozygous and 

 showing dominance, then the two indepen- 

 dently produced 3 : 1 ratios may be combined 

 in the progeny as shown in Figure 

 This may be read: among the offspring, the 

 "ich are round (because of segregation, 

 random fertilization, and the dominance of 

 R in the cross Rr ■ Rr will also be yellow 

 2 4 of the time and gre. d die time 



cause of segregation, random fertilization, 

 and the dominance of J' in the cross 

 ?. of all progerr. 11 be 



round yellow and s 1<5 round greet 



see, men, mat independent segrega- 

 tion by two pairs of genes results in the 



' ' ' - ■ '-' - - - i - ; ' ■-.:.'. : ' ■-.'.'. 



In the present case, the F 3 dihybrid (hetero- 

 zygote for two of all the gene pairs under 

 consideration ) received bom R and Y from 

 .:..-.-'-. ■'._■ 



HV_ • ' - ? -'-... . . ':- 



parent and rY from the other, the gametic 

 recombinations would have been RY and 



. . - - - 



Regardless of how die genes enter the in- 

 dividual, then, the dihybrid forms four, 

 equally frequent, genetically different gam- 



The types and frequencies of gametes 



: "' -- '. "-'- '■ : --'-. ..- - . ..- 



• . . , ~ - . . - .-----.--. • . . -. - - 



double recessive individual, Le.. an individ- 

 ual homozygous recessive for both pairs of 

 genes concerned. In the cross of Rr Yy by 

 ----- . :-.-:--. - 

 - - . -. - . - . ::'--/-. - _ 



pected to produce four dif fe r e nt and equally 

 :: "-'-.'- ■- -' R '':' \ -.::'-:- 



r - r - ~ '- *--"-; : f s r r. - 1 ± 



cross (Figure 4—6) that very nearly 1 



----- " " :-- If -_-: 



r . . f r 25 . _ 



"1 ' " ' - ---..:. ---- .:--•; ;- 



PARENTS Rr Yy x Rr Yy 



.; Round 



<- I ellow 



OFFSPRING < 



I '/« Wrinklec <^ 



Greei 



• - - 



Gfeei 



lound elow 



Rc-= G-ee- 

 •Vrinlcled Yellow 

 •'•-■ e r 3 '£ £ - 



FlGU? I — S Phenotypic results of a cross between identical dihybrids. 



