THE MENDELIAN METHOD 



Now all these statements can be put much more simply if we say 

 that the zygotes are double, and the gametes single, in respect of 

 something which determines each of the characters and differences 

 Mendel was studying. We must then say that the two alternative 

 determinants or factors of each kind in a zygote may be alike or 

 different; but even if different, these alternatives, or allelomorphs 

 as they arc called, separate, or segregate, from one another unchanged 

 to give pure gametes; that is each gamete has only one of them, 

 and that one untainted and unblended. This is Mendel's so-caUed 

 first law of inheritance. In applying this law we may note that it 

 is quite indifferent for its transmission whether a factor is dominant 

 or recessive, whether it comes from the male or the female parent, 

 or whether it goes to male or female gamete. 



On this basis we can, as Mendel did, make certain predictions. 

 When the Fj is crossed, back-crossed as we say, to one of its parents, 

 offspring will be produced of two kinds in equal numbers. One kind 

 wiU be like the parent in question and the other like the F^. With 

 the dominant-carrying parent the two types will look alike, 

 although they will breed differently {RR and Rr). With the 

 recessive-carrying parent they will look different, as well as breed 

 differently {rr and Rr). 



This expectation was realized. In a back-cross of an Fi to its round- 

 seed parent, Mendel obtained 192 round seeds, and on testing 177 

 of these further 87 bred true and 90 proved to be of the Fj mixed 

 type. In the back-cross to the wrinkled parent, he obtained 208 seeds ; 

 102 were wrinkled seeds breeding true, i.e. homozygous for the 

 factor rr, 106 were round seeds, all of the mixed type, i.e. hetero- 

 zygous for the factor Rr. 



So much for single differences. Where the homozygous parents 

 differed in two characters, the factors determining the two differences 

 were independent. They were independent equally in transmission 

 and in action. Thus the F^ showed both dominant characters whether 

 these came from the same or from different parents. And in both 

 cases the ¥., showed a proportion which was the square of a 3 : i 

 proportion, in other words 9:3:3:1. 



Thus Mendel crossed round yellow by wrinkled green peas 

 {RRYY X rryy) and obtained a round yellow F^ (Rr Yy). This gave 

 him an Fg of 556 plants in the following numbers: 



40 



