DT-HYBRID CROSSES 187 



All the following zygotic combinations are, then, 

 equally likely ; 



ABAB ABAb ABaB ABab 



AbAB AbAb AbaB Abab 



aBAB aBAb aBaB aBab 



abAB abAb abaB abab 



Altogether there are sixteen combinations. The 

 result can be expressed more shortly in the form 

 (A+zAa + a) (B + 2Bb + &),* which will be found to give 

 the above terms when expanded. Thus the combina- 

 tion of the Mendelian formulae for F 2 when each of 

 the pairs of allelomorphs is considered separately, 

 gives us the formula for the two pairs of allelomorphs 

 considered simultaneously. 



The same result may also be written in the form : 



4 A ] ( i B iBb ib 



8 Aa \ combined with \ 2 B 4 Bb 2 b 

 4 a J I i B 2Bb i b 



or 



AB 2 ABb Ab 



2 Aab 4 AaBb 2 Aab 

 aB 2 aBb ab* 



* It is customary to condense these expressions as far as 

 possible by never repeating the same letter more than once 

 in each term. Thus, A stands for A A, B for BB, and so on. 

 On expansion, i.e., multiplying together the contents of the two 

 brackets, AxB gives ABAB, Ax.Bb gives ABAb, and so on 

 for all the other terms of the expression. 



