DI-HYBRID CROSSES 187 
All the following zygotic combinations are, then, 
equally likely : 
ABAB ABAb ABaB ABab 
ALAB AbAb AbaB Abab 
aBAB aBAb aBaB aBab 
aAB abAb abaB abab 
Altogether there are sixteen combinations. The 
result can be expressed more shortly in the form 
(A +2Aa+a) (B+2Bb +b),* which will be found to give 
the above terms when expanded. Thus the combina- 
tion of the Mendelian formule for F, 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 : 
4A IB 2 Bb 16 
8 Aa [ combined with } 2B 4 Bb 26 
4a 1B 2 Bo 1d 
or 
AB 2 ABS Ab 
2 Aab 4 AaBb 2 Aad 
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 AA, B for BB, and so on. 
On expansion, 7.¢., multiplying together the contents of the two 
brackets, AxB gives ABAB, AxBb gives ABA}, and so on 
for all the other terms of the expression. 
