424 GENETICS IN RELATION TO AGRICULTURE 



which can be used to describe any pedigree. The scheme described 

 below is essentially Pearl's system with some minor changes. In the dia- 

 gram (p. 423) solid lines with circles containing numbers indicate matings 

 between the individuals represented by the letters which they connect. 

 Dotted lines lead from the matings to the individuals produced. The 

 order of precedence in nomenclature may be varied to harmonize with 

 the former practice of the investigator. Table LVI indicates every pos- 

 sible combination of F z individuals to produce F 3 . By priming the 

 mating numbers reciprocal back crosses can be indicated and the kinds 

 of individuals produced may be similarly distinguished. For example, 

 following the order of precedence suggested below the diagram, Z X Y 

 is mating 8 and produces E but Y X Z is mating 8' and produces E' 

 while E or E' X X is mating 26 and produces, let us say, N and X X E or 

 E' is mating 26' and produces N r . The order of mating in intra-fraternal 

 crosses will be immaterial except in the case of sex-limited characters 

 when individuals may be distinguished by subscripts. Pearl explains 

 the arrangements of mating numbers in the table as follows: 



A word should be added in regard to the system by which the numbers 

 have been assigned to the matings. It might at first sight appear as 

 though the arrangement were an entirely haphazard one. It is not. 

 On the contrary the numbers will be found to conform to the following 

 general principles, which seem likely to be of aid in practical work, as 

 tending to make it easy to recall from a number just what its particular 

 pedigree looks like. 



1. All even numbers refer to back-cross matings. 



2. All odd numbers refer to co-fraternal or intra-generation matings 

 (not back-crosses). 



3. Matings below 2 are of parental generation individuals; between 2 

 and 8 inclusive are of Fi individuals; matings over 10 are of F z individuals. 



4. Even numbers from 10 to 36 inclusive designate back-crosses of 

 F 2 individuals with their grandparents, or individuals of the grand- 

 parental generation. 



5. Even numbers from 40 up designate back-crosses of F 2 individuals 

 on Fi individuals. 



6. In the case of the odd numbers from 11 up it is, in a general way, 

 true that the smaller the designating number of a mating the more closely 

 related to each other are the two individuals entering that mating likely 

 to be. This principle of assigning the numbers could not be so precisely 

 followed as the preceding five, but still is perhaps worth a little. 



In using such a system it is of course necessary to have the basic 

 table always at hand. The diagram is quickly drawn and the typewritten 

 tables may be pasted in note books or both diagram and table may be 

 printed on cardboard for use in breeding pens or plots. 



