586 GENETICS IN RELATION TO AGRICULTURE 



3. The use as breeders of such males only as are known to be the sons of high- 

 producing dams, since only from such males can we expect to get high-producing 

 daughters. 



4. The use of a pedigree system, whereby it will be possible at least to tell what 

 individual male bird was the sire of any particular female. This amounts, in ordinary 

 parlance, to a pen pedigree system. Such a system is not difficult to operate. In- 

 deed, many poultrymen, especially fanciers, now make use of pen pedigree records. 

 It can be operated by the use of a toe-punch. All the chickens hatched from a par- 

 ticular pen may be given a distinctive mark by punching the web between the toes in 

 a definite way. If one desires to use a more complete individual pedigree system, he 

 will find the system described in Bulletin 159 of the Maine Agricultural Experiment 

 Station a very simple and efficient one. It has been in use at this Station for 7 

 years, with entire satisfaction, on the score of both accuracy and simplicity. 



5. The making at first of as many different matings as possible. This means the 

 use of as many different male birds as possible, which will further imply small matings 

 with only comparatively few females to a single male. 



6. Continued, though not too narrow inbreeding (or line-breeding) of those lines 

 in which the trapnest records show a preponderant number of daughters to be high 

 producers. One should not discard all but the single best line, but should keep a 

 half dozen at least of the lines which throw the highest proportions of high layers, 

 breeding each line within itself. 



In the above set of directions two things will challenge the student's 

 interest most, namely the emphasis which is laid upon constitutional 

 vigor and vitality in the selection of breeding birds, and the fact that a 

 system of line-breeding or inbreeding is used in order to increase 

 fecundity. 



The relation of the above directions to the genotypic behavior are 

 not difficult to point out. Of females there are two different types 

 (ZM}WLL and (ZM)WLl which are high producers; the remainder are 

 either mediocre or low producers. It is assumed that by trap-nest 

 records, it has been possible to segregate out a certain number of such 

 high-producing hens from a mixed flock of low, high, and medium pro- 

 ducers. When these are mated to males from the same lot, a variety of 

 results will be produced according to the genetic constitution of the males. 

 In Table LXXIX are collected the results which follow when females of 

 the two high-producing genotypes are mated with the nine possible kinds 

 of males. Now if the numbers of females of genotypes (ZM)WLL and 

 (ZM) WLl in each pen are approximately equal in practice those of geno- 

 type (ZM}WLl would probably be in excess then it will be practically 

 impossible to distinguish matings of types (1) to (3) and possibly (4) and 

 (5) unless the number of daughters tested from each pen be relatively 

 large. In this connection we recall the fact, as a further difficulty, that 

 mo difiability in egg production is relatively very great. If now an 

 equal number of pens from matings (1) to (5) should happen to have 



