148 MAINE AGRICULTURAL EXPERIMENT STATION. I9II. 



Each character of an organism which is inherited must, in 

 some manner, be represented by a factor in the germ cell. For 

 each of these gametic factors Johannsen proposes the name 

 gene. A gene then is that factor in the germ cell or gamete the 

 presence of which (and therefore its taking part in gametic re- 

 actions) is connected with the existence of a particular somatic 

 character or set of characters. All of these unit factors or 

 genes taken together constitute the genotype of the organism. 

 The genotype then represents the hereditary or gametic consti- 

 tution of the individual as distinguished from the somatic. 



Johannsen's experiments show that in the organisms with 

 which he has worked genotypes cannot be modified by selection. 

 That is, given a group of a thousand individuals all of the same 

 genotypical constitution, and no' amount of selection of somatic 

 variations within this group will produce any permanent or 

 inherited effect. The offspring of the selected individuals will 

 be, on the average, like the offspring of other individuals which 

 had the same genotypical constitution. The sole function of 

 selection then becomes the isolation and propagation of strains 

 composed of individuals having the desired genotypical consti- 

 tution. 



It may be said that confirmation of one or another of the 

 essential features of the genotype concept of Johannsen, has 

 come not only from his work with beans and barley, but also 

 from the work of Nilsson-Ehle, Jennings, Shull, East, Hanel, 

 Roemer and others on a variety of plants and animals. 



Suppose this general conception of the mechanism of in- 

 heritance to be taken as a working hypothesis (to be tested by 

 experiment) in attempting to increase or decrease &gg produc- 

 tion in the domestic fowl by breeding. To what sort of picture 

 of the make-up of a flock does it lead? First of all it may be 

 assumed that a number of distinctly different genotypical con- 

 stitutions will be represented in the flock. By way of concrete 

 illustration let us suppose a population or flock to be made up 

 of individuals representing seven marked genotypical differ- 

 ences. Each set of individuals of like genotypical constitution 

 may be considered to form a "line" or "strain" in the breeder's 

 sense. There will then be seven lines which may be designated 

 2iS A, B, C, D, B, P,2ind G. (See fig. 79). The general average 

 production for the population as a whole is 130. The geno- 



