THE NATURE AND EXPRESSION OF MENDELIAN FACTORS 131 



cells during division, or in various other peculiar phenomena. The ab- 

 normal variety also displays a certain degree of sterility, probably asso- 

 ciated with abnormal cell division. In spite of all the differences both 

 external and internal which this mutation displays when compared with 

 the normal variety from which it arose, its behavior in inheritance shows 

 clearly that only a single factor difference is involved. When crossed with 

 the normal type, the Fi is intermediate, and in F 2 segregation is in 

 approximately the ratio 1 abnormal: 2 intermediate:! normal. The F 2 

 homozygous segregants are exact duplications of the original pure forms, 

 the normal segregants are in every respect as normal as the normal parent 

 and the abnormal segregants are no less abnormal than those of pure 

 abnormal races. The heterozygous forms are throughout clearly dis- 

 tinguishable from abnormal homozygotes on the one hand and normal 

 homozygotes on the other. Taken as a whole it would be difficult to find 

 a better example of the profound effects which may result from a single 

 factor difference. 



Lethal factors also exist which affect vital organs and result in the 

 death of individuals homozygous for them. Excellent examples of such 

 disturbing factors are those which affect the production of chlorophyll 

 in plants. A number of species of plants at times produce races in 

 which under experimental conditions approximately one-fourth of the 

 seedlings are yellow or white instead of green and hence die soon after 

 germination. Such strains are particularly common in cereals, and in 

 maize in almost any variety when a large number of self-fertilized ears 

 are tested, a number of strains may be found which produce seedlings 

 about one-fourth of which die as soon as the food supply of the endosperm 

 is exhausted on account of deficiency in chlorophyll production. 



Since the homozygous recessive forms of albino strains die soon after 

 germination, it follows that such strains must be propagated by means 

 of the heterozygous individuals. The operation of such a scheme is 

 illustrated in the following case. The original self-fertilized ear gave 

 on germination 3 fully green seedlings to one which was pure white and 

 which died shortly after germination. If we call the albino factor g 

 in this case and its normal allelomorph present in the green plants G, we 

 may assume that this ear was produced by a heterozygous green plant 

 of the constitution Gg. Half the pollen grains of such a plant carry the 

 factor G and half the factor g', and likewise in the ovules half bear the 

 factor G, and half g. By self-pollination of such a plant, random fertili- 

 zation of the ovules by the pollen grains results in grains in the ratio 

 lGG:2Gg:lgg. Although grains of these different genotypes are indis- 

 tinguishable in appearance, those of the genetic constitution GG and Gg 

 produce fully green plants, while those which are gg produce albino 

 seedlings which are incapable of independent existence on account 



