320 EVOLUTION AND GENETICS 



just described, for example, color is produced by the factors C 

 and R, but a factor B occurs in some sweet peas which modifies 

 the color by adding l)lue. 



Rabbits have a complex series of color and pattern genes which 

 illustrate both supplementary and complementary factors. In 

 them the factor C is necessary for the production of color. A cc 

 individual is always an albino, although it contains factors for 

 yellow, black and brown pigment. The albino may also carry a 

 factor A, or agouti, which determines the distribution of pigments 

 in individual hairs; this is characteristic of the cotton-tail rabbit 

 and is known as the wild gray type. A black rabbit crossed with 

 an albino bearing the agouti factor produces offspring of the 

 wild gray type. Other factors influence the depth of color and 

 its distribution on the body (Fig. 185). 



Lethal Factors. Factors have appeared in a few cases whose 

 presence in the proper combination results in death. Among 

 animals these factors are usually evident through reduction of 

 the usual number of progeny but among plants the congenital 

 absence of chlorophyll is a lethal factor which does not interfere 

 with the development of the seedling until food stored in the seed 

 has been exhausted. Several kinds of plants, including corn, are 

 known to produce these individuals. The character is in all 

 cases, whether plant or animal, a recessive which can be perpetu- 

 ated through heterozygous individuals. 



Several lethal factors have been discovered in Drosophila. The 

 effects of a sex-linked lethal character in these flies is shown in 

 Figure 186. Since the male bears only one x chromosome it 

 cannot be a carrier, but dies if recessive for the lethal character. 

 When a female carrier, the only individual capable of perpetuating 

 such a character, is mated, two-thirds of her offspring are females 

 instead of one-half as under normal conditions, and of these 

 females one-half are normal and one-half carriers. In the figure 

 L indicates the normal condition and 1 the lethal recessive. 



The Effect of X-rays on Genes. In connection with the study 

 of heredity in Drosophila Professor H. J. Muller has recently 

 shown that the treatment of flies with X-rays has a direct effect 

 on the behaviour of hereditary characters. His experiments prove 

 that the rays affect germ cells at any stage, even including the 

 spermatozoa contained in the seminal receptacle of the female. 

 Mutations appeared in the flies developed from gametes so treated 



