274 GENERAL ZOOLOGY 



segregation. Often the Fi shows a condition intermediate 

 between the parental characters and sometimes an entirely new 

 character is formed. Correns, one of the rediscoverers of 

 Mendel's laws, showed that when the white variety of the four-o'- 

 clock, Mirabilis jalapa, is crossed with the red variety, the Fi is 

 pink. Inbred the pink-flowered plants produce in the F 2 a ratio 

 of 1 red: 2 pink: 1 white, a modification of the 3 : 1 ratio, resulting 

 from the fact that the heterozygotes are all pink. If a black 

 Andalusian fowl is crossed with a white-splashed-with-black fowl, 

 the Fi is slaty blue. These inbred produce in the F 2 on the aver- 

 age a ratio of 1 black: 2 blue: 1 white-splashed-with-black. In 

 cases of incomplete dominance the heterozygotes are distinguish- 

 able externally from the homozygotes. 



Universality of Mendelian Heredity. — The basic principles 

 brought to light by Mendel's experiments have been found to 

 apply to the heredity of animals and plants generally, which 

 means that all heredity, except cytoplasmic heredity, is Men- 

 delian heredity. On this basis it becomes possible to analyze the 

 heredity of an organism in terms of genes, which on the whole 

 remain remarkably stable from generation to generation as a 

 result of their immunity from the effects of disturbing environ- 

 mental conditions of various sorts. They may, however, be 

 affected by certain external influences under experimental condi- 

 tions, as has been shown, and they may also change spontaneously 

 to produce what are called mutations, some examples of which 

 will be discussed presently. 



A common human trait whose inheritance follows along the 

 usual Mendelian lines is eye color. Difference in eye color is due 

 to difference in amount and distribution of pigment in the iris 

 and not to difference in the color of the pigment. In dark-brown 

 and black eyes the pigment is present in the outer and inner sur- 

 face of the iris and in the region (stroma) between the two 

 surfaces. Light-brown, gray, and green eyes have less pigment 

 in the outer surface. Blue eyes have pigment only in the inner 

 surface of the iris. These various grades of eye color represent 

 allelomorphic conditions. Thus brown is dominant to blue; 

 so that if one parent has homozygous brown eyes and the other 

 blue, the children will all have heterozygous brown eyes. If one 

 parent has heterozygous brown eyes and the other blue, there is 

 one chance out of two for blue eyes in the children. If both 



