254 PRINCIPLES OF ANIMAL HIOLOCY 



are produced. A spore develops asexually into a thread-like structure 

 (protonema) on which a number of moss plants grow. All the moss 

 plants from a single protonema are, in certain species, of the same sex. 

 The spore evidently contains something which determines the sex of the 

 plants growing from its protonema. What that something is has not 

 been determined. There is no evidence that sex is associated, in these 

 mosses, with the number or kind of chromosomes, although such an 

 association has lately been discovered in another plant. 



Sex-linkage. — In many animals in which sex is dependent upon the 

 number of certain chromosomes present, there is linkage of sex with 

 certain other inherited characters. As a result, in certain crosses the 

 males and females inherit the body characters unequally. A case of this 

 kind is found in the eye color of the fruitflj^ Drosophila. These flies are 

 normally red-eyed, but white-eyed individuals are known. When a 

 white-eyed male is mated with a red-eyed female, the Fi are all red-eyed. 

 InFo, however, though there are three reds to one white, the two sexes do 

 not share equally in the two colors. The F2 females are all red-eyed, 

 while half of the males are red-eyed, half white-eyed. Plumage charac- 

 ters in fowls are not infrequently linked with sex in similar manner. 

 A detailed explanation of sex-linkage would be out of place here. It is 

 enough to say that it results from the fact that the genes for the sex-linked 

 characters (eye color in flies, plumage characters in fowls) are located in 

 the chromosomes which determine sex. All the characters whose genes 

 are represented in Fig. 189 are sex-linked,, since these chromosomes are 

 the sex-chromosomes. 



Mechanism of Heredity : a Retrospect. — In the foregoing discussions 

 it has twice been asserted dogmatically that the genes are in the chromo- 

 somes. The evidence for this statement is abundant, but only a small 

 part of it can be advanced here. In sea-urchin eggs, certain chromo- 

 somes have been lost in the course of experiments, and with them were 

 lost some of the characters of the larval skeleton. Still better evidence 

 is found in the fact that the chromosomes behave, in maturation, in 

 such a way as to furnish an explanation for many of the fundamental 

 facts of heredity, if only the assumption be made that the genes are in 

 the chromosomes. Some of these fundamental facts may be referred to. 



First, the segregation of the genes in the germ cells is accounted for if 

 the genes are in the chromosomes. It has been shown that, in matura- 

 tion, the chromosomes come together in pairs. Presumably the two genes 

 for a given kind of character are in these two chromosomes. In a 

 heterozygous black guinea-pig (Bb) the gene B is assumed to be in one 

 chromosome, b in another. These two chromosomes pair with each 

 other. In a heterozygous rough guinea-pig (Rr), R is in one chromosome, 

 r in another, and these chromosomes pair early in maturation. Like- 

 wise in a homozygous organism {BB or rr), the two genes that produce 



