268 Experimental Transformation of Species 



ous characters of the eye, the wings, the legs, the abdomen, 

 the hairs, and so on, seem to be Hnked together. Thus, in 

 certain strains of flies, black body, vestigial wings and pur- 

 ple eyes go together (Fig. jj) . Whether the particular 

 characters studied are dominants or recessives, they seem to 

 stay together after segregation, instead of forming entirely 

 new combinations (Fig. 78 ) . As the chromosomes divide pre- 

 paratory to cell division the genes making up one chromo- 

 some are split lengthwise: each cell of the body therefore has 

 a gene corresponding to another particular gene in each of 

 the other cells. When germ cells are about to be formed, the 

 two members of each pair of chromosomes become separated, 

 one going to each of the resulting germ cells and carrying its 

 set of genes. This corresponds to the close association of va- 

 rious characters in distinct groups. Such an association of 

 characters is called a linkage. Linkage is supposed to be due 

 to the adhesion of neighboring genes. Studies have shown 

 linkage of three, four or more characters. 



Linkage, however, is not absolute. In the course of 

 experiments in heredity, certain groups of characters 

 usually travel together from generation to generation. 

 Sometimes, however, they become separated. Yellow wings 

 and white eyes are recessive in the fruit flies; and ex- 

 periments have shown these two characters to be closely 

 linked. If a male fly with these characters is mated with 

 a wild female, having red eyes and gray wings, all the 

 offspring will have gray wings and red eyes — the dominant 

 characters (Fig. 79). If one of the hybrid females is now 

 mated with a male of the father type, that is, one having 

 recessive yellow wings and white eyes, there will be produced 

 four kinds of offspring. If there were perfect linkage we 

 should expect, in accordance with Mendelian ratios, two 

 types of offspring — one having both eye and wing char- 

 acters dominant (red and gray) and the other having both 

 characters recessive (white eye and yellow wing). On the 

 other hand, if there were no linkage the four types would 

 appear in equal proportions (Fig. 80) . In actual experience 

 these four types occur in the ratio of 99 grandparent types 



