236 



PRINCIPLES OF ANIMAL BIOLOGY 



this same phenomenon is found in birds, butterflies, and moths, for in 

 these groups the sex-determining chromosomes are so arranged that the 

 female produces two kinds of eggs and the male only one kind of sperma- 

 tozoon. The distribution of the sex-linked genes in these animals is 

 precisely .like that in Drosophila except that the sexes are reversed. 

 What is true of the male in Drosophila is true of the female in birds, for 

 example. An opportunity to work out the situation in birds is afforded 

 by one of the problems at the end of the chapter. 



Autosomal Linkage. — The chromosomes other than X and Y are 

 known as autosomes. When two genes for different characters are located 



in the same autosome, they have a 

 strong tendency to remain together for 

 a while, going to the same germ cells. 

 How many successive generations they 

 stay together depends on how far apart 

 the genes are in the chromosome. 

 The chromosomes break more or less 

 at random and homologous chromo- 

 somes recombine their pieces in new 

 ways. If the breakage occurs between 

 two pairs of genes, the genes enter into 

 new combinations. The genes which 

 had been going to the same germ cells 

 now go to different germ cells. Natu- 

 rally the farther apart they are, the 

 more often the breaks occur between 

 them. 



Linkage operates to distort the 

 expected ratios of different kinds of 

 individuals. This effect is illustrated 

 in Fig. 204. The two pairs of genes 

 involved are v (vestigial wing) con- 

 trasted with V (long wing), and h 

 (black body) as against B (gray). The chromosome composition of the 

 two flies is shown at the top of the illustration. The chromosomes in 

 their respective germ cells are pictiu*ed between the parents, and the Fi 

 female fly below. This Fi fly is hetei'ozygous for both color and wing 

 length and affords an opportunity to discover the breakage of the chromo- 

 somes. It produces four kinds of eggs, as shown at its right. The first 

 two of these {Bv and hV) are produced if the two pairs of genes are not 

 separated; and the genes are near enough together so that this happens 

 in about 83 per cent of all cells. In the other 17 per cent the two pairs of 

 genes are separated by breakage of the chromosomes, resulting in the 



Fig. 204. — Linkage of body color 

 and wing length in Drosophila. Left, 

 above, gray vestigial-winged male; 

 right, black long-winged female. 

 {From Morgan, "Physical Basis of 

 Heredity," J. B. Lippincott Company.) 



