1 88 COMPARATIVE PHYSIOLOGY 



by the experiment just described is known as linkage. Linkage 

 may be partial or complete. 



Both types of linkage are illustrated by the cross between 

 the recessive mutants of the fruit fly known respectively as 

 black (already mentioned) and vestigial. When a black fly 

 with long wings is crossed with a gray fly with vestigial wings 

 all the off'spring as in the foregoing experiment are gray with 

 long wings ; and the same is true if a black fly with vestigial 

 wings is crossed with a fly that is homozygous for the gray body 

 colour and long- winged condition. But whereas, when the 

 F.I male from the cross between gray- vestigial and black- 

 long is mated to the double recessive (black- vestigial) female, 

 one-half of the off'spring are gray-vestigial and the other half 

 black-long ; when the F.i male of the cross between black- 

 vestigial and gray-long are mated to the double recessive female 

 one-half of the progeny are black- vestigial and the other half 

 gray-long. The genes re-emerge in the same combinations 

 as those in which they were present in the original parent. 

 Here linkage is complete. 



Partial Hnkage is seen when the F.i females are crossed to 

 the double recessive males. The off'spring of the mating 

 between the double recessive male and F.i female from the 

 cross between black-vestigial and gray-long are not fifty per 

 cent, black- vestigial and fifty per cent, gray-long but 41*5 per 

 cent, black- vestigial, 41-5 per cent, gray-long, 8*5 per cent, 

 black-long and 8*5 per cent, gray- vestigial. Similarly the 

 offspring of the mating between the double recessive male and 

 the F.I females from the cross between black-long and gray- 

 vestigial are 41*5 per cent, black long, 41*5 per cent, gray- 

 vestigial, 8'5 per cent, black-vestigial and 8*5 per cent, gray- 

 long. In seventeen per cent, of the offspring there has been 

 '' crossing over " ; the genes for black-gray and long- vestigial 

 have become detached, though not to such an extent as to 

 segregate with complete independence. 



Linkage has been studied in several hundred mutants of 

 Drosophila, and two important general results emerge from 

 these researches of Morgan's school : (i) if a gene a is linked 

 with a gene b which is also linked with a gene c, then a and c 



