THE MECHANISM OF HEREDITY 9 



transcend the number of the chromosomes. As a matter of fact, several 

 strains are known in which the number of Mendelian characters is greater 

 than the number of chromosomes ; but just here a remarkable phenomenon 

 has come to light that shows in certain cases that many of the characters 

 do not segregate independently, but are linked to each other as though they 

 belonged to some common system. As a result, peculiar ratios appear, 

 that differ from the expected Mendelian ratios in so 'far as that expecta- 

 tion rests on independent assortment. The question here raised has, 

 therefore, taken on a new aspect, and it has become essential to discover 

 whether there are as many, or more, or fewer groups of linked char- 

 acters than there are kinds of chromosomes. 



Correns appears to have been the first to call attention to a case in 

 which peculiar ratios appeared which he attributed to coupling. Bateson 

 and his coworkers have described several instances of the same kind. 

 They have found that certain characters in sweet peas do not fulfill the 

 expectation for independent assortment of different pairs of characters, 

 although they do show Mendelian segregation when each pair is taken 

 separately. This phenomenon has been described as coupling or repul- 

 sion. 4 I shall refer to it as linkage. 



It was found, for instance, in sweet peas that when plants with blue 

 flowers and long pollen grains were crossed to plants having red flowers 

 and short pollen grains in the grandchildren, the blues had for the most 

 part long pollen and the reds short pollen. Again when blue and red 

 flowers with erect and hooded standards were used all the red grand- 

 children had erect standards. 



We have met with these same phenomena at nearly every step in our 

 studies of heredity in the fruit fly, Drosophila. Over one hundred 

 mutants have appeared from many of which pure races have been 

 formed. At present we have studied fifty-nine of these sufficiently to 

 show that they fall into three great groups. 



The characters in the first group show sex-linked inheritance. They 

 follow the sex chromosomes. The second group is less extensive. Since 

 the characters in this group are linked to each other we say that they 

 lie in a second chromosome. The characters of the third group have 

 not as yet been so fully studied, except to show that they are linked. We 



4 Bateson and Punnett formerly denned coupling as the association of two 

 factors and repulsion ("spurious allelomorphism") as the condition where 

 two factors are usually not associated in the same gamete. They point out that 

 the same idea is expressed by saying that if two dominants come from one 

 parent and two recessives from the other coupling is observed; but if one domi- 

 nant and one recessive come from each parent, repulsion will be found. For the 

 fly, Drosophila, we have pointed out (Jour. Expt. Zool., 1911) how both these 

 results can be accounted for on the hypothesis that the factors concerned are 

 carried by the chromosomes. Bateson has more recently changed his conception of 

 coupling and repulsion. 



