NA TURE 



117 



THURSDAY, APRIL 6, 1916. 



HEREDITY AND CHROMOSOMES. 



Tiie Mechanism of Mendelian Heredity. By Prof. 

 T. H. Morg-an, A. H. Sturtevant, H. J. Muller, 

 and C. B. Bridges. Pp. xiii + 262. (London: 

 Constable and Co., Ltd., 1915.) Price 125. net. 

 '"PHIS latest book of Prof. Morgan and his 

 ^ collaborators should be sure of a welcome 

 from a wide circle of readers. In his preface Prof. 

 Morgan deplores a tendency to regard heredity as 

 a subject for specialists only, and states expressly 

 that the present volume has been written for the 

 biologist at large as well as for those who are 

 more actively engaged in these studies. He has 

 produced a book which should present no difficul- 

 ties to anyone with the elements of a biological 

 training, while at the same time it sets forth 

 clearly and within reasonable compass the latest 

 deductions and speculations of genetic research. 



Prof. Morgan's book is avowedly an argument 

 in favour of what is known as the chromosome 

 theory of heredity. He points out that the mech- 

 anism revealed in the process of the maturation 

 of the germ cells is also a mechanism which 

 fulfils the requirements of the mode of distribu- 

 tion of Mendelian factors. A further argument 

 is provided by the fact that in certain insects two 

 kinds of sperms, differing in the number of the 

 chromosomes which they contain, are associated 

 respectively with the formation of a male and a 

 female individual, and this argument was greatly 

 strengthened when the discovery of the heredity 

 of sex-linked characters provided independent evi- 

 dence that the difference of sex could be expressed 

 in terms of Mendelian factors. 



-As the result of a remarkable series of experi- 

 ments with the pomace fly (Drosophila ampelo- 

 phila) Morgan and his collaborators are able to 

 add a striking piece of evidence in favour of the 

 chromosome theory. In the course of these ex- 

 periments more than one hundred characters of 

 various kinds were shown to exhibit Mendelian 

 inheritance, but the chief point of interest lies in 

 the fact that they fall into four groups. The 

 members of any given group exhibit linked in- 

 heritance with regard to one another, but are 

 transmitted quite independently of the members of 

 the other three groups. The importance of this 

 point becomes evident when it is stated that the 

 number of chromosomes in the pomace fly is four. 

 If the chromosome theory is true and if the num- 

 ber of chromosomes is less than the number of 

 factors exhibiting Mendelian heredity, it is clear 

 that the factors must exist in groups correspond- 

 ing to the number of the chromosomes. The large 

 number of workable factors in Drosophila, coupled 

 with the small number of chromosomes, has 

 rendered possible an exhaustive test of this point 

 such as is at present out of the question for any 

 other species of animal or plant. The result 

 clearly bears out the chromosome theory, and it 

 .IS further strengthened by the fact that the mem- 

 bers of only one of the four groups cf characters 

 NO. 2423, VOL. 97] 



show sex-linked inheritance, these being presum- 

 ably those borne by the chromosome that also 

 bears the sex-determinant. 



There is, however, a complication which Prof. 

 Morgan deals with in a most ingenious manner. 

 When a cross is made between an individual con- 

 taining two factors, A and B, lying in the same 

 chromosome pair and another individual whose 

 corresponding chromosomes contain the allelo- 

 morphs a and b, then in all subsequent genera- 

 tions proceeding from the AB x ab the two factors 

 A and B should always hang together; in other 

 words, there should be complete linkage between 

 them. This, however, is not so, but in Fg there 

 appear a small proportion of individuals which 

 may be represented as Ab, together with a corre- 

 sponding proportion of the form aB ; that is to 

 say, the linkage is generally incomplete. To get 

 over this difficulty Prof. Morgan suggests an 

 explanation based on the observations of Jannsens 

 that at certain stages of meiosis the homologous 

 chromosomes belonging to a given pair twist 

 round one another, and supposes that in a certain 

 proportion of cases the chromosomes break on 

 separation, so that both members of the resulting 

 pair contain a portion of each of the two original 

 chromosomes. This conception of "crossing- 

 over," which is clearly explained and illustrated, 

 involves the supposition that every factor has a 

 definite locus in the chromosome in which it 

 occurs, and Morgan claims that if the values of 

 the crossing-over for A and B and also for B and 

 C have been experimentally determined it is 

 possible to predict the value of the crossing-over 

 for A and C. Indeed, he has been able to con- 

 struct a map of the four chromosomes of Droso- 

 phila showing the positions thus deduced of many 

 of the factors. 



The development of the chromosome theory in 

 its present form is clearly incompatible with the 

 presence and absence theory of factors as usually 

 accepted ; for unless each member of a pair of 

 homologous chromosomes contains the same 

 number of corresponding factors arranged in the 

 same sequence the " crossing-over " could not 

 occur in an orderly manner. Morgan points out 

 that several cases now known are open to the 

 simple interpretation that three factors are in- 

 volved, any two of which are allelomorphic to one 

 another. In Drosophila, for example, red and 

 eosin eye form a simple pair, as also do eosin 

 and white. Nevertheless, red and white also give 

 a simple Mendelian result, no eosins making their 

 appearance in Fo. These systems of multiple 

 allelomorphs, which are not necessarily confined 

 to three members, open up problems of great 

 interest, to which the reader will find a chapter 

 of the book devoted. 



Though Prof. Morgan has succeeded in stating 

 a strong case for the chromosome theory, there 

 are nevertheless some gaps in the argument. AVe 

 do not. for instance, know at present whether 

 Drosophila shows the peculiar twisting of the 

 homologous chromosomes round one another, such 

 as was described by Jannsens in Batracoseps. 

 The existence of such twisting is, of course, essen- 



