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NATURE 



[September 23, 1920 



germ-cell is made up of pairs of elements or genes 

 which segregate during maturation. In recent 

 years an entirely unexpected and important dis- 

 covery has been made in regard to segregating 

 pairs of genes (allelomorphs). In an ever-increas- 

 ing number of cases it has been found that there 

 may be more than two distinct characters that act 

 as allelomorphs to each other. For example, in 

 mice, yellow, sable, black, white-bellied grey, and 

 grey-bellied grey are allelomorphs— i.e. any two 

 may be present (as a pair) in an individual, but 

 never more than two. It may be noted, too, that 

 at an early stage in his argument the author 

 rejects the criticism that Mendelian characters are 

 necessarily superficial characters. He also points 

 out what critics of Mendelism sometimes fail to 

 understand, that species differ from one another, 

 not by a single Mendelian difference, but by 

 many. 



Mendel's second law describes the free and 

 independent assortment of the genes. If an im- 

 mature germ-cell contains the factors or genes — 

 "tall," "short," "colour," "white"— then at 

 maturation the genes "tall" and "colour" {sit 

 venia verbis) will go to one daughter-cell, "short " 

 and " white " to the other, or " short " and 

 " colour " to one, and " tall " and " white " to the 

 other. Now the cytological evidence is that each 

 pair of chromosomes, just before the reduction 

 division, consists of a maternal and a paternal 

 member, and there is a free or random assort- 

 ment of 'Some maternal chromosomes to one pole 

 and some to the other, and similarly for the 

 paternal chromosomes. 



But further investigation is disclosing an in- 

 creasing number of cases in which free assortment 

 does not occur. Many characters have been found 

 to keep together in blocks in successive genera- 

 tions instead of assorting freely. This is what is 

 called Linkage, one of the post-Mendelian elabora- 

 tions. 



Correlated with the idea of linkage is that of : 

 Crossing Over, which means an interchange of j 

 blocks of genes between homologous pairs of ! 

 chromosomes. It may be frequent or rare ; it may ; 

 occur in the germ-cells of one sex, but not in ' 

 those of the other; the size of the blocks may 

 vary at different temperatures and for internal 

 reasons ; and a break in one region of the chromo- 

 some may interfere with a break in another 

 region. It is altogether very curious and intri- 

 cate. The genes of a pair do not jump out of one 

 chromosome into the other, so to speak, but are 

 changed by the thread breaking as a piece in front 

 of them or else behind them, but not in both places 

 at once. It is possible that there is a limiting 

 NO. 2656, VOL. 106] 



value for crossing over, and if thij can be estab- 

 lished it may lead to the discovery of the lower 

 limit of the size of the gene. It seems that the 

 crossing over may be effected at the time of the 

 conjugation. 



The data in regard to linkage and crossing over 

 lead to the conclusion that the genes are arranged 

 in linear order, standing at definite levels in the 

 chromosomes and definitely spaced. Another in- 

 teresting suggestion is that the correspondence 

 seen in Drosophila melanogasier between the 

 number of linkage groups and the number of 

 chromosomes may hold good generally. Yet 

 another suggestion is to be found in the curious 

 fact, too frequently illustrated to be a coincidence, 

 that one species may have twice as many chromo- 

 somes as a closely related one. 



In regard to the view that the difference 

 between the sexes is connected with the distribu- 

 tion of particular chromosomes, Prof. Morgan dis- 

 cusses two opposing interpretations. In certain 

 species a female organism develops from a fer- 

 tilised ovum with two X-chromosomes, while a 

 male organism develops from a fertilised ovum 

 with only one X-chromosome, with or without a 

 Y-chromosome in addition. The question is 

 whether the presence of the two X-clyomosomes 

 causes a female to develop, or whether XX is 

 merely an index of sex. According to the author 

 the evidence is now conclusive that sex follows the 

 chromosomes, not the other way round. He 

 shows ingeniously how the chromosome theory 

 of sex may be applied to the interpretation 

 of inter-sexes, gynandromorphs, and allied 

 phenomena. 



The conception of a gene or factor seems to 

 become increasingly complex. A gene is to be 

 thought of as a certain amount of material in the 

 chromosome that may separate from the chromo- 

 some in which it lies, and be replaced by a corre- 

 sponding part (and none other) of the homologous 

 chromosome. But it is now becoming clear that 

 a gene is causally associated with manifold effects ; 

 that different genes may produce characters that 

 are indistinguishable, such as whiteness in 

 poultry ; that each character is the product of 

 many genes, and so on. Moreover, the varia- 

 bility of a character is not necessarily due to varia- 

 bility in the gene; much may be due to variability 

 in the environmental conditions of development. 

 Critics of Mendelism must read this book, not 

 necessarily to criticise less, but to criticise more 

 wisely. 



There are so many illuminating discussions in 

 Prof. Morgan's book that we find it difficult to 

 stop. Regarding mutations it is pointed out that 



