384 



SCIENCE 



[N. S. Vol. XXXIV. No. 873 



SPECIAL ABTICLES 



RANDOM SEGREGATION VERSUS COUPLING IN 

 MENDELIAN INHERITANCE 



Mendel's law of inheritance rests on the 

 assumption of random segregation of the fac- 

 tors for unit characters. The typical propor- 

 tions for two or more characters, such as 

 9:3:3:1, etc., that characterize Mendelian 

 inheritance, depend on an assumption of this 

 kind. In recent years a number of cases 

 have come to light in which when two or 

 more characters are involved the propor- 

 tions do not accord with Mendel's assumption 

 of random segregation. The most notable 

 cases of this sort are found in sex-limited in- 

 heritance in Abraxas and Drosophila, and in 

 several breeds of poultry, in which a coupling 

 between the factors for femaleness and one 

 other factor must be assumed to take place, 

 and in the case of peas where color and shape 

 of pollen are involved. In addition to these 

 cases Bateson and his collaborators (Punnett, 

 DeVilmorin and Gregory) have recently pub- 

 lished^ a number of new ones. 



In order to account for the results Bateson 

 assumes not only coupling, but also repulsions 

 in the germ cells. The facts appear to be 

 exactly comparable to those that I have dis- 

 covered in Drosophila, and since these results 

 have led me to a very simple interpretation, 

 I venture to contrast Bateson's hypothesis 

 with the one that I have to oiler. 



The facts on which Bateson bases his inter- 

 pretation may be briefly stated in his own 

 words, namely : " that if A, a and B, h are two 

 allelomorphic pairs subject to coupling and 

 repulsion, the factors A and B will repel each 

 other in the gametogenesis of the double 

 heterozygote resulting from the union 4b X 

 aB, but will be coupled in the gametogenesis 

 of the double heterozygote resulting from the 

 union AB X ah," and further, " We have as 

 yet no probable surmise to offer as to the 

 essential nature of this distinction, and all 

 that can yet be said is that in these special 

 cases the distribution of the characters in the 

 heterozygote is affected by the distribution in 

 the original pure parents." Bateson further 

 points out that since " sex in the fowls acts as 



^Proc. Royal Soc, Vol. 84, 1911 



a repeller of at least three other factors, . . . 

 some of them may be found able to take 

 precedence of the others in such a way as to 

 annul the present repulsion with subsequent 

 coupling as a consequence." 



In place of attractions, repulsions and or- 

 ders of precedence, and the elaborate systems 

 of coupling, I venture to suggest a compara- 

 tively simple explanation based on results of 

 inheritance of eye color, body color, wing 

 mutations and the sex factor for femaleness in 

 Drosophila. If the materials that represent 

 these factors are contained in the chromo- 

 somes, and if those factors that " couple " be 

 near together in a linear series, then when 

 the parental pairs (in the heterozygote) con- 

 jugate like regions will stand opposed. There 

 is good evidence to support the view that 

 during the strepsinema stage homologous 

 chromosomes twist around each other, but 

 when the chromosomes separate (split) the 

 split is in a single plane, as maintained by 

 Janssens. In consequence, the original ma- 

 terials will, for short distances, be more likely 

 to fall on the same side of the split, while 

 remoter regions will be as likely to fall on 

 the same side as the last, as on the opposite 

 side. In consequence, we find coupling in 

 certain characters, and little or no evidence 

 at all of coupling in other characters; the 

 difference depending on the linear distance 

 apart of the chromosomal materials that repre- 

 sent the factors. Such an explanation will 

 account for all of the many phenomena that 

 I have observed and will explain equally, I 

 think, the other cases so far described. The 

 results are a simple mechanical result of the 

 location of the materials in the chromosomes, 

 and of the method of union of homologous 

 chromosomes, and the proportions that result 

 are not so much the expression of a numerical 

 system as of the relative location of the 

 factors in the chromosomes. Instead of ran- 

 dom segregation in Mendel's sense we find 

 " associations of factors " that are located 

 near together in the chromosomes. Cytology 

 furnishes the mechanism that the experi- 

 mental evidence demands. 



T. H. Morgan 



September 10, 1911. 



