294 Morgan, Are the germ-cells of Mendeliaö hybrids "Pure"? 



In my Eorcner paper I made an attempt to aeconni Cor the 

 exceptional behaviour of yellow mice as given by Cuenot <>n the 

 assumption that he was dealing with extracted dominante and 

 dominant-recessives, and therefore his material was contaminated, 

 hence he could not expect to obtain yellow mice having pure yellow 

 gametes represented by the formula YY, but only those represen- 

 ted by the formulae Y(G) and Y(G)B. Nevertheless if the yellow 

 mice behave in inheritanee in the same way as do the other col- 

 ors there should be obtained a race of extracted -dominants, Y(G') 

 that would breed true inter se, provided Cuenot's statement, 

 that yellow always dominates all other colors is correct. In regard 

 to the failure of all yellow mice to breed true it seemed to nie 

 that Cuenot's statement was not clear, for, he laid his main em- 

 phasis on the impossibility of obtaining yellow mice with pure 

 gametes, YY, (which if my Interpretation is correct would not bc 

 the case under the conditions of his experiment), rather than on 

 the failure to get a race of yellow mice, that produce only yellow 

 mice. I wrote to Professor Cuenot and he informs nie that he 

 found that none of his yellow mice breed true, but in all cases 

 throw a certain number of black or of gray or of qhocolate mice, 

 aecording as their ancestryj,ias been contaminated by these colors. 

 This aecords with the interpretation that Professor Wilson gave 

 of his results in Opposition to my own (Science XXIII. 1906), 



Cuenot interpreted his results to mean that extracted domi- 

 nants are not produced in the case of yellow mice. In other 

 words, the first term of the Mendelian proportion is lacking. He 

 offers the following ingenious hypothesis to aecount for this suppo- 

 sed result. Y^ellow-bearing spermatozoa never fertilize yellow- 

 bearing eggs. In other words, selective fertilization of the germ- 

 cells oecurs for mice of this color. He cites, as possibly a 

 parallel case, that of the aseidian. Oiona, studied by Castle and 

 myself, in which self-fertilization does not oeeur, or only in a small 

 number of cases. The results are obviously not exactly the same, 

 for, while the spermatozoa of Oiona do not fertilize the eggs of 

 the same individual, they fertilize readily the eggs of other indivi- 

 duals, while in the yellow r mice the sperm of a given individual 

 fails to fertilize the yellow bearing eggs of all other individuals, 

 on Cuenot's assumption. However, Cuenot meant probably no 

 more than to point to a case where selective fertilization has been 

 shown to take place. 



Why do not yellow mice form a true race as do mice of other 

 colors? Is it really due to the failure of ihc yellow-bearing sperm 

 to fertilize the yellow-bearing eggs. as Cuenot suggests, or can any 

 other explanation bc offered? I could not but feel that the Suggestion 

 of selective fertilization is improbable, and for that reason I made an 



