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THE AMEBIC AN NATURALIST [Vol. XL VIII 



We could distinguish such gametes individually from those 

 obtained by the ordinary arrangements only if linkage were in- 

 volved, for then we should sometimes obtain results indicating 

 that the chromosomes containing A and A 1 had crossed over 

 with one another, and so had probably behaved as homologous 

 chromosomes at the reduction division. However, we could also 

 determine whether this mode of pairing occurred or not merely 

 by determining the relative numbers of the different kinds of 

 gametes formed. For, if the third type of pairing occurred, we 

 should obtain 4Aa gametes in addition to the 2 A A, 4Aa and 2aa 

 derived from the other two types of pairing. The ratio of 

 gametes would then be five containing a dominant to one reces- 

 sive, there being 1AA : 4Aa : laa, as opposed to the ratio 

 1AA : 2Aa : laa obtainable on either of the other modes of segre- 

 gation. 4 The latter or more usual ratio is the only one considered 

 by Gregory, who apparently takes it for granted that in so far 

 segregation must be of the same sort as in diploid forms. 



Let us see which ratio is more in accord with his experimental 

 data. As the ratio of offspring in a back-cross is the same as the 

 gametic ratio, it will be seen that a back-cross of — - by a reces- 

 sive should give 3 A : la plant on Gregory's view, the 3A's consist- 

 ing of 1 : 2 . On the other view, a back-cross should result 



in 5A : la, the 5A 's consisting of 1 — — : 4 - -. On inbreeding an 

 — — plant, however, owing to the random fertilization of 

 gametes, Gregory's 3:1 gametic ratio would result in a 15:1 

 ratio among the offspring (which correspond to F 2 ) and our 

 own 5 : 1 gametic ratio would give a 35 : 1 ratio of A to a among 

 the offspring. 



A summary of his back-crosses of P x heterozygous thrum-eyed 

 plants of the type to recessive pin-eyed plants - a gives 

 the result 61 thrum: 6 pin (10:1, as compared to the two ex- 

 pectations 3:1 and 5:1). Among the F x thrums there should 



