ON SUCCESSIVE DUPLICATE MUTATIONS. 215 



that exchanges of chromosomes of different pairs but without 

 change in number would take place under these conditions. 

 By such a mismating or exchange of mates on the part of two 

 chromosome pairs, a plant which was homozygous (RR) for one 

 factor would give rise to plants which were heterozygous for 

 duplicate factors (RrR'r'). This is then what has probably 

 occurred in the cases where plants from a 3:1 family have given 

 rise to 15:1 ratios. The frequency with which such mismating 

 occurs in (Enothera may thus be estimated. 



It is known that the chromosomes of (Enothera are in pairs 

 (doubtless of paternal and maternal origin) throughout the 

 somatic divisions, and the paired arrangement is probably a 

 feature of the first mitosis after fertilization. If, then, a plant 

 which would have been homozygous for a single factor (RR) 

 becomes transformed into one which is heterozygous for dupli- 

 cate factors (RrR'r') and so gives a ratio 15:1 in its offspring, 

 the most likely assumption is that at the time of fertilization the 

 two R chromosomes, instead of becoming paired with each other, 

 each paired with another (r) chromosome. Hence in this case 

 the regrouping of chromosomes probably occurred not during 

 meiosis where it would have to occur simultaneously in all the 

 germ cells, but as a feature of fertilization or the first mitosis of 

 the embryo. It will be shown later, however, that mismatings 

 of the chromosome pairs probably also occur during meiosis and 

 so modify the 3:1 ratio. To sum up, it appears that when a 

 15:1 family is derived directly from a plant in a 3:1 family, the 

 remating of the chromosomes must have occurred at fertilization 

 or soon afterwards; but when, for example, a 4:1 or a 5:1 family 

 is derived from a 3:1 family, this may be accounted for by a cer- 

 tain amount of remating of chromosomes during meiosis. 



The method above described will also apply to the origin of 

 duplicate and triplicate factors in wheat and is perhaps more 

 probable than the successive chemical transformation of dif- 

 ferent chromosomes. There is, however, a method of testing 

 between these two possibilities. If the duplicate condition 

 arises through a regrouping of the chromosome pairs, then, as 

 has been mentioned, a race or a plant homozygous (RR) for one 

 factor will give rise to a plant heterozygous for two factors 



