October 22, 1920] 



SCIENCE 



389 



quency in our cultures of this si)ecies during 

 the past six years. The majority of these 

 12 mutants have been already briefly described 

 or fignred elsewhere.^ 



The twelve have certain characteristics 

 which distinguish them from the normal stock 

 from which they arose. They are of feebler 

 growth than normals and have a relatively 

 high degree of pollen sterility, while pollen 

 from normals is relatively good with less than 

 5 per cent, obviously imperfect grains when 

 examined in unstained condition. The breed- 

 ing behavior of the twelve is peculiar in that 

 the mutant character is transmitted almost 

 entirely through the female sex. Usually 

 about one quarter or less of the offspring only 

 from a given mutant reproduce the parental 

 mutant type. The jwllen entirely fails to 

 transmit the mutant character, or transmits it 

 only to a small percentage of its offspring. 

 This is concluded from the fact that normal 

 female plants crossed with mutant pollen pro- 

 duce no mutant offspring or only a small per- 

 centage, and from the fact that the pollen of 

 any of the 12 mutants seems to be no more 

 potent in reproducing the mutants than pollen 

 from normals. 



Another type of mutant, provisionally 

 called "liTew Species" because of the diffi- 

 culty or impossibility of crossing it with 

 normals has relatively good pollen and breeds 

 true. 



A study has been begun by the present 

 authors of the relationship which exists in 

 Datura between the cytological condition and 

 the related phenomena of mutation and Men- 

 delian inheritance. The cytological findings 

 are based on counts of over 350 groups of 

 chromosomes. We can confirm the report of 

 others as to the presence of 12 pairs of 

 chromosomes in the somatic cells of normal 

 jimsons. The somatic number is accordingly 

 twenty-four in contrast to the gametic num- 

 ber twelve. Chromosomal counts from the 

 first division of pollen mother cells show that 

 the gametic number in all the 12 mutants is 



1 Blakeslee, A. P., and Avery, B. T., "Mutations 

 in the Jimson Weed," Jour, of Heredity, X., 111- 

 120, Pigs. 5-15, March, 1919. 



apparently 12 and 13 giving a calculated 

 somatic number of 25 instead of the 24 found 

 in normals. Whereas in normals all the 

 gametes have 12 chromosomes, in our dozen 

 mutants presumably half the gametes have 12 

 and half have 13 chromosomes. Apparently 

 in the 13-chromosome gamete the extra chro- 

 mosome is brought in by a duplication of one 

 of the regular twelve. 



The suggestion lies near at hand that each 

 of our 12 mutants is associated with, if not 

 actually determined by, the duplication of a 

 different individual chromosome to make up 

 the calculated total of 25 characteristic of 

 their somatic cells. 



If each of oiu- dozen mutants is character- 

 ized by the presence of an additional chromo- 

 some in a definite one of the 12 chromosome 

 sets, it should be possible by breeding tests 

 to identify the mutant which has as its extra 

 chromosome the one which carries the gene 

 for any particular Mendelian character. This 

 we apparently have been able to do for two 

 of the twelve sets. 



The mutant Poinsettia (1) which appears 

 to be caused by a duplication of one of the 

 chromosomes carrying determiners for purple 

 or white flower color will serve as an example. 

 Poinsettia plants have 2 chromosomes in all 

 the sets except in the one carrying the gene 

 for flower pigtnentation, which has three. 

 Considering only the latter, we may have 

 Poinsettia mutants, as regards their purple 

 pigment, either triplex PPP, duplex PPp, 

 simplex Ppp or nulliplex ppp. 



A duplex purple Poinsettia with the formula 

 PPp should, if the chromosomes assort at 

 random, be expected to form egg cells of the 

 following types: 2P i+ p -^ pp + 2Pp. The 

 pollen grains should have the same consti- 

 tution; but, since the Poinsettia character 

 fails to be carried by the pollen to any signifi- 

 cant extent, the effective male gametes are 

 2P -|- p. Combining male and female gametes 

 in selfing we expect the following zygotes: 

 4PP + 4Pp -F pp -h 2PPP 4- 5PPp 4- 2Ppp. 

 The zygotes with 2 chromosomes in the set 

 are normals, the zygotes with 3 chromosomes 

 are Poinsettia mutants. We should have 



