292 Varietäten, Descendenz, Hybriden. 



of the following C, R, H, K, HK, CH, CK, CHK, RH, RK, 



RHK, or they may lack all. 



When a hoary plant is heterozygous in regard to any of the 



four factors C, R, H, K, it will yield, on seifing, a mixed offspring 



in which the proportion of hoary to glabrous will depend on whether 



the parent was heterozygous in 1, 2, 3, or 4 of the factors. 



Thus if the parent is heterozygous in 1 factor there will be 3 H to IG 



„ „ „ ., „ „ 2 factors „ „ „ 9 H „ 7 G 



j) •) i) ji » n^ „ „ „ „ // £1 „ oi vj 



v » o ji n >» "+ D )) II „ ol n ,, I/O Li 



Abundant evidence had been previously brought forward re- 

 garding the first two of the above ratios; in the present paper 

 work relating to the third and fourth cases is discussed. Theagree- 

 ment between expectation and the results actually obtained is 

 sufficientl}'' convincing. 



The inter-relations between the half-hoarjr stock and the glab- 

 rous or fully hoary types are interesting in that they reveal for 

 the first time a difference in the relations of the two factors H and K 

 to a third factor. In the results hitherto obtained, H and K could 

 be interchanged for one another provided the substitutions were 

 made throughout. Such is not the case in meetings where the half- 

 hoary form is employed. 



The relations found to occur from breeding experiments may 

 be explained under the following scheme. 



1. The half-hoarj'- type lacks H but contains K together with 

 an additional factor (J) which reacts with R to produce thecondition 

 of hoariness characteristic of this type. 



2. When the amount of the dose of J is equal to that of K, H 

 being absent, the result is the condition termed half-hoary. 



3. When, on the other hand, one dose of J goes with a double 

 dose of K, H as before being absent, the result is to dilute the 

 effect of J and to produce the condition described as „quarter-hoary". 



4. The converse case, where the dose of J is double that of 

 K, still awaits experimental investigation. 



5. When the JK pair is combined with H, the colour pair, CR, 

 being also present, the füll degree of hoariness due to the com- 

 bination CRHK masks the presence of J. 



6. Whether J reacts with K independently of C and R, or other- 

 wise, has yet to be determined. 



A convenient table is appended to the paper giving a restate- 

 ment of earliest experiments. W. Neilson Jones. 



Trow, A. H„ A criticism of the hypothesis of „Linkage" 

 and „Crossing over". (Journ. Genetics. V. p. 281—297. 1916.) 



This paper is a criticism of the hypothesis of linkage and 

 crossing over as set forth in „The Mechanism of Mendelian Here- 

 dity" by Morgan, Sturtevant, Muller and Bridges. The author, 

 whilst paying tribute to the skill and industry with which a great 

 deal of valuable experimental data has been collected, rejects the 

 hypothesis of „linkage and crossing over". He argues that, although 

 superficially simple, the difficulties involved in the „linkage and 

 crossing over" hypothesis become very great as soon as it is 

 attempted to explain the more complex Mendelian ratios thereby. 



The conditions obtaining when there are one, two, or more 

 crossings over are analysed in considerable detail and both mathe- 



