302 MISCELLANEOUS STUDIES 



The early type is probably due to a single dominant mutant factor 

 segregating normally from the corresponding Snowflake factor; the 

 quantitative nature of its differences from Snowflake, however, makes 

 positive determination of this point a matter of great difficulty. 



At least five other types plainly reproduce themselves, but about 

 50 to 70 per cent of the progeny are usually Snowflake; no true- 

 breeding individual of any generation of any of these types has yet 

 been tested. Genetic work with most of these types has been much 

 hampered or even prevented by low vigor and fecundity, and the 

 aggregate data from progeny of parents of four types strongly indi- 

 cate selective viability at germination. It has been determined by 

 crossing that in three of the types the mutant factor (or factors) is 

 carried both by eggs and by sperms. From these facts it seems prob- 

 able that homozygotes of the mutant types are non-viable, and that 

 severe selective elimination occurs during embryonic development; 

 or, in other words, that the mutant factor is imperfectly recessive for 

 a lethal effect. 



In three types there appears to be linkage with the factor pair for 

 singleness and doubleness of flowers, the mutant factor being coupled 

 with singleness in the tested apparent mutants of two types, and with 

 doubleness in the third type. AA 7 ith two other types these factors 

 seem to be independent. No reversal of coupling has been found in 

 later generations of the former two types, but on the scheme presented 

 crossover singles should be scarce. 



For one type (crenate-leaved) a hypothesis based on the facts stated 

 gives very closely the ratio obtained from selfed parents. Reciprocal 

 crosses with Snowflake conform less closely to the requirements of the 

 hypothesis, but do not definitely contradict it. The slender type, 

 which shows similar apparent linkage, seems to disagree definitely 

 with the hypothesis; there is strong evidence, however, that slender 



individuals may differ genetically among themselves. 



r 



A more complex scheme providing also for the usual origin of these 

 types from Snowflake by segregation is briefly outlined. 



The selfing ratios are very suggestive of duplication of a chromo- 

 some (non-disjunction), as in Oenothera lata, but it is hard to 

 reconcile the cases of apparent linkage with this hypothesis. It seems 

 probable that these three linked types have originated and are trans- 

 mitted in the same general way as the double-flowered type, and that 

 all of these four mutant factors (including double) represent changes 

 of some sort within a chromosome of the same .pair, which may be 



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