MUTATION IN MATTHIOLA 



263 



fact is that the parental (smooth-leaved) type appears not in three- 

 fourths of the progeny, but in only about one-fourth. 



The extracted Snowflake parents tested behave like pure recessives, 

 showing no influence of their smooth-leaved ancestry. Only the 

 aberrant ratio seems inconsistent with the assumption that the smooth- 

 leaved individuals tested were ordinary heterozygous dominants. 



The relatively weak growth of this type and the apparently poor 

 germination of the seed produced by it suggest that normal segregation 

 may be masked by selective elimination. Possibly the smooth-leaved 



TABLE 25 

 Smooth-leaved type: heredity. Summary. 



* Respectively 63.8 and 22.7 per cent of the numbers of seeds planted. 



factor is lethal when homoz'ygous, as is often the case (Muller, 1918) 

 with dominant mutant factors in Drosophila; the data for germi- 

 nation, however, indicate that two-thirds of the mature embryos can 

 hardly belong to the mutant type. "We might expect, in view of the 

 weak growth of smooth-leaved plants, that partial elimination of 

 heterozygotes would also occur. That this is the case is suggested, 

 though the numbers are small, by the lower proportion of the mutant 

 type with poor germination (table 25; see also tables 39 and 40) ; it 

 should be noted, however, that transferring the first lot of table 24, 

 the only lot between 50 and 73 per cent, to the "poor" total, makes 

 the percentages practically identical. 13 



See also table 2 and the second paragraph under "Occurrence of Mutants." 



[121] 



