16 Mutations and Evolution. 



Thus it would appear that many recessive Mendelian characters 

 originate either as single individuals from the union of two 

 mutated germ cells, or with a frequency approaching 25% in the 

 offspring of individuals which have in turn arisen as a heterozygous 

 combination of a mutated and a non-mutated germ cell. 



In races or species where self-fertilization is the rule, a viable 

 recessive mutation is very Ukely to " come out " in two or three 

 generations, but in many animals, including man, where a large 

 amount of inter-crossing of strains takes place, recessive mutations 

 may accumulate over a considerable period, and some of them may 

 then come out as soon as in-breeding begins. Thus in the case of 

 cross-breeding organisms, it is impossible to say, when a recessive 

 Mendelian mutation appears, how long it may have been carried in 

 the germ plasm in a heterozygous condition without getting a chance 

 to express itself. The premutation may have taken place many 

 generations earlier. 



As one more case of a presumptive Mendelian mutation in 

 CEnothera we may consider (E. brevistylis. Although it has never 

 actually occurred as a mutation in controlled cultures, yet the fact 

 that it was found by de Vries growing wild with (E. lamarckiana 

 although it ripens practically no seeds, indicates that it must have 

 been derived from lamarckiana, with which its pollen crosses freely. 

 In this way it maintained itself for seventeen years. When seeds 

 can be obtained it is shown to breed true, and the chromosomes 

 number 14. De Vries (1913) found it to behave as a simple 

 Mendelian recessive, and Davis (1918) confirms this from a more 

 extensive series of crosses. He finds that the reciprocal crosses 

 with lamarckiana are uniform and indistinguishable, the characters 

 of lamarckiana being strongly dominant. But measurements show 

 that in heterozygous individuals brevistylis has an influence in 

 shortening the styles, slightly broadening the leaves and bracts, 

 making the bud cones a little thicker and the sepal tips somewhat 

 shorter. The resulting segregation in P a is sharp and complete, 

 but the percentage of brevistylis is usually below expectation in all 

 crosses, owing to brevistylis zygotes being apparently less viable. 



This is a very good example of a type which was able to 

 survive for an indefinite period in competition with its parent form, 

 because while producing practically no viable seeds it was 

 perpetuated by occasional crosses of its pollen giving rise to 

 heterozygous lamarckiana plants. 



