MENDELISM 1(H 



5. SOME FURTHER INSTANCES OF "MENDEL'S LAW" 



Since the rediscovery of Mendel's law the ratio of 

 3 to 1 in the second hybrid generation has been found 

 by a number of different investigators to be constant in 

 a large array of characters observed both in animals 

 and plants of diverse kinds when these are cross-bred 

 with reference to the characters in question. 



Botanists have an advantage perhaps in this matter, 

 as they deal with forms which usually produce a large 

 number of offspring from a single cross, a very desir- 

 able condition in estimating ratios. On the other hand, 

 they are handicapped by being unable usually to obtain 

 more than one generation in a year, while zoologists 

 may secure from animals like rabbits and mice several 

 generations in a year, although ordinarily the number 

 of progeny is much smaller and the ratios obtained 

 have a larger chance of error than is the case with the 

 more numerous plant offspring. 



Semi-microscopic animals, as, for example, the pom- 

 ace fly, Drosophila, which produces a large progeny 

 every two weeks or so, may combine the general ad- 

 vantages mentioned for the two groups of organisms 

 indicated above, but they have the disadvantage of 

 being so small that the detection of their distinctive 

 phenotypic characters is attended with considerable 

 technical difficulty. 



What the modern experimenter in genetics desires is 

 an organism, first, which possesses conspicuous distinc- 

 tive somatic characters, and, second, that will come to 

 sexual maturity early and breed either in captivity or 



