CAUSE OF HYBRID VIGOR 179 



Certain factors have even been recognized, and in the 

 case of Drosophila ^^^' ^^« have been located in the chromo- 

 some mechanism, which are so injurious that they cause 

 the death of the individuals possessing them, unless pro- 

 tected by the factors being in combination with a normal 

 allelomorph. A well-known case of this kind is the yellow 

 mouse. Lethal factors, in order to be recognized easily, 

 must be recessive in their lethal action and must show a 

 visible effect on the soma when in combination with their 

 allelomorphs, since only in that case can the heterozygotes 

 be detected. In the yellow mouse there is associated with 

 color another effect which causes the death of the animals 

 when they are pure for that factor. This has been demon- 

 strated by the altered ratios obtained. Yellow mice are 

 mated together ; instead of getting a ratio of 3 yellow to 1 

 non-yellow, the ratio is more nearly 2:1; that is, (1) : 2 : 1, 

 in which the pure recessives are eliminated.^^ This as- 

 sumption is further corroborated by actually finding the 

 missing number of animals in stages of dissolution in 

 early embryonic life. Of the more than one hundred and 

 twenty-five mutations which have been described in Dro- 

 sophila by far the greater majority of them are recessive, 

 and nearly all of them are less favorable to the develop- 

 ment of the fly than the wild-type characters. The effect 

 of the recessive factors even seems to be cumulative, be- 

 cause when many of them are combined together the flies 

 are extremely difiScult to maintain. Much the same con- 

 dition is true for domesticated animals and plants. The 

 majority of the variations which have occurred are reces- 

 sive, and are seldom beneficial and often deleterious. 

 / One may be led to inquire why it is that most of the 

 experimentally observed mutations are recessive and less 



