156 MUTATION. 



when mated to males without G. This proved, that both these 

 animals were homozygous for G, whereas at least one of them 

 must at some time have produced at least one gamete from 

 which G was lacking. In another instance two mice were tested 

 by mating them to animals lacking in the factor F, a gene 

 which fully-coloured mice have, and silvered lack. The male 

 in these test-matings gave 24 young with F, the female gave 

 12 young with F, and none without, when their respective 

 mates were lacking in F. Therefore these two animals must 

 have been both homozygous for F, FF. Nevertheless we found 

 that one of their young, when they were bred together, a 

 male, was heterozygous for this same factor Ff. So here again 

 one of the homozygous, FF animals must have given off at 

 least one gamete, in which this gene was not present. To prove 

 a real case of mutation in animals, it is absolutely necessary 

 to show that an individual, which by suitable test-matings has 

 been proved to be pure for a certain gene, nevertheless produ- 

 ces a germ-cell without it. It might be thought, that strict in- 

 breeding would necessarily show whether in a given group of 

 animals all were pure for a certain gene. But we will see, that 

 the production of a new recessive form can be the result of a 

 perfectly normal Mendelian segregation and still take a num- 

 ber of generations to realize. It even sometimes happens, that 

 one half of the number of the individuals of a group are impure 

 for a gene, which has a marked influence on animals of their 

 biotype, without the production of the corresponding recessive 

 type. These cases are those, in which for any reason two 

 gametes without the gene can not combine. In a umber of ani- 

 mals, it has been noted, that a mutual icpulsion exists between 

 a certain gene and that which females have more than males. 

 If in such a case a female is impure for the gene in question, 

 whereas the male is pure, only two kinds of off-spring are pro- 

 duced, females heterozygous tor the gene, and males pure for 

 it, because all those germ-cells, into which the sex-determining 

 gene enters, and which will ultimately give females, will lack 

 the gene. A male can never be without this gene, or even im- 



