MUTATIONS OF GENES 383 



all be located in the same relative position, but of course 

 any particular chromosome can have but one of them. 

 Thus we have in such a case a system instead of a pair of 

 genes, all due to different changes of the same gene. 



Mutations of Genes. — If the wild red-eyed vinegar 

 fly is actually the original type it is clear that all other 

 eye-colors have been derived from it by a change or 

 mutation of some one or more of the genes concerned in 

 the production of red. In the mutations which have 

 given rise to white, cherry, and eosin we have seen that 

 the mutation occurred at the same locus or position 

 in the chromosome. Other eye-colors, e.g., vermilion, 

 have arisen by mutation of other genes in the same 

 chromosome-pair. When white-eyed males and ver- 

 milion-eyed females are crossed the Fi males are 

 vermilion, like the mother, and the Fi females are red, 

 because they receive the normal red gene corresponding 

 to white from the mother and the normal gene corre- 

 sponding to vermilion from the father. An inspection 

 of the accompanying figure (114) will show that they are 

 hybrid for both genes, white and vermilion, and therefore 

 have red eyes, because the normal genes are both domi- 

 nant. Mutations of genes always precede and are in fact 

 the cause of mutations in characters of plants and 

 animals. 



The Gametic Ratios of Hybrids. — The reader 

 already knows that the hybrid between parents differing 

 in a single pair of genes and characters produces just two 

 kinds of gametes, and further, that the hybrid between 

 parents differing in respect to two pairs of genes produces 

 four kinds of gametes in equal number, provided the 

 genes in question are in different pairs of chromosomes 

 and hence not linked. It is easy to go a step further 

 and generalize this. If parents 1 and 2 differ in respect 

 to the independent genes a and a' and b and b', the hybrid 

 has the genotype aa\ bb' and can produce equal numbers 

 of gametes ab, ab' , a'b, a'b'. Now suppose a third pair 



