2o8 GENETICS 



the parents differ in one or more gene pairs, whether in X, 

 the autosomes, or in Y. 



/. Parents differing in a single pair of autosomal genes. — 

 As seen in the preceding chapter, the red color of the eye in 

 the fruit-fly depends on at least 19 gene pairs scattered in 

 various parts of the four pairs of chromosomes (figure 41 ) . 

 An individual with the usual red eyes has all these genes in 

 the normal or unmodified condition. Suppose that such an 

 individual is mated with another that has all but one of these 

 19 gene pairs in the normal condition. This individual has 

 the two genes of the pair at II, 54.5 (figure 41 ) so modified 

 as to cause the eyes to be purple in color. We may call these 

 two modified genes aa, while the corresponding two unmodi- 

 fied genes may be designated AA. The chromosomes of the 

 two parents will then be represented as in figure 44, P; they 

 differ only in the genes at II, 54.5. 



When germ cells are formed in the usual way by such in- 

 dividuals (one chromosome of each pair passing to each 

 germ cell), and the germ cells from the two parents are 

 mated, we of course find that all the individuals (zygotes) 

 of Fi have both the dominant normal gene A and the re- 

 cessive gene a, so that they have the normal red eyes (figure 

 44, Fi). 



When two of these individuals Aa of Fi form germ cells 

 and are mated in the usual way they yield in F2 individuals 

 of three different genetic constitutions, in the proportions 

 AA -j- 2A« -\- aa, or 3 dominant red to i recessive purple. 

 The red and purple are inherited in the typical Mendelian 

 or autosomal ratios for a single-gene difference, or as "unit 

 characters." 



(It is obvious that to obtain the correct constitutions and 

 ratios in later generations no account need be taken of the 

 parts of the genetic system that are alike in the two parents. 

 All that is required is to represent by different letters the 



