RULES AND RATIOS OF INHERITANCE 211 



two parents differ in but one pair, out of the many gene 

 pairs on which depends the character examined. Thus in 

 probably all cases in which characters give "unit character," 

 or "single factor" inheritance ratios, the character is in fact 

 affected by many diverse gene pairs, but only one pair differs 

 in the two individuals that mate. The expressions "unit char- 

 acter" and "unit character inheritance" are still much used, 

 but they should always be interpreted as meaning "unit dif- 

 ference"; that is, as inheritance resulting from difference in 

 a single pair of genes. 



But If the parents differ in more than one pair of the genes 

 on which the given character depends, the simple Mendelian 

 or sex-linked "unit character" ratios will not be produced, 

 but more complex ratios. This may be Illustrated for the 

 same character, red eye-color, as follows: 



J. Parents differing in two pairs of autosomal genes, in 

 different chromosomes. — The red-eyed parent is mated with 

 an individual that has two of the 19 eye-color genes modi- 

 fied; one pair in chromosome II at 54.5, giving recessive 

 purple eyes; another pair in chromosome III at 48.0, giving 

 recessive pink eyes (see the maps, figure 38). 



As before, we need to take account only of the genes in 

 which the two parents differ. Call the two recessive genes 

 that give purple eyes aa, the two that yield pink eyes, bb. 

 Then the corresponding dominant genes in the other parent 

 may be called AA and BB, these giving red eyes. It is neces- 

 sary to keep in mind the eye color in the different types of in- 

 dividuals. Those having only the two recessive genes aa 

 have purple eyes; those with only bb have pink eyes; those 

 with both aa and bb have purple-pink eyes. Those with nei- 

 ther pair of recessive genes have red eyes; these are AABB. 



When the red-eyed parent AABB is mated with the 

 purple-plnk-eyed parent aabb, the results are those given in 

 the analysis of page 115, Chapter V, for parents differing in 

 two autosomes. The Fi generation all have the constitution 



