692 General and Applied Biology 



completely dominated. In so-called incomplete dominance the Fi does 

 not resemble either parent exactly for the trait in question, neither gene 

 of the pair completely dominating the other. An example is the four- 

 o'clock flower (Fig. 339) in which homozygous white is crossed with 

 homozygous red and the Fi is pink. The genetic content and ratios are 

 shown. Note that the phenotype ratio in the F2 is identical with the 

 genotype ratio. Incomplete dominance, with one pair of genes, is illus- 

 trated by the blue Andalusian fowl (Fig. 340). The Fi shows incom- 

 plete dominance by being neither black nor white but an intermediate 

 shade called ''blue," which is always heterozygous. When two blue fowls 

 are crossed, the offspring show a ratio of 1 white : 2 blue : 1 black. When 

 a blue and black are crossed, the ratio is 1 black: 1 blue; when a blue 

 and a white are crossed, the ratio is 1 white: 1 blue. 



8. Multiple Genes and Interaction of Genes. — There are many traits 

 which are determined by more than one pair of genes and the specific 

 methods of inheritance vary, but the following may give a general idea 

 of some of these genetic phenomena. When a quantitative character 

 (one with various degrees of trait expression) is the result of several, 

 duplicate, cumulative genes, such genes are known as multiple genes 

 (multiple factors). The hair color in wild rabbits is the result of no 

 less than thirteen pairs of different genes located in various chromosomes. 

 Some of these genes are recessive, but a majority in this case are domi- 

 nants. 



Another example of multiple genes (more than one pair for a trait) is 

 the production of skin color in which Negroes differ from whites in two 

 pairs of genes. These two pairs of genes interact cumulatively and show 

 incomplete dominance. Explanation: Negro, AABB; dark mulatto, 

 AABb or AaBB; medium mulatto, AaBb, AAbb, or aaBB; light mulatto, 

 Aabb or aaBb; white, aabb. Using these gene symbols, if a pure Negro 

 (AABB) and a pure white (aabb) are crossed, the Fi offspring are 

 medium mulatto (AaBb) . Another crossing may be shown as follows: 



Parents Medium mulatto (male) X Medium mulatto (female) 



(AAbb) (AaBb) 



i ^ 4. i \ 



Gametes Ab AB Ab aB ab 



Fi 1 dark mulatto (AABb), 2 medium mulattoes (AAbb) (AaBb), 



1 light mulatto (Aabb). 



When three pairs of cumulative genes interact and possess incomplete 

 dominance, there result various degrees of trait expression. For example. 



