MENDELIAN HEREDITY 



281 



In the four-o'clock white flowers crossed with red produce in 

 the Fi generation only pink flowers. Inbreeding of this generation 

 produces all thrtn^ colors in the ratio of 1 white: 2 pink: 1 red 

 (Fig. 160). Animal characters are well exemplified by the classic 

 case of the blue Andalusian fowl. These chickens are produced 

 by crossing black and splashed white Andalusian types, and never 



Ft 



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% 



Fig. 160. — Diagram to illustrate the results of crossing white and red flowered 

 races of four o'clocks (MirahilU jnlapn.) The somatic condition or pheno- 

 type is shown graphically; the small circles represent the genes involved. 

 (From Woodruff.) 



breed true. They are the heterozygous individuals of any genera- 

 tion and the race can be maintained only by constant hybridizing 

 or by the elimination of the homozygous individuals of every 

 generation. The condition of the blue Andalusian has been inter- 

 preted as a mosaic of finely divided colors. 



Dihybrid Ratios. When two pairs of characters are associated 

 in a hybrid the possibilities of reassociation in the F2 generation 

 are greatly increased, since not only the expression of each is 

 variable but the association of the unrelated characters as well. 

 There are four possibilities of distriljution of the characters as 



