MENDELIAN LAWS 



389 



mixed hybrids will continue to breed dominants and recessives in 

 the ratio of 25% dominant, 50% mixed, and 25% recessives. In 

 crossing white Andalusian fowls with black ones, the hybrids are a 

 bluish type (really white and black splashed feathers). The blue 

 hybrids, if crossed, produce 25% black, 50% blue, and 25%, white. 

 The whites will always breed whites, the blacks will breed black, 

 while the blues, if crossed, will con- 

 tinue to give the ratio of 25% black, 

 50% blue, and 25% white. Here 

 black is evidently a dominant and 

 white a recessive character. The 

 production of plants or animals 

 having dominant and recessive 

 character is based on the laws of 

 chance. In hybrid peas (i^i, page 

 386), for example, half the pollen 

 grains would bear germ cells con- 

 taining the determiner of ^'smooth" 

 (R) and half the determiner of 

 " wrinkled " (r), while in the ovule, 

 half would contain the determiner 

 ''smooth'' and half ''wrinkled." 

 Crossing them, we have in the F^ 

 generation, the result shown on 

 page 386. 



Dihybrids and Others. — Though 

 breeding for one pair of characters 

 is comparatively easy to under- 

 stand, we often find breeders cross- 

 ing for two or more pairs of characters. This is purely a matter 

 of mathematics (on paper) as the diagram on page 386 shows, but 

 it is too difficult to study in an elementary course in biology. At 

 the present time most of the really valuable work in plant and 

 animal breeding is being done by this method of Mendel. Not 

 only does this enable breeders to fix the unstable hybrid char- 

 acters, but also it enables them to combine worth-while characters 

 such as immunity to diseases of various kinds, early ripening, 

 color or size of fruit, and many other characters. 



H. NEW CIV. BIOL. — 26 



Law of segregation iHu^l^rated in 

 three generations of rats. Here the 

 gray color is a dominant character 

 and the white is recessive. (Explain 

 from the text.) 



