Chap. 20 THE PHYSICAL BASIS OF HEREDITY 393 



The 3tol Ratio Oemonstroted 



PUREBRED BLACK ^ Jfj^ PUREBRED WHITE 



M 



BLACK GREY CREY WHITE 



Fig. 20.4. The result of crossing fowls of pure lines, one with an unmixed 

 ancestry for black and the other for white feathering; Fj, incomplete dominance 

 of black resulting in dapple gray. Crossing of dapple grays produces a generation 

 {¥.,) in ratio of 1 black, 2 dapple gray, 1 white. The blacks are pure black, and the 

 whites are pure white like their grandparents. (Courtesy, Public Affairs Pamphlet 

 No. 165. New York, Public Affairs Committee, Inc., 1950.) 



ones of the next generation (Fi). Mendel had experience with incomplete 

 dominance for he crossed pure early flowering peas with pure late flowering 

 ones and produced an Fi generation of plants with a flowering time half way 

 between those of their parents. 



Mendel's principles have held true. Since his time, thousands of experi- 

 ments have been made in plant and animal breeding and the results of the 

 great majority have upheld his principles. 



Cellular Basis of Genetics 



Chromosomes. In 1902 an American biologist, W. S. Sutton, pointed out 

 that chromosomes are mechanisms that carry out the Mendelian principles. 

 It may be well to review the characteristics of chromosomes in connection 

 with their role in genetics (Figs. 20.5, 20.6). The behavior of chromosomes 

 shows a striking parallel to the dominant and recessive body characters. The 

 chromosomes of the body cells are paired; so are dominant and recessive 

 characters. A character is an inherited quality, e.g., the color black. A factor 

 is the gene or genes that are responsible for it. A gene is a minute part of 

 a chromosome. Factor and gene are used as synonyms. Experimental cross 



