THE ORIGIN OF VARIATION 



Brown 



Black 



Parents 



b|; Gametes (|B| 

 X 



Figure 71. A Monohy- 

 BRiD Cross Between 

 Brown and Black- 

 coated Guinea Pigs. 

 ( From Villee, "Biology, 

 The Human Approach," 

 3rd Ed., W. B. Saunders 

 Co., 1957.) 



F. Generation 



M) °'' C0) Gametes (M) or (Jb 

 X 



Eggs 



F^ Generation 



Black 



Block 



Block 



« 7i^jU~^ 



chance, deviations from them within the Hmits of statistical probabiHty 

 are obtained. 



An example may be discussed with the aid of Figure 71. In guinea pigs, 

 black coat color ( B ) is dominant over brown {h). If homozygous strains 

 are crossed, all of the offspring (Fi, or first filial generation) must be 

 heterozygous (Bl)), and black because of dominance. If these are then 

 interbred, each parent will produce two kinds of gametes (B and b) in 

 equal numbers. Random fertilization of the two kinds of eggs liy the two 

 kinds of sperm results in offspring of which 25 per cent are homozygous 

 black (BB), 50 per cent are heterozygous (Bb) and black because of 

 dominance, and 25 per cent are brown (bb). Thus the F2 generation con- 

 sists of 3 blacks to 1 brown. An example without dominance is aff^orded 

 by flower color in Four-o'clocks. If red flowering plants (RR) are crossed 

 with whites (rr), then the Fi plants are all hybrids (Rr) and are pink- 

 flowering. Again, these produce pollen and ovules of two types (R and r) 



202 



