EVOLUTION, HEREDITY, EUGENICS 521 



(c) The cross bred may have a character entirely different from 

 either parent. The hybrid may possess its own character. 



In the case of the Blue Andalusian fowl we have a good example. 

 The parents are: Dominant — black; recessive — white with black 

 splashes. The hybrid is blue. When two hybrids are crossed the 

 offspring are found to be )4 black; ^ splashed white; and }4 blue. 



MendeVs Seco7id Law: The Law of Segregation or Puj-'ity of 

 Gametes^ Called the Law of the Splitting of Hybrids. — Both body 

 cells and germ cells of the Fi parent, before reduction divisions, 

 contain the determiners (genes) of both alternative characters and 

 are hybrid in character, but during maturation the alternative 

 genes (allelomorphs) are segregated, and we have the law that the 

 hybrid, whatever its character, produces ripe germ cells which 

 bear only the pure characters of one parent or the other. A gamete 

 then is never hybrid with reference to any single character. 



Dihybrids. — The usual Mendelian proportion for crosses where 

 the parents have two pairs of characteristics each is: 9 : 3 : 3 : i. 



YR crossed with GW results in offspring in the (first filial) 

 generation, YR (GW). The germ cells are of four types, excluding 

 the obviously impossible RW and YG,'" and may be arranged in 

 squares to indicate the crossing: 



Sperm gametes YR: YW: GR: GW: 

 Egg gametes YR: YW: GR: GW: 



YR YR YR YR 

 YR YW GR GW 



YW YW YW YW 

 YR YW GR GW 



GR GR GR GR 

 YR YW GR GW 



GW GW GW GW 

 YR YW GR GW 



While there are sixteen possible combinations, called genotypes, 

 we must note that recessive characters are latent in the presence of 

 dominant, so that we have but four types of peas so far as appearance 

 goes. These are called phenotypes (see page 538). 



The possible combinations, when added together, result in 9 YR 

 plus 3 YW plus 3 GR plus I GW (YWGR is really a YR, and so on). 



1" Every germ cell is pure as regards any given character. (See Second Law.) 



