52 



Heredity and Eugenics 



characters, short and dark hair, but able to transmit the 

 alternati\T conditions, long and white hair. At repro- 

 duction by such an individual, L will separate from S, and 

 W will separate from D, passing into a different cell prod- 

 uct, but it will be a matter of chance whether L is asso- 

 ciated with W as originally, or with D. Hence the chances 

 are even for the production of the four kinds of gametes 

 showm in the diagram, LW, SD, LD, and SW, the visible 



expression of which would 

 produce individuals in 

 character, respectively long 

 white, short dark, long dark, 

 and short white, as actually 

 obtained by experiment. 



If the individuals crossed 

 are pure and differ in three 

 particulars, color, length, 

 and roughness of the coat, 

 then their grandchildren 

 will be of eight sorts, repre- 

 senting all possible combi- 

 nations of three independent unit-character pairs, each of 

 which has its basis in a different material body in the cell. 

 A great many of the characters of animals and plants 

 behave as simple units in heredity, yielding a 3 : i ratio of 

 dominant to recessive individuals in the second genera- 

 tion from the cross. I have shown that this is true of 

 certain hair characters in guinea-pigs, namely, blackness, 

 roughness, and length of the hair. We have no idea how 

 numerous such characters are until they happen to be lost 

 in one individual or another. Then a new variation, a 

 sport or mutation, is observed. It is by this means, acci- 



FlG. 29. — Diagram to show the result 

 of crossing a long-haired, white guinea- 

 pig (L W) with a short-haired, dark one 

 (SD). 



