166 



GENETICS 



The crosses which were made are represented in 

 the table below, in which the expectation according 

 to the Mendelian dihybrid ratios is given in paren- 

 theses after the actual results of each cross. 



It will be seen that the actual results, even when 

 such small totals are concerned, approximate very 

 closely the expectation and are entirely consistent. 



e. Castle's Brown-eyed Yellow Guinea-pigs 



Recently Castle has shown that in guinea-pigs 

 there is an independent factor for extension of pig- 

 ment distinct from the uniformity factor already 

 mentioned. The absence of this extension factor 

 ("restriction ") is manifested by a lack of black 

 or brown pigment everywhere except in the eyes 

 and to a slight extent in the skin of the extremities, 

 while the distribution of yellow is wholly unaffected 

 by it. 



That such "extension" and "restriction" factors 

 really exist, is proven in the following way : 



When a brown (chocolate) guinea-pig is crossed 

 with an ordinary black-eyed yellow one, the young 

 are all black pigmented, but by cross-breeding 



