286 EVOLUTION AND GENETICS 



characters are said to be linked. Linkage does not imply that 

 the characters are in any way dependent upon each other or that 

 they are related in any way other than simple association in the 

 same individuals. They may involve very different parts of the 

 body, as in the case of bar eyes and small wings in Drosophila, 

 the fruit fly. Characters may also be definitely associated with 

 sex, and are then said to be sex-linked. Among these are color- 

 blindness in man and various characters in birds, insects and 

 other organisms. 



The effect of linkage is a simplification of the ordinary hybrid 

 ratios, for a group of linked characters give only a monohybrid 

 ratio because of their normal inseparability. 



Modem Investigations. During the twentieth century Men- 

 del's discoveries have been repeatedly verified and considerably 

 extended by breeding experiments. Both plants and animals 

 have been used as materials, and in all cases satisfactory results 

 have been secured, although accuracy is most nearly attained in 

 plants and such animals as produce large numbers of young. 

 Laboratory animals like the guinea-pig, rabbit and rat are suffi- 

 ciently prolific to be useful, but not to give approximately accurate 

 Mendehan ratios unless several litters of the same cross are used. 



According to the law of chance, if a given phenomenon may 

 occur in a number of ways and is allowed to occur a sufficient 

 number of times, it will occur in all possible ways. The allelo- 

 morphs in a monohy])rid can combine in only four ways, three 

 of which are different. If animals produce only single young, 

 obviously one litter cannot express the monohybrid ratio. There 

 is a possibility that a litter of four may do so. A dihybrid ratio 

 cannot be completely expressed by less than sixteen individuals, 

 which is beyond the size of litters produced by most mammals, 

 although the four phenotypes represented in this ratio may 

 readily be produced in snialler litters. 



Corn has come into use in the last few years as a laboratory 

 illustration of Mendclian ratios, and because of the large number 

 of grains on a single ear it usually approximates dihybrid ratios 

 fairly well and monohybrid ratios very closely. Since each seed 

 is a potential plant, the seed characters of a field of corn can be 

 studied in a few ears. The characters available include purple 

 and white aleurone and starchy and sugary endosperm, as well 

 as other color characters. A single ear representing the F2 genera- 



