04 



GENETICS IX RELATION TO AGRICULTURE 



example peloric snapdragons with flowers flesh-colored on yellow are 

 least frequent, but they all breed true on self-fertilization. In the case 

 of the most frequent class, however, the zygomorphic red on ivory only 

 one plant in 27 is homozygous for the three factors involved and con- 

 sequently would breed true. These are features of Mendelism which 

 have a direct practical application. 



Multi-factor Hybrids. Very few cases have been worked out which 

 demonstrate conclusively, that more than three pairs of independently 

 Mendelizing characters were involved. Not only are the experimental 

 difficulties in such cases too great, but the scientific interest attached to 

 them is not considerable. From a scientific standpoint accuracy of analy- 

 sis is of chief importance, and accuracy is best attained by working with 

 small numbers of factors at a time. 



Little and Phillips, however, have conducted an experiment involving 

 four pairs of independently Mendelizing factors in mice. The factors 

 and the character expressions which they produce are listed below : 



A factor for agouti coloration. In this type of coloration the pigment is disposed 

 in bands in the hairs giving the peculiar gray or agouti coloration of the wild mouse. 

 The allelomorph o conditions a uniform distribution of pigment in the hairs. 



B a factor for black coat color. In this experiment the allelomorph b conditions 

 the production of brown coat color. 



D a factor for intensity of coat coloration. Animals with the factor D were full 

 colored, whereas those with d were "dilute" colored. 



P a factor for eye coloration. P conditions a dark eye coloration; the allelo- 

 morph p pink-eye color. 



TABLE XIV. FOUR-FOLD FACTOR SEGREGATION OP MICE (From Little and Phillips) 



