Self-Sterility 



275 



because of the elimination of certain gametes. The first case 

 of such linkage was discovered by Brieger and Mangelsdorf, who 

 found that the basic color gene, C, in Nicotiana was incom- 

 pletely linked with the self-sterility alleles. Other cases of such 

 linkage are genes for pollen color and for length of the corolla 



Fig. 80. Results of all self- and cross-pollinations among self-sterile plants 

 of six different genotypes. Three of the plants are homozygous for one of 

 the alleles. S, sterile, and F, fertile combinations. Female gametes at 

 left, male at top. 



tube in Nicotiana, for basic white flowers and for pink flowers 

 in the red clover, and for buff and bicolor flowers in Nemesia 

 strumosa. 



If a pair of alleles, A and a, are linked with self-sterility al- 

 leles, and if two Aa plants are mated, the offspring will segregate 

 into a 3 : 1 ratio if there is no self-sterility allele common to the 

 two parents. If, however, there is a common allele, the ratio of 

 A to a will not be 3 : 1 but will vary according to the manner 

 and degree of linkage. If the linkage is complete, and if the 

 A gene is on the same male chromosome as the common self- 

 sterility allele, there will be a ratio of lA : la. Such a cross is 

 s'^A / s^a X s'^A I s^a. Because of the elimination of the s^ male 

 gametes, the two resulting types of offspring will be s^A / s^a and 

 s^a / s^a. If linkage is not complete, the ratio will be less than 

 3 : 1 and greater than 1 : 1 and will depend upon the amount 



