276 



Multiple Alleles 



of crossing over. If the recessive morphological gene is coupled 

 with the common self-sterility allele in the male, all the offspring 

 will be dominant if linkage is complete. Thus the cross 

 s'^A / s^a X s'^a / s^A will give s''-A / s^A and s^a / s^A offspring. 



Fig. 81. The results obtained from testcrosses involving the gene bi (bi- 

 colored flowers) in Nemesia strumosa and the self-sterility alleles if there is 

 no common self-sterility allele as in the cross s^ bi / s~ bi X s^ Bi / s^ bi and 

 if a common self-sterility allele is coupled with Bi as in 5I bi / s^ bi X ^^ 

 Bi / s^ bi or with bi as in s^ bi / s^ bi X s^ bi I s^ ^i- The expected ratio in 

 the offspring is different in each of the three crosses. The reciprocal of 

 each cross, however, would yield a 1 : 1 ratio. 



If linkage is not complete, the ratio will be greater than 3:1. 

 Figure 81 illustrates linkage between self-sterility alleles and 

 the gene for bicolor in Nernesia strumosa. 



If testcrosses are made between Aa and aa individuals, and if 

 these genes are linked with the self-sterility alleles, the results 

 of reciprocal crosses may be different. If the heterozygote is 

 the male, and if linkage is complete, the offspring will all be Aa 



