METHODS FOR TESTING LINKAGE 



57 



linkage in question. Consider that two loci A, a and B, b are to be tested for linkage. 

 The double heterozygote is then either ABjab or AbjaB, depending on whether the 

 recessive alleles, a and b, came to the heterozygote from the same parent or one from 

 each parent. In the former case the linkage is said to be in the coupling phase, in the 

 latter in the repulsion phase. 



Taking the coupling phase as an example, one can see that four kinds of gametes 

 may be formed by the double heterozygote, AB and ab, the parental combinations, and 

 Ab and aB, the recombinations. In the absence of linkage the parental combinations 

 and recombinations should occur with equal frequency and the proportion of re- 

 combination should be 1/2. In the presence of linkage the parental combinations 

 should occur in excess of the recombinations and the proportion of recombination 

 should be less than 1/2. The purpose of a linkage mating is to provide a way of 

 estimating the proportion of recombination occurring during gamete formation in the 

 double heterozygote, and of determining whether the proportion so estimated differs 

 significantly from 1/2. 



There are three kinds of matings which provide this information, the double 

 backcross, the single backcross or mixed cross, and the intercross. If there is dominance 

 at both loci, all three kinds of matings produce offspring of the same four phenotypes, 

 A-B-, aabb, A-bb, and aaB-. If these types are represented by the numbers 1, 2, 3, 

 and 4 respectively, the outcome of the various matings can be represented as in table 13. 



Table 13 



Kinds of offspring produced by recombinant and nonrecombinant gametes in 



three kinds of matings 



1 = AB, 2 = ab, 3 = Ab, 4 = aB. 



Heterozygous parent 



Other parent 



Genotype 



Gametes 



Double 

 backcross 



aabb 



Single 

 backcross 



Aabb 



Ab ab 



Intercross 

 AaBb 



AB 



ab 



Ab 



aB 



AaBb 



AB 

 ab 

 Ab 

 aB 



The upper two rows represent the parental combinations if the heterozygous 

 parent is in the coupling phase, and the recombinations if the heterozygous parent is in 

 the repulsion phase. In the double backcross and mixed cross, recombination cannot 

 be detected in the other parent, but in the intercross the other parent is also a double 

 heterozygote and produces detectably different parental and recombinant gametes. 



