Multiple Alleles; Mtdtigenic Traits 



59 



I B i X U and 12 is I "I" X w. Note that wc 

 have made a new supposition with regard 

 to genes. In the former case the alternative 

 allelic form of / is /', whereas in the latter 

 case it is / /; . This must mean that / ' and / /; 

 are also alleles. The results of marriage 

 types 7 and 8 confirm this hypothesis, the 

 heterozygote I A I B showing no dominance and 

 appearing as AB blood type. All the results 

 indicated in the table are now explained ge- 

 netically. Note that l A and /" produce qual- 

 itatively different antigenic effects. We 

 have, therefore, proved that the gene can 

 exist in any one of two or more alternative 

 genetic states, so that a gene can have mul- 

 tiple — different — alleles. Of course, any one 

 person normally carries no more than two 

 of all the multiple alleles possible. 



2. Blood Type Isoalleles. It has been 

 shown that persons with A blood type really 

 have one of three different subtypes, result- 

 ing from slightly different allelic forms of 

 /J __/-«, ia2 ja3 Three s iig ht i y different 



allelic alternatives are known also for / /{ , 

 producing three subtypes within the B blood 

 group. Thus, alleles which at first seem 

 identical may prove to be different when 

 tested further. Such alleles are said to be 

 isoalleles. Other examples of isoalleles have 

 been detected because different alleles show 

 varied responses to the presence of non- 

 allelic genes, to environmental changes such 

 as temperature and humidity, or to agents 

 which modify mutation rates. Of course, 

 the number of isoalleles detected will de- 

 pend upon how many different phenotypic 

 criteria are employed to compare alleles, 

 and how small a phenotypic difference is per- 

 ceptible. 



In the case of ABO blood type, it is 

 sometimes adequate to classify individuals 

 on the basis of alleles that produce A-type, 

 B-type, or neither type of antigen. There- 

 fore, only three alleles need to be considered. 

 When one studies the genetic relationships 

 among individuals in detail, however, it is 

 often necessary to deal with all seven alleles. 



figure 5-2. Distribution of ABO blood group 

 phenotypes in different human families. 

 * In some families. 



3. Isoalleles in Drosophila. In different 

 wild Drosophila populations, designated as 

 1, 2. and 3, the venation of the wings is 

 complete and identical. In the hybrids pro- 

 duced by all possible crosses between these 

 populations, the venation is unchanged. 

 This result suggests that all three popula- 

 tions are genotypically identical in this re- 

 spect. The venation in a mutant strain is 

 incomplete, the cubitus vein being inter- 

 rupted (ci = cubitus interruptus) in homo- 

 zygotes (Figure 5-3). Hybrids formed by 

 crosses between ci ci and wild populations 

 1 or 2 have complete venation, so that the 

 gene for normal venation, ci + , in these pop- 

 ulations is completely dominant to ci. But 

 the hybrid between ci ci and wild flies from 

 population 3, c7 + :! ci, shows the cubitus vein 

 interrupted. Furthermore, the lack of domi- 

 nance of ci + : '' over ci can be shown to be 



