Chapter 5 



MULTIPLE ALLELES; 

 MULTIGENIC TRAITS 



A L 



ll the phenotypic results dis- 

 cussed in preceding chapters 

 can be explained genetically 

 by dividing the genetic material into gene 

 pairs. Since no proof has been presented 

 that a particular gene can occur in more than 

 two different states, one could maintain at 

 this point that the only alternative for a given 

 gene (causing round seeds) is the absence of 

 that gene (thus causing wrinkled seeds). 

 This may be called the presence-absence 

 view of gene alternatives. It is clear that it 

 requires finding a gene with more than two 

 alternatives to prove that not all mutations 

 remove an entire gene, and that genes can 

 mutate to alternative gene forms. If, as as- 

 sumed, genes arise only by gene replication, 

 one would expect to find multiple alleles, 

 since each individual carries many different 

 nonallelic genes, many of which must have 

 been derived from a common ancestral gene 

 in past evolution. 



Multiple Alleles 



1. Human Blood Types. Numerous family 

 studies of blood type provide us with data 

 regarding the number of alternatives possible 

 for an allele. However, before discussing 

 these studies, it is necessary to learn what 

 is meant by a blood type or blood group. 



Human blood contains red blood cor- 

 puscles (cells) carried in a fluid medium, 

 the plasma. The corpuscles carry on their 

 surfaces substances called antigens, whereas 

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the plasma contains, or may form, substances 

 called antibodies. An antibody is a very 

 specific kind of molecule capable of reacting 

 with and binding a specific antigen. This 

 reaction may be visualized as a lock (anti- 

 body) which holds or binds a particular key 

 (antigen). If a rabbit is injected with suit- 

 able antigenic material — foreign red blood 

 cells, for example — to which it has never 

 before been exposed, certain antibody-pro- 

 ducing cells of the rabbit will manufacture 

 an abundance of antibodies, which will ap- 

 pear in its plasma. Some of these antibodies 

 will be used to react specifically with the 

 antigenic component of the foreign red blood 

 cells. If, on some later occasion, the same 

 antigen is injected into the rabbit's blood- 

 stream, specific antibodies will already be 

 present to bind the antigen. The antigen- 

 antibody complex then formed often causes 

 the blood to clump, or agglutinate. It is 

 simple to arrange the procedure so that this 

 reaction may be observed in a test tube or 

 on a glass slide. 



Red blood corpuscles from different peo- 

 ple are injected into different rabbits, with 

 the result that the rabbits form antibodies 

 against the antigens introduced. The iso- 

 lated rabbit's blood, centrifuged free of cells, 

 can then serve as an antiserum, containing 

 antibodies that will clump any red blood cells 

 added to it carrying the original types of 

 antigens. It is found 1 that two very dis- 

 tinct antisera are formed by these rabbits, 

 and that any person's blood cells tested with 

 these two antisera can react in one of three 

 ways: the red blood cells are agglutinated 

 or clumped either in one antiserum (arbi- 

 trarily called anti-M), or in the other (called 

 anti-N), or in both of these antisera. All 

 people can be classified by their blood cell 

 antigens as belonging to either M, or N, or 

 MN blood group, respectively. 



1 Based upon work of K. Landsteiner and P. 

 Levine. 



