Chapter 9 



MULTIPLE ALLELES AND LETHALS 



li 



N ORDER to explain the transmis- 

 sion genetics of ABO blood group 

 .type it was necessary to postulate 

 (Chapter 5) that a gene can exist in any one 

 of several different allelic forms. Many cases 

 are known where the gene may be one of a 

 whole series of different alleles, so that each 

 of these genes forms a multiple allelic series. 



ABO Blood Group 



Human beings can be classified into A, B, 

 AB, and O blood types based upon a series 

 of three alleles, /•^ F, / (Chapter 5). Of 

 course, any person carries only any two of 

 these alleles. South American Indians are 

 almost all //'. Certain Indian tribes in North 

 America have predominantly A or O blood 

 type. All other populations contain not 

 only these types, but B and AB also. It has 

 been shown that A blood type is really com- 

 prised of three different subtypes resulting 

 from slightly different forms of /' which can 

 be called h^', l^\ I^^. Three different alterna- 

 tives are known also for P, producing three 

 subtypes within the B blood group. Only 

 one type O is known. (Recently, / has been 

 found to produce a unique type of antigen.) 

 Does it seem confusing that we first said 

 there are three alleles for ABO blood type, 

 and then later stated that there are seven? 

 The number of alleles discovered will depend, 

 of course, upon how carefully or in how 

 much detail one wishes to study phenotypes. 

 Let us imagine that the multiple allelic series 

 under consideration produces pigmentation 

 instead of blood antigens. Let /^^ /-^^, 

 62 



/'^•^ produce light, medium, and dark blue 

 pigment, respectively, and P', P-, P-^ pro- 

 duce light, medium, and dark green pigment, 

 respectively, all showing no dominance with 

 respect to each other, but all dominant to 

 /, which produces no pigment at all. For 

 some purposes we may be perfectly willing 

 to classify individuals with respect to their 

 carrying an allele capable of producing blue, 

 green, or no pigment. In this event we would 

 need to recognize only three alleles. But if 

 we wanted to study more detailed genetic 

 relationships among individuals, or the 

 detailed basis for phenotypes, we would need 

 to recognize all seven alleles. So, for many 

 practical purposes, people are classified just 

 as if the ABO blood group had only three 

 alleles. 



It should be recalled that the /-^ and P 

 alleles produce qualitatively different anti- 

 genic effects. 



Eye Color in Drosophila 



One of the series of multiple alleles which 

 occurs in the fruit fly, Drosophila, involves 

 eye color. In this case the different alleles 

 can be arranged in a series that shows a grada- 

 tion effect on eye color, ranging from dull 

 red to white: dull red (h'+), blood (vv*'), coral 

 (vi""), apricot (u"'), bujf {m*-^), and white iyv). 

 The VV+ allele is dominant to the others hsted, 

 and is the allele commonly found in flies 

 living in nature. (Note that a slightly differ- 

 ent system of symbolizing genes is being used 

 here. This and other conventions for the 

 use of symbols are described on p. 111.) 

 You can think of the different alleles as all 

 producing the same kind of phenotypic effect, 

 but less of it in proceeding from h'+ to w, the 

 white allele being completely inefficient in 

 this respect. 



Coat Color in Rabbits 



The multiple allelic series for coat color in 

 rabbits combines properties of both of the 

 allelic series already discussed. On the one 



