INTRODUCTION TO BIOCHEMICAL SYSTEMATICS 



51 



used to test a number of related species. Gene homology is implied in 

 each case where such antiserum is active against the serum furnished 

 by the test species. One series of species-specific antigens, through 

 appropriate matings, has been defined as the product of genes occupy- 

 ing homologous loci in four different species (Stimpfling and Irwin, 

 1960b). The antigens behave as contrasting characters in backcross 

 hybrids and are considered to be products of genes that had a com- 

 mon origin but underwent subsequent change. They then constitute 

 a series of multiple alleles considered in this situation at the generic 

 level. Additional complexity of the locus, at least in the serological 

 expression, is indicated by the fact that, within a species, variants of 

 the species-specific allele occur. Furthermore, another antigen, in this 

 case from a different genus, has been shown to have some serological 

 affinities with the series discussed above, and it may represent another 

 allelic variant which, if confirmed, would elevate the character to the 

 family level. In this last case, the serological affinities of the allelic 

 series are greater among species of the same genus than with the 

 extrageneric related antigen. 



We hardly know where enzyme studies will have arrived by 

 ten more years. The following discussion is indicative of the course of 

 future progress, and discloses the potential refinements of biochemical 

 systematics. 



In recent years much progress has been made in the analysis 

 of the amino acid sequences within certain protein molecules. The 

 classic example is the work of Sanger on the insulin molecule. Applica- 

 tion of these techniques to genetics appears in the work of Ingram on 

 sickle cell anemia and altered hemoglobin. In a recent book by Anfin- 

 sen (1959), The Molecular Basis of Evolution, some of this work has 

 been summarized. A few of the pertinent facts follow: 



(1) Insulin from five different species has been studied (beef, 

 pig, sheep, horse, and whale), and only insulins of pig and 

 whale were found to be identical. 



(2) Adrenocorticotropic hormone (ACTH) of sheep, beef, 

 and pig has been examined, and that of pig diff'ers from 

 the other two. 



(3) Sheep and beef ribonucleases differ. 



(4) Vasopressin (with only eight amino acids) of beef has 

 arginine while that of hog has lysine.^ 



(5) Ferriporphyrin peptides from cytochrome C of pig, horse, 

 beef, and salmon are alike, but in chicken, serine replaces 

 alanine. 



2 Addendum from A. C. Allison, 1959. 



