420 CORRADO BAGLIONI 



the ba^i^ of the afcuinulatiiig e\idence of siinilarities in .structure ami 

 amino acid sequence among the human hemoglobin chains. 



Ingram (1961 a) (lc\-eloi)cd an evolutionary sclieme which accounts 

 for the similarities between the hemoglobin chains. In the course of 

 evolution a gene or a chromosomal fragment, carrying a hemoglobin 

 gene, has undergone duplication. Mutations of the two initially identical 

 genes result in single or multiple amino acid substitutions, or possibly 

 in in^•ersions of the amino acid sequence of part of the peptide chains. 

 These events are favored or discarded in the course of natural selection. 

 The duplicated genes then evolve independently, under the selective 

 pressure of the environment, and may be separated by chromosomal 

 inversions or translocations. Several duplications followed by inde- 

 pendent evolution would account for the presence of the four hemoglobin 

 chains, for which an independent genetic control has been shown. The 

 primaiy sequences of the a, (3, y, and S peptide chains have been shown 

 to be determined by different stmctural genes; these genes undergo 

 mutational events which affect only the corresponding peptide chains 

 (see Section IV, A, E, and F). On the contrary, the peptide chains of 

 the minor component Hb-As have been shown to be not under independ- 

 ent genetic control. The chemical studies on Hb-A-, (Muller, 1961) 

 provide evidence that the peptide chains of Hb-A and of Hb-A:j are 

 under the control of the same structural genes and have an identical 

 primary sequence. 



E. Hb-Aa 



This minor component migrates in electrophoresis at pH 8.6 faster 

 than Hb-A, but is not completely resolved from Hb-A in starch block 

 electrophoresis (Kunkel and Wallenius, 1955). Considerable evidence 

 was obtained by these authors that Hb-A.-, is a heterogeneous component, 

 which may be further resolved into at least two fractions. 



Hb-As is most likely a derivative of Hb-A (Kunkel and Wallenius, 

 1955) ; Hb-As increases in amount in old samples, particularly if Hb-A 

 is present as methemoglobin. The rate of labeling of Hb-Ag after Fe"^^ 

 administration was studied by Kunkel and Beam (1957) ; Hb-Aj showed 

 a lower specific activity than Hb-A or Hb-Ao. This and several other 

 observations have shown that the Hb-A,-, fraction, which can be further 

 resolved by column chromatography (Schnek and Schroeder, 1961). 

 contains a mixture of by-products of Hb-A, formed by alterations of 

 the hemoglobin molecule during the aging of the red cells. 



The fingerprint of Hb-A.-, isolated by column chromatography has 

 shown a single difference from the finger])rint of Hb-A (Muller, 1961). 

 A peptide reacting to the ninhydrin and sulfur test was present only in 

 the fingerprints of Hli-A.-j and not in the fingerprints of pui'ified Hb-.\; 



