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CHAPTER 10 



as adolescents or young adults; this allele, 

 therefore, almost always functions as a re- 

 cessive lethal. 



It is found that the red blood cells of these 

 homozygotes may become sickle-shaped in- 

 stead of being disc -shaped (Figure 10-2). 

 Sickle-shaped cells may clump and clog blood 



I 



• • 9 





vessels in various parts of the body leading 

 to the malfunctions of all the organs already 

 mentioned; in addition, since these cor- 

 puscles are defective, they are destroyed by 

 the body, which as a consequence becomes 

 anemic. 



We see, then, that the wide variety of appar- 

 ently unrelated phenotypic effects of the gene 

 for sickling are but consequences of the sick- 

 ling of red blood cells. Moreover, studies 

 at the biochemical level show that the sickling 

 behavior itself is the result of the presence of 

 an abnormal type of hemoglobin which sickle 

 cell homozygotes carry in their red blood 

 cells. There is, then, a pedigree of causes for 

 the multiple effects of the gene for sickling. 

 The first cause is the gene, the second is the 

 abnormal hemoglobin it produces, the third 

 is the sickling that follows, the fourth is the 

 subsequent red cell clumping and destruction 

 which produce gross organic defects and 

 anemia. 



In this case all the multiple effects of the 

 gene are attributed to a single or unitary effect 

 which is of a biochemical, perhaps enzymatic, 

 nature. This single effect then affects many 

 varied chemical reactions which are involved 

 in the production of different, at first appar- 

 ently unrelated, traits. We may even hy- 

 pothesize that most, if not all, genes have a 

 single primary phenotypic effect. It may 

 yet be found that the pleiotropic effects de- 

 scribed in the mouse and Drosophila are 

 tertiary or even further removed effects in a 

 pedigree of causes, whose primary cause is 

 genie and whose single secondary cause is 

 still undetermined. Replying to the question 

 with which this section started, the simplest 

 hypothesis is that most, if not all, genes have 

 one primary phenotypic effect following which 

 a pedigree of causes ends in pleiotropism. 



FIGURE 10-2. Silhouettes showing various types 

 of human red blood cells : normal, in normal homo- 

 zygote (A), sickle cell trait, in mutant heterozygote 

 (B), sickle cell disease, in mutant homozygote (C). 



