Pleiotropism, Penetrance and Expressivity 



69 



follows: two strains of Drosophila were ob- 

 tained that were practically identical genetic- 

 ally (isogenic) except that one was pure for 

 the gene for dull red eye color (h'+) and the 

 other was pure for its allele white (vr). Then 

 some other trait was chosen for examination 

 in these two strains, a trait which is appar- 

 ently unconnected with that for color of 

 eyes. The trait selected was the shape of an 

 organ, located internally, called a sperma- 

 theca, which is found in females and is used 

 to store the sperm that they receive. The 

 ratio of the diameter to the height of this 

 organ was determined for the two strains. 

 This index of shape was found to be signifi- 

 cantly different in the dull red as compared 

 to the white strain. From this result it can 

 be concluded that the eye color gene studied 

 is pleiotropic. The results of other studies 

 have shown that many different genes are 

 morphologically pleiotropic. 



Another example ^ may be taken from 

 Drosophila. There is a recessive lethal gene 

 ^ Based upon E. Hadom's work. 



called lethal-translucida which causes pupae 

 to become translucent and die. Using suit- 

 able techniques, it is possible to compare the 

 kinds and amounts of chemical substances 

 in the blood fluid of normal larvae and pupae, 

 with those found in the recessive lethal homo- 

 zygotes (Figure 10-1). When this is done, 

 some substances are found to be equal in 

 amount in both genotypes (peptide III), others 

 are more abundant in the lethal than in the 

 normal individual (peptide I, peptide II, and 

 proline), still others are less abundant (glu- 

 tamine) or absent (cystine) in the lethal. 

 This case illustrates that pleiotropism can 

 occur at the biochemical level. 



One of the most instructive studies of 

 pleiotropism involves the genetic disease in 

 man called sickle cell anemia. Homozygotes 

 for a certain allele show the following differ- 

 ent effects, either singly or in any combina- 

 tion: anemia, enlarged spleen, heart trouble, 

 paralysis from brain damage, kidney trouble, 

 and skin lesions. As a consequence, homo- 

 zygotes for the gene for sickling usually die 



FIGURE 10-1. Pleiotropism at the biochemical level. 

 {After E. Hadorn.) 



PEPTIDE I PEPTIDE II CYSTINE PEPTIDE III GLUTAMINE PROLINE 



