Genetics 315 



of lighter and darker material can be seen. These bands may be the 

 region of the location of the various genes. This is purely supposition, 

 as the gene has never been accurately identified even by use of the 

 electron niicroscooe. 



CYTOPLASMIC INHERITANCE 



At the time of fertilization, the entering sperm brings very little if 

 any cytoplasm into the egg ; thus the developing embryo has genes from 

 both parents, but essentially has cytoplasm from only one parent, the 

 mother. This poses the interesting problem as to whether or not the 

 cytoplasm carries any hereditary material. 



In the crosses which have been discussed those characters which 

 were carried on the autosomes influenced the offspring equally whether 

 or not they were carried by the male or female parent. In the 

 cross between a vestigial-winged and a long-winged Drosophila, the re- 

 sulting Fi generation will have long wings regardless of which parent 

 carried the gene for long wing. Obviously this trait is completely con- 

 trolled by the genes of the autosomes. 



There are other characters, however, which are not controlled by 

 the chromosomal genes and through careful experimentation have been 

 shown to be carried in the cytoplasm. One example is that of CO2 sen- 

 sitivity in Drosophila. Matings of sensitive and resistant strains of 

 flies resulted in offspring that were not alike in their resistance. Whether 

 or not they were sensitive to the CO- depended upon whether or not 

 the mother was. This sensitivity is controlled by a special gene in the 

 cytoplasm known as a plasmagene. A few animals did result which 

 were not like the mother, but this can probably be explained by the fact 

 that occasionally the sperm does carry some cytoplasm with it. 



Another plasmagene has been studied in Paramecium, a one-celled 

 animal. This plasmagene is functional only in the presence of a gene 

 in the nucleus, thus differing in this way from the plasmagene for CO2 

 sensitivity in Drosophila. The gene in the nucleus, is known as the 

 killer gene, but it can only act to destroy other strains of paramecia in 

 the presence of Kappa material (the plasmagene) found in the cyto- 

 plasm. The killer gene follows the usual methods of inheritance, while 

 the plasmagene is definitely shown to be in the cytoplasm. 



