Heredity — Genetics 683 



slightly larger than the others. Internally the composition of the cyto- 

 plasm may differ in the two sides of the cells, and there may be more 

 yolk granules in one side (lower) than in the other (upper). Hence, 

 there are two types of cells (smaller, upper ones, and larger, lower ones) 

 with the cytoplasm differing between the two poles of the cells. This is 

 known as differentiation of the cytoplasmic contents of cells, and this 

 phenomenon may lead to additional differentiations within cells and 

 between certain adjacent cells. Hence, the identical genes in the nuclei 

 of the various cells are surrounded by different cytoplasms as well as 

 different gene-products and nongenic contents within the different nuclei. 

 The organization and differentiation of an embryo depend upon genie 

 actions in cells as well as the interactions between cells and between the 

 various regions of the embryo and its surroundings. 



How does genie action produce the traits or characteristics displayed 

 by living organisms? A trait in a developing or fully developed organ- 

 ism may be any observable structure or function such as a biochemical 

 property, a structure or function of a cell, tissue, or organ, a mental 

 characteristic, etc. It is to be expected that no simple connection exists 

 between genes and most observable, developed traits, but usually there 

 may be several steps in sequence in the process. Most traits arise 

 through complex interactions of numerous genes as well as interactions 

 between genes and cytoplasmic influences, so that a single gene may 

 often influence the development of more than one trait. The statement 

 that a trait may depend on the interactions of several genes may seem 

 to contradict the statement that a certain gene is responsible for a trait. 

 A single gene, by being part of this network of developmental inter- 

 actions, then may be indirectly responsible for the eventual development 

 of a particular trait which is to say that a particular gene may not 

 directly develop that trait but does so in an indirect manner. In other 

 words, that particular trait might not have developed specifically as it 

 did if that "one" gene had not been associated in the complex network 

 of gene interactions. If "another" gene had been present instead of the 

 "one," the resulting interactions might have been quite different and 

 the trait developed might have also been quite different. The particular 

 way in which genie and nongenic actions take place in a network of 

 interactions might well be influenced by the presence of a single gene 

 of a specific type. 



5. Mendel's Experiments and Laws. — Gregor Mendel, a monk in 

 Austria, in 1864 gave the first scientific interpretation of the heredity 

 mechanism through his experimental crossings of pea plants in the gar- 



