HEREDITY AND VARIATION 



in Drosophila. Conversely, a single pair of genes may influence more than one 

 character. In Drosophila the genes for rudimentary wings affect characters of 

 the legs and the number of eggs laid. 



Genes are the functional units that determine inheritance in organisms. 

 At least two allelomorphic genes can be associated with the appearance of 

 each heritable character of an individual. In many instances it is known 

 that three or more allelic states of the gene for a particular character exist. 

 No more than two of these multiple allelomorphs are present in a particular 

 individual. There may be many pairs of allelomorphic genes that interact 

 to produce a given character. A single pair of genes may also influence the 

 appearance of more than one character. Certain environmental factors 

 during development of a character limit the effect of the genes. 



Microorganisms such as bacteria and molds are now widely used in studies 

 concerning the nature of the gene and the way in which hereditary factors 

 produce their effects; such investigations are sometimes known as physiological 

 genetics. The possibility of exact control and the ease of modification of their 

 environment, together with their rapid rate of reproduction, make these 

 simple plants desirable material for genetic studies. This serves to point up 

 the fact that no fundamental diflferences in the mechanisms of heredity and 

 variation have been found to exist between animals and plants, unicellular 

 or multicellular. The tentative hypothesis concerning the nature of genes 

 proposes that they are molecules of desoxyribonucleic acid (DNA), possibly 

 complexed with distinctive protein moieties. It should be understood that the 

 number of structurally unique DNA molecules is, theoretically, almost un- 

 limited, as is true of protein molecules. These distinctive chemical units, 

 the genes, are conceived to be held together in linear order by physico- 

 chemical forces. The effects of the genes arranged in these gene strings, or 

 genonemata, are believed to be conditioned not only by their own unique 

 chemical structure but, also, by that of contiguous hereditary units, as well 

 as the reactant substrate outside the chromosomes. Long chain reactions 

 occurring between the constituents of the cytoplasm in any particular region, 

 triggered by an enzyme produced by one particular gene and modified by 

 enzymes formed by other genes, are thought to result in the differentiation 

 of the hereditary characteristics of organisms. You may want to refresh your 

 memory concerning enzyme-directed chain reactions in cells by referring 

 again to Figure 2.9, p. 36. 



Human Inheritance 



Man's characteristics, determined by genes at more than 5000 loci, are 

 inherited from generation to generation, as are those of other living organ- 

 isms. The course of heredity is well understood for many characteristics, 

 and much information has been accumulated. For the inheritance of human 

 eye color there is a pair of genes, the dominant member of which must be 



197 



