530 - Heredity and Evolution 



of the mutation, as a single recessive gene, 

 can be followed readily in the next genera- 

 tion, when a mutant strain is allowed to con- 

 jugate with a nonmutant. The diploid sporo- 

 cvtes in the sporangium of such a sexually 

 produced mycelium (Fig. 12-10) will possess 

 the mutant gene balanced against a non- 

 mutant allele. But these genes soon become 

 segregated from one another during meiosis, 

 which follows immediately. Therefore, by 

 collecting the individual spores from each 

 sporangium and by culturing the spores sep- 

 arately it is possible to determine that half 

 I he spores possess the mutant (usually defec- 

 tive) gene and half carry the normal non- 

 mutant allele. 



Normally Neurospora grows excellently on 

 a medium that contains only inorganic salts, 

 sugar, and one vitamin, namely biotin (p. 

 353). After irradiation, however, various mu- 

 tant strains are found that cannot grow un- 

 less the mold is provided with one or more 

 additional vitamins, such as niacin, thiamine, 

 etc. The data show that each genie deficiency 

 is inherited as a simple recessive and is re- 

 lated to the loss of a particular enzyme. The 

 synthesis of niacin, for example, passes 

 through a series of intermediates: anthranilic 



Ei E L , E 3 



acid — > — * indole: indole -\~ serine — ► trypto- 



E 4 E, E„ E T 



plwn — » — » ?>-hydroxxanthranilic aeid — » — * 



niacin. Each of these seven steps is catalyzed 

 by a different enzyme (E x , E 2 , . . . E 7 ). But 

 the important point is that each time a single 

 gene is damaged by the radiation, one and 

 only one enzyme is lost in the series. Probably 

 most of the individual genes of an organism 

 carry the code for the production of a par- 

 ticular enzyme. However, some genes may 

 determine the synthesis of other proteins. 



One more example of a one gene, one 

 enzyme relation will suffice, although many 

 others could be cited among higher as well 

 as lower organisms. Certain flounders are 

 known to lack black pigment in their me- 

 lanophores, and this lack, which prevents 

 the fish from changing its shade in accord- 



ance with the background of the environ- 

 ment, is determined by a recessive allele of 

 the normal "pigment gene." Accordingly, 

 when heterozygous pigmented flounders are 

 inbred, they produce pigmented and unpig- 

 mented offspring in a ratio of 3:1. But the 

 significance of this study is revealed by fur- 

 ther data. When the melanophores of the 

 nonpigmented individuals are tested, these 

 cells are found to lack one specific oxidizing 

 enzyme (dopa oxidase), which is present in 

 all normal melanophores. Thus the black 

 pigment, melanin, which is an oxidation 

 product derived from dopa, a colorless pre- 

 cursory compound, cannot be formed in the 

 melanophores of the nonpigmented fish. 

 From this experiment it may be concluded 

 that the normal "pigment gene" achieves its 

 phenotvpic effect in the melanophores of the 

 flounder by determining the production of a 

 specific metabolic enzyme; a similar situation 

 has been found in reference to the formation 

 of black hair pigment in guinea pigs and 

 other mammals. 



Determining, as they do, the enzymes of an 

 organism, the genes are a tremendously po- 

 tent force. They govern not only the metab- 

 olism of the cells of the adult, but also the 

 metabolism of the cells throughout all stages 

 of embryonic development. This, indeed, is 

 generally believed to be the principal mech- 

 anism bv which the complex and beautifully 

 integrated processes of development are 

 guided. However, the presence of a particular 

 gene in a particular cell does not mean that 

 the enzyme templated (indirectly) by that 

 gene must necessarily also be present in the 

 cell. Genes may remain dormant until acti- 

 vated by the conditions of their intracellular 

 environment. A central problem in experi- 

 mental embrvology is, therefore, to find out 

 how the intracellular environment provides 

 for the intricate system of checks, balances, 

 and controls for genie activity throughout 

 development. 



Enzyme Induction Phenomena. A study of 

 enzyme induction, particularly among bac- 

 teria, has provided some clue as to one 



