Heredity and Development Second Edition (1972) / Chapter Skim
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6 Genetics - Old and New
6 Genetics - Old and New
Pages 140-151
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From page 140... ...
In the following list, the generally established concepts of classical genetics appear in italics; expansions or exceptions to each concept -- which for the most part have not been mentioned before -- are in roman type.
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From page 141... ...
Some have the expected diploid number but others may have twice this number, (c) Sex chromosomes, as in the case of XO males, offer still another exception to chromosomes existing in homologous pairs, 7.
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From page 142... ...
A fruitful hypothesis, though unproved before 1940, was that each gene controls the production of a specific enzyme, which in turn controls a specific biochemical reaction. From these simple propositions one can deduce most of the phenomena of classical genetics.
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From page 143... ...
Garrod recognized alkaptonuria as a genetic disease and spoke of it as an ‘inborn error of metabolism.' He suggested that individuals with alkaptonuria lack an enzyme. This enzyme is present in normal individuals where it converts homogentisic acid to simpler substances, which are excreted in normal urine.
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From page 144... ...
Very special methods had to be devised to discover intermediate reactions passed through so quickly. It was found, for example, that a poison may produce its effects by destroying or inhibiting a specific enzyme.
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From page 145... ...
The standard procedure for a geneticist had been to determine what the genes do: they produce blue eyes, round seeds, or vestigial wings. It was reasonable to assume that these phenotypic expressions of genes were, themselves, consequences of various biochemical reactions.
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From page 146... ...
Spores from the irradiated colonies were placed on the culture medium that contained the minimum variety of molecules necessary for growth. Most of the spores grew, showing that if mutations had occurred they did not prevent the Neurospora from synthesizing its constituent molecules from the few simple chemicals of the basic medium.
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From page 147... ...
This can be done by crossing the presumed mutant strains to normal Neurospora of the opposite mating type. Spores obtained from such a cross were grown in separate tubes.
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From page 148... ...
Thus the reaction can be abbreviated: If Neurospora has a similar metabolic pathway, one should be able to see how the seven mutants are involved, since the failure of each specific enzyme would have a different consequence. If something was wrong with enzyme Ex, the reaction would end with precursor X;
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From page 149... ...
Instead ornithine probably changes to some unknown intermediate, in a reaction catalyzed by one enzyme, and then from the unknown intermediate to citrulline under the influence of a second enzyme. Finally, one strain was found that would grow only on arginine: neither citrulline nor ornithine would permit growth.
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From page 150... ...
Suggested Readings H.J.Muller's Pilgrim Trust Lecture of 1945 to the Royal Society of London, entitled ‘The Gene,' is reprinted in Readings in Heredity and Development. It is a masterful synthesis of what could be induced about the nature of the gene from the data of classical genetics.
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From page 151... ...
6. The meiotic and mitotic divisions that produce the eight spores of Neu rospora occur in a geometrically exact manner.
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