Genetics / D.J. Cove.

  • Cove, D. J.
Date:
1971
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics / D.J. Cove. Source: Wellcome Collection.

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    haploid organisms 5 medium. Many différent types of mutant were found. Some for example would only grow when a particular amino acid was added to the medium, some only when a vitamin was added, and so on. They reasoned that the mutation must have led to the strain carrying it being unable to synthesise the particular biochemical that was required for growth. A more detailed analysis, testing the mutant strains for growth on the known precursors of the required bio- chemicals, lead Beadle and Tatum to propose that biochemical mutations resulted in the absence of a single enzyme from the cells of the strain which carried them. Suppose a biochemical X is syn- thesised from a precursor A, through intermediates B, С and D, a four-step process catalysed by four enzymes, a, b, с and d as follows : A typical mutation might lead to a strain being able to grow provided X ox D от С were added to the medium but not if В ox A were added. This growth pattern is consistent with the strain lacking enzyme b. Another mutation might lead to a strain carrying it being able to grow only if X were added, and not if either A, or B, or С or D. This strain appears to lack enzyme d. These examples are illustra¬ tive of many actual cases and often the conclusion can be confirmed by direct enzyme measurements. As a result of their work Beadle and Tatum proposed that genetic information was used by cells to synthesise enzymes. They were not in fact the first workers to have proposed this, but the value of their contribution lies in that they showed that the genetic determination of enzymes was a much more widespread phenomenon than it had been thought to be hitherto. We are now equipped with sufficient detail to consider what can be deduced about the process of inheritance from simple breeding experiments. One of the simplest breeding experiments which can be performed with Aspergillus is to cross a mutant strain to the wild type. The results of such a cross are simple but nevertheless informative. If for example a yellow-spored mutant strain is crossed to the green-spored wild- type strain, the resulting ascospores develop into progeny of two types only, resembling one or other of the two parents. Furthermore these two t5фes are produced in approximately equal numbers. Similar results are obtained if other types of mutant are used. When a strain