Genetics / D.J. Cove.

  • David J. Cove
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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    8 Patterns of inheritance I: At this point, it is convenient to introduce the term gene. It is best to define a gene as a unit of hereditary information. It will be seen later that there is little point in being more explicit than this. No more precise definition is possible as it would not cover everything which is usually called a gene. In the breeding scheme represented in figure 3, we can say that A and a represent alternative forms of the same gene. Such alternative forms are called alleles. Now let us examine what occurs when we do a shghtly more complex breeding experiment. What happens if two different mutant strains are crossed together? The results are again simple. If for example a yellow-conidiospored strain of Aspergillus is crossed with a slow- growing strain, four classes of progeny are obtained in equal numbers. These classes are the two parental types, one producing yellow conidiospores but having the normal growth rate, and the other producing wild-type, green conidiospores but having a slow growth rate. Two new types are also obtained, one with green conidiospores and a normal growth rate, identical in fact to the wild-type strain from which the two parental strains were derived by mutation, and the other with yellow conidiospores and a slow growth rate, which can be described as being doubly mutant. These two new non- parental types are called recombinants (see figure 4). The first thing to note from this experiment is that with respect to the individual characters involved, i.e. rate of growth and spore colour, all the conclusions that were drawn from the previous breeding experiments still hold. There is for example no blending of information with respect to an individual character, and no yellowy-green conidio- spored progeny, or ones having an intermediate rate of growth occur. It can again be said that the individual alleles of the two genes involved remain distinct from one another during the process of zygote formation and reduction division. What in addition emerges from this experiment is that the formation of a zygote by the gametes of the two different strains, followed by the production of ascospores must provide an opportunity for the shuffling or reassortment of genetic information affecting different characters. Progeny are obtained which have some information derived from one parent and some from the other. A complete set of genetic information is called a genome, and so this last conclusion can be rephrased in more technical terms by saying that progeny derive their genes from both of their parents' genomes. The single character cross did not enable any predictions to be made as to whether this
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