The Genetic Situation

The kinds of foods that are grown are determined by their desirability for direct consumption or by their suitability for consumption after manufacturing operations. Each particular kind of food exists in many varieties, some of superior, some of inferior, quality, and new, superior varieties are continually being produced by the breeder. It is an advantage to the breeder to have as much information as possible about the separate variable properties which are concerned in the over-all quality of a product. In the previous section we have discussed some of these properties, and the various ways in which flavonoid compounds may affect them. 

Some of the genetic variations in which flavonoid compounds are concerned can be discerned by mere visual inspection such variations, for instance, as the presence or absence of red or yellow color in the skin or flesh of fruits others, such as astringency, can be discerned by simple organololeptic test. But when, as is frequently the case, the genetic situation is complicated, it may be important to the breeder to know, for instance, exactly what pigments are present in a particular strain, or how to detect variants possessing undeveloped characters which may be elicited in further crossings. 

Their occurrence is determined by the genes A, B, I, and Y, as indicated. A and B differ in the depth of anthocyanin coloration they determine. In an abnormal white form, still another pigment, which gives a deep mahogany color when fumed with ammonia and which is possibly a flavanonol, is present. The genetic status of this form has not yet been ascertained, but it may perhaps represent the recessive condition, abyi. 

A particularly important aspect of Lawrence and Scott-Moncrieff s work is that they showed the following interactions of genes  

The position in D. variabilis is complicated by the fact that it is a polyploid, and each gene can therefore be multiplex. Some of the genes are additive in their effects, others fully effective in the simplex condition. Similar or even greater complexity can be expected in large numbers of cultivated plants, so many of which are hybrid and polyploid in their genetic makeup. 

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Quite in contrast are the comments on the Tibetan manual by Lama Anagarika Govinda. His opening statement at first glance would cause a Judaeo-Christian psychologist to snort in impatience. But a close look at these phrases reveals that they are the poetic statement of the genetic situation as currently described by biochemists and DNA researchers. 

Yellow flesh color is due to carotenoid pigment (McKinney, 1937), so that the genetic situation as between white and yellow flesh does not concern us here. 

Mitochondria are unique organelles in that they contain their own DNA (mtDNA), which, in addition to ribosomal RN A (rRNA) and transfer RN A (tRNA)-coding sequences, also encodes 13 polypeptides which are components of complexes I, III, IV, and V (Anderson et al., 1981). This fact has important implications for both the genetics and the etiology of the respiratory chain disorders. Since mtDNA is maternally-inherited, a defect of a respiratory complex due to a mtDNA deletion would be expected to show a pattern of maternal transmission. However the situation is complicated by the fact that the majority of the polypeptide subunits of complexes I, III, IV, and V, and all subunits of complex II, are encoded by nuclear DNA. A defect in a nuclear-coded subunit of one of the respiratory complexes would be expected to show classic Mendelian inheritance. A further complication exists in that it is now established that some respiratory chain disorders result from defects of communication between nuclear and mitochondrial genomes (Zeviani et al., 1989). Since many mitochondrial proteins are synthesized in the cytosol and require a sophisticated system of posttranslational processing for transport and assembly, it is apparent that a diversity of genetic errors is to be expected. 

One property expected of the genetic material is a constancy of amount in every cell of the body under every environmental situation. DNA, not RNA or protein, fulfills this expectation. Its content per nucleus is the same in every cell except the germ cells, which have exactly half that found in the somatic cells. Again, this is expected if progeny obtain half their characteristics from each parent. This constancy is so dependable that the measurement of the DNA concentration in a tissue can be used to calculate die number of nuclei and thus the number of cells. This works well for diploid cells such as those of the kidney, but corrections must be made for polyploid mammalian liver or cancer cells. 

In deciding what to put in and what to leave out, preference has been given to items that are thought likely to be useful to the reader or give an understanding of the current situation. This leads to information on nutrition being included while the genetics of yeast have been left out. 

Mutations may also occur in a single cell within an early embryo. As all the cells divide during growth and development, the individual will have some cells with the mutation and some cells without the genetic change. This situation is called mosaicism. 

Fig. 7 Genes coding for PVA degradations are organised in an operon structure. The scheme (simplified from Kawai [70]) shows the situation in two well-studied PVA-degrading strains Pseudomonas and Sphingopyxis). The genetic organisation in other strains has not yet been examined in such detail...

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