Genetics for the clinician / C.A. Clarke.

  • Clarke, Cyril A. (Cyril Astley)
Date:
1964
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics for the clinician / C.A. Clarke. Source: Wellcome Collection.

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    2 GENETICS FOR THE CLINICIAN the patterns being specific for any given chromosome. These are the chromomeres and they are certainly associated with the genes since irradiation of Drosophiia frequently results in a mutation known as 'notched wing' which is often (though not always) paralleled by absence of a specific chromomere in a specific rod-shaped chromosome (Stem, i960). It is necessary now to consider whether the arrangement of the genes is of importance. That this is the case is shown again in Drosophiia where observable inversions or translocations (pp. 22-23) can take place, with the result that some of the genes must have altered their position with respect to some of their neighbours. When this happens their manifesta¬ tions may also be altered, a phenomenon known as the 'position effect'. We next have to see whether the action of the genes can be demon¬ strated to have a physico-chemical basis, and for many years there has been evidence that this is so. For example, the European flour moth nor¬ mally has black eyes, but occasionally a recessive mutant occurs in which the pigment is red. If wild-type (see glossary) organs such as testis, ovary or brain are implanted into 'red' larvae the eyes of the resulting moths are no longer red but black. On crossing such 'implanted' mutant females to mutant males the eye-colour is again wild-type, although normally the mating MdU produce aU red-eyed insects (Kühn et al, 1935). The reason for these results is that the wild-type allele is responsible for an enzyme which enables tryptophane to be oxidized to kynurenine, a diffusible hormone which is the precursor of the dark pigment; in the mutant the oxidative step from tryptophane to kynurenine is blocked, so that red pigment is formed instead of dark. It is interesting to compare this with the effects of a mutation in Man, In alkaptonuria (see p, 221) where the urine often darkens on standing and there is pigmentation in cartilage the abnor- mahty is due to the absence of a normal enzyme, homogentisic acid oxidase. We next have to consider how the cells of the body know what particu¬ lar functions to carry out, and it has been shown that the instructions originate in the deoxyribonucleic acid (DNA) of the chromosomes. The evidence is as follows: Certain strains o£pneumococci form 'smooth' (S) colonies and these tend to be virulent and to have capsules with differing antigenic properties. Conversely, the rough (R) strains are nonencapsulated and less pathogenic. As long ago as 1928 Griffith showed that when mice were injected with living (R) pneumococci mixed with killed (S) strains, living (S) pneumococci could be recovered from the animals. In other words, some of the (R) pneumococci had been 'transformed' to the (S) type and the latter continued to reproduce themselves in subsequent generations 'as if a new gene had