Genetics for the clinician / C.A. Clarke.

  • Clarke, Cyril A. (Cyril Astley)
Date:
1964
Catalogue details

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

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

    79/438 (page 49)
    SOME EFFECTS OF GENE INTERACTIONS 49 We first of all consider the female forms of a particular race, and Plate 4.1 shows those found in South Africa. Breeding experiments have shown that the differences in the colour and wing pattern between one form and another of this race are controlled by a single gene, and that these are at the same locus and constitute an allelomorphic series similar to the ABO blood groups in Man. At the bottom of the plate is the 'bottom recessive' (No. i) and as we proceed upwards each form is completely dominant to the one below—until we reach No. 5, which is the recognizable hetero¬ zygote between Nos. 3 and 4. Except for this heterozygote we know that dominance is complete because we can show experimentally (unlike the usual situation in humans) that the heterozygotes and homozygotes are identical. We next take insect No. 4 (dark orange) from the South African race (Plate 4.1) and cross it with an extremely different form (pale yellow) found in the Abyssinian race (Plate 4.2, No. i). Plate 4.2, No. 3 shows the resulting hybrid which is clearly a very different insect from either of its parents, and it is abundantly clear that the effects of the gene controlling the dark orange colour of the South African butterfly are quite different from what they were in their native gene-complex. Moreover, it is also clear that there is no dominance of one form over the other. The reason for the difference between butterfly No. 2 and butterfly No. 3 in Plate 4.2 must be that there are modifiers in the former which are perfecting the dominance, and a corollary of this is that the dominance itself must have been evolved. In the particular instance of Papilio dardanus we know why these modifiers have accumulated—it is in order to perfect the mimetic pattern since the butterfly strikingly resembles a similar insect (the model) which is distasteful to birds, and the more complete the resemblance the better will dardanus be protected. It is of interest that these mimetic wing patterns are sex-limited to the female, the males everywhere being similar to one another and quite unlike the females (Plate 4. i, No. 6). The evolution of dominance is bound up with the advantage of the heterozygote. Genes always have more than one effect (pleiotropy) and consequently when a gene previously rare becomes an asset some effects will be advantageous and others less so. Selection will tend to make ad¬ vantageous effects dominant and those unfavourable, recessive. Thus both homozygotes will be at a disadvantage as compared with the hetero¬ zygotes—the homozygous normals because they are without the ad¬ vantageous dominant mutant and those homozygous tor the mutant because they are also homozygous for the disadvantageous recessive effects of the mutant gene (Sheppard, 1953). Here then is a convincing explana¬ tion for the doctrine of heterozygous advantage and the only surprising