Human genetics and medicine / by Cyril A. Clarke.

  • Cyril Clarke
Date:
1970
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Human genetics and medicine / by Cyril A. Clarke. Source: Wellcome Collection.

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    24 inversions giving rise to chromosomal polymorphism § 4.5 4.5 Chromosomal inversions giving rise to chromosomal poly¬ morphism Another example of the way in which a collection of genes can operate as a unit is when there is a chromosomal inversion giving rise to chromosomal polymorphism. It has been demonstrated in Drosophila pseudo-obscur a and D. persimilis that chromosome polymorphism can be I maintained by heterozygous advantage (heterosis). Numerous instances I have been found in wild populations in which the numbers of inversion | heterozygotes exceed expectation, assuming equal viability for all three I genotypes. An entirely independent proof of heterosis is provided by the fact that when a population of larvae carrying the inversions is reared in the laboratory under optimal conditions, the three genotypes in the adults have the normal proportions expected from the Hardy-Weinberg law (see page 31). When, however, the larvae are in competition for a restricted food supply it is found that the proportion of heterozygotes exceeds expectation among the adults to which they give rise. I In some cases the polymorphism consists in the number of chromo- ^ somes. For example in Nicandra physaloides^ distantly related to the tobacco plant, the fertilized eggs with one chromosome fewer than normal are subject to delayed germination and the variation so produced is of advantage in contending with environmental fluctuations. 4.6 Heterozygous advantage and the evolution of dominance The usual way in which polymorphisms are maintained is through the selective advantage of the heterozygotes over both homozygotes—^this will keep the alternative alleles in the population. This heterozygous advantage can arise in two ways. First, as ford (1965) very clearly explains, any gene which begins to be favoured must exist almost entirely in the heterozygous state in the initial stages of its increase since there is little chance of matings between the rare heterozygotes. If the mutant gene has a slight advantage, recessive lethals or semi-lethals which are carried on the same chromosome can be sheltered from elimination by their proximity to the advantageous mutant, as long as they are in a heterozygous individual. As the newly successful super-gene increases in frequency, homozygotes appear and will be handicapped by these harmful récessives (now homozygous and conse¬ quently active). Secondly, major genes always appear to have multiple effects and if one of the features for which a mutant is responsible gives it an advantage and others do not, selection will tend to make the beneficial effect dominant and the harmful ones recessive (sheppard, 1953). In these circumstances the homozygotes will have both advantages and disadvan¬ tages since they will be homozygous for the deleterious recessive genes as | well as for the successful mutant, while the heterozygotes will bear the \ advantageous dominant allele but only one dose of the disadvantageous recessive ones. i