[OG bub]

The overdominance hypothesis, proposed independently by East and by Shull in 1908 and 1909, postulated a physiological stimulus to development that increases with the diversity of the uniting alleles of the same locus in heterozygotes-that is, that many single loci exist at which the phenotype of heterozygotes is superior to that of corresponding homozygotes. In Mendelian terms this implies that there are single genetic loci at which particular heterozygotes are superior to corresponding homozygotes and that such stimulus increases with greater differences between the interacting alleles in heterozygotes. This idea has been variously designated as single-gene heterosis, superdominance, or stimulation of divergent alleles; however, the term usually applied to the concept has been overdominance. East elaborated this idea further in 1936 when he proposed a series of alleles at individual loci a1, a2, a3, a4.... of gradually increasing divergence in physiological function; thus, an a1a2 heterozygote was postulated to be less vigorous than a1a3, < a1a4, and so forth. At the time East formulated this hypothesis there was no direct evidence that any heterozygote lay outside the range of the corresponding homozygotes. Single-locus "heterosis" (Aa > AA, aa) has subsequently been inferred from measurements of seed size in heterozygotes versus homozygotes at some known major loci (e.g., chlorophyll mutants in barley) and from estimates of the fitnesses of heterozygotes relative to corresponding homozygotes in populations of corn, barley, beans, and other species. Others argued, however, that such apparent overdominance might be due to closely linked loci, an issue that is difficult to resolve experimentally. Regardless, clear-cut cases of single-locus heterosis have not appeared to this day, and the absence of such cases has been a deterrent to general acceptance of the overdominance hypothesis.

Another perhaps equally widely accepted genetic hypothesis to explain "inbreeding depression" and the apparent conversely related phenomenon of "hybrid vigor" also had its beginnings soon after the rediscovery of Mendel's work. This hypothesis, proposed by Davenport (1908) and Bruce (1910), started with the widely accepted postulate of the time that cross-fertilizing populations are made up of large numbers of genetically different individuals, many carrying deleterious recessive alleles concealed in heterozygotes. When such individuals are inbred, there is perforce an increase in homozygosity and various morbid homozygous recessive types appear, including plants that have defective flowers, defective seeds, lack of chlorophyll, and the like. These types are unable to survive or to reproduce effectively and so are quickly eliminated from inbred populations. Other characters also came to light that do not necessarily lead to immediate extinction of the afflicted individuals in inbred populations (e.g., dwarfness and partial chlorophyll defectiveness), but nevertheless impose serious handicaps. The appearance of such types on selfing appeared to provide an explanation for part of the injurious effects of inbreeding. Segregation occurs at a decreasing rate within lines as inbreeding progresses and sooner or later brings about near homozygosity, ultimately resulting in lines that carry different alleles and different complexes of alleles of different loci. Some lines by chance apparently receive more favorable and fewer unfavorable alleles than others, accounting for the differences observed in degree of inbreeding depression in different lines. Inbreeding depression, according to this hypothesis, is therefore not intrinsically a process of degeneration but a straightforward consequence of Mendelian segregation. Thus, the injurious effects of inbreeding are not produced by the process of inbreeding itself, as believed by many early biologists and philosophers, but the magnitude of such effects are directly related to the number and kinds of Mendelian "factors" in the original population. Under this hypothesis the intercrossing of inbred lines should sometimes lead to the formation of hybrids in which deleterious alleles, usually recessives contributed by one parent, are immediately hidden, as they were in the original open-pollinated stock by alleles contributed by the other parent. The precise degree of response to crossing should therefore be a function of the genotypes of particular inbreds. Some genotypes, when crossed, should complement each other nicely to produce hybrids better than the average of the original open-pollinated variety, whereas others might not "nick" well owing to the particular unfortunate combinations of alleles they happened to receive by chance during the haphazard segregation that occurred during the inbreeding process.

Objections to the dominance hypothesis were quickly raised on two grounds. First, if the hypothesis were correct, it should be possible to obtain inbred lines homozygous for all favorable dominant alleles. Such lines should be like the F1, in vigor, and they should be true-breeding. However, high-yielding inbred lines were not found in very extensive inbreeding experiments. Jones (1917) reconciled this apparent discrepancy by pointing out that many Mendelian alleles affect growth and that each chromosome would be expected to carry many such alleles. In addition, any single chromosome would be expected to carry some favorable dominants and some unfavorable recessives. A series of precisely placed crossovers and fortunate segregation distributions would, therefore, be required to obtain all favorable alleles in one gamete, a process that would almost certainly require very large numbers of sexual cycles to achieve. Thus, Jones was apparently the first to recognize the important point that short chromosome blocks, not single alleles, are the units of genetic transmission in sexual reproduction. The second objection to the dominance hypothesis was directed at the apparently symmetrical distributions that were observed for heterotic phenotypes during inbreeding. If heterosis were due solely to dominance of independent genetic factors, distribution curves for such characters should be skew rather than symmetrical, because dominants and recessives would be expected to be distributed according to expansion of the binomial (3/4 + 1/4)<N>. Jones reconciled the symmetrical distributions actually observed with the dominance hypothesis on the basis that linkage between favorable and unfavorable alleles within chromosome blocks should be counterbalancing and thus should lead to symmetrical distributions. However, Collins (1921) pointed out that even in the absence of linkage within chromosome blocks, skewness would be difficult to detect if many loci were involved and, moreover, that the chances of recovering any completely homozygous types would be small so long as the numbers of genes involved were at all large. These several ideas came to be called the dominance or dominance of linked genes hypothesis.

[ethereal]

very important info! tagged for breakfast read! thx again for all the wonderful stuff u have 'stickied'

[nycdfan042]

yea great read thanks..

[beta]

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