SUMMAKYThe probability distribution of the heterogenic (non-identical by descent) fraction of the genome in a finite monoecious random mating population has been derived. It was assumed that in any generation the length of both heterogenic and homogenic segments are exponentially distributed. An explicit expression is given for the expected number of 'external junctions' (sites that mark the end of a heterogenic segment) per unit map length in any generation. The latter necessitates the introduction of two higher-order identity relations between three genes, and their recurrence relations. Theoretical results were compared with the outcome of a series of simulation runs (showing a very good fit), as well as with the results predicted by Fisher's 'theory of junctions'. In contrast to Fisher's approach, which only applies when the average heterogeneity is relatively small, the present model applies to any generation.
A model is proposed for the non-selective displacement of flowering time in closely adjacent plant populations. Numerical results obtained on a single locus model as well as a polygenic simulation model demonstrate that an environmental difference may trigger genetic divergence of flowering tune. This divergence results because there is non-random migration with respect to flowering time, which has effects like those demonstrated by Thoday and Gibson (1970) in an experiment in which selective migration alone gave genetic divergence between habitats with respect to sternopleural bristle number in Drosophila melanogaster.In view of the results it is suggested that the evolution of reproductive isolation may sometimes start through a selectively neutral process, which can secondarily enhance the adaptation to divergent selection regimes in adjacent plant populations.
SummaryIn any partially inbred population, ' junctions' are the loci that form boundaries between segments of ancestral chromosomes. Here we show that the expected number of junctions per Morgan in such a population is linearly related to the inbreeding coefficient of the population, with a maximum in a completely inbred population corresponding to the prediction given by Stam (1980). We further show that high-density marker maps (fully informative markers with average densities of up to 200 per cM) will fail to detect a significant proportion of the junctions present in highly inbred populations. The number of junctions detected is lower than that which would be expected if junctions were distributed randomly along the chromosome, and we show that junctions are not, in fact randomly spaced. This non-random spacing of junctions significantly increases the number of markers that is required to detect 90 % of the junctions present on any chromosome : a marker count of at least 12 times the number of junctions present will be needed to detect this proportion.
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