Some HGPRT spontaneous revertants were isolated from a mutant line (E2) of V79 Chinese hamster cells and phenotypically characterized. Dot–Blot hybridization with a 32P-Iabelled HGPRT probe revealed an increase in the number of HGPRT sequences in some of these revertants, suggesting the occurrence of gene amplification. Cytogenetic analysis performed in three of these revertants showed a characteristic abnormally banding region (ABR) on the elongated p arm of the X chromosome. In Situ hybridization in one revertant (RHE2) showed that the amplified sequences reside on the p+ arm of the X chromsome in two different localizations. Because of the very probable clonal origin of the revertant, these features indicate that the amplified sequences might rearrange after their integration into the chromosome.

Skin and fleece traits have been characterized in four lines of Merino sheep selected for high- and low-fibre diameter (D±) and staple length (L±) from a medium-woolled flock. Over a period of 20 years, each line responded in the desired direction, producing fleeces composed of thick or thin fibres and long or short wool staples. However, variations in the amounts of wool grown that might be expected from these procedures were compensated by changes in unselected characters. Thus a predicted difference in fleece weights between high and low staple length lines was reduced by an increase in fibre crimp frequency in L− sheep. Similarly, changes induced in fibre diameter in the D lines resulted in small effects on fleece weight in comparison to the large (and inverse) effects on follicle numbers. Towards the end of the selection regime, mean follicle density in D− sheep was twice that of D+ sheep. This intriguing response within the follicle population was examined further: an analysis of the relationship between follicle density and fibre diameter amongst the four lines revealed a highly significant, negative linear correlation. The implication of this statistical association is that the numbers of follicles initiated in skin during foetal life had a direct bearing on the sizes of wool fibres eventually produced. It was concluded that both features must be under the control of a single developmental mechanism. Since the expression of each of the characters is separated in time, the mechanism must be activated during the earlier event, i.e. at or before the phase of follicle initiation.

The number of children produced by a modern woman is usually below her total reproductive capacity and is determined by circumstances other than natural selection. It is, therefore, practically impossible to detect differences in natural fertilities associated with different types (e.g. phenotypes, genotypes) of women. This does not mean, however, that natural selection at the reproductive level cannot at all be detected today. If women of a particular type have high natural fertility, this usually means that they reproduce (become pregnant) at a higher rate than women of a type with lower natural fertility. Hence, when there is a limit on the number of children, women of the first type will reach the limit at an earlier age than women of the second type. As a result, types that have a higher natural fertility should be overrepresented among pregnant women of younger ages and, consequently, underrepresented among older ones, as compared to types with a lower natural fertility. Based on this notion, a model of age-related differences between distributions of types among pregnant women is suggested. The model is applied to data on MNSs-blood group and PGM1 (phosphoglucomutase) types in a sample of pregnant women and an evidence of natural selection at the reproduction level associated with these genetic markers is obtained.

High levels of chromosomal heterosis have previously been detected in Drosophila using the balancer chromosome equilibration (BE) technique, in which single wild-type chromosomes are introduced into population cages along with a dominant/lethal balancer chromosome. The balancer chromosome is rarely eliminated in such populations, showing that the fitness of chromosome homozygotes must be low by comparison with chromosomal heterozygotes. As with all cases of chromosomal heterosis, the underlying cause could either be deleterious recessives at various loci or generalized overdominance. The experiment of the present paper examines the first of these explanations. Population cages containing just two wild-type chromosomes (dichromosomal populations) were set up and allowed to run for many generations. Single chromosomes were then re-extracted from these populations, and their fitness measured using the BE technique. Our expectation was that the gradual elimination of recessive genes from the dichromosomal populations ought to result in an increase in the fitness of such re-extracted chromosome homozygotes. Yet in two replicated experiments we were unable to demonstrate an; unequivocal increase in fitness. We have estimated the rate of increase of fitness under multiple locus dominance and partial dominance models. The principal unknown parameter in these calculations is the selection intensity per locus, s. The expected increase is approximately proportional to s, and we estimate that values of s around 1/64 should be detectable in our experiments. However linkage is expected to reduce the efficiency of the dichromosomal procedure We show by computer simulation that this reduction is by a factor of approximately 2, thus increasing the detectable level of s to approximately 1/32. Consideration of mutation-selection balance models shows that this is a feasible selection intensity only if dominance is nearly complete. Thus we are unable to rule out the notion that the genes responsible for heterosis are maintained by a simple mutation-selection balance, but the experimental results constrain the parameters of such a model to a narrow range.

Identity disequilibrium, ID, is the difference between joint identity by descent and the product of the separate probabilities of identity by descent for two loci. The effects of ID on the additive by additive (a * a) epistatic variance and joint dominance component between populations and in the additive, dominance and a * a variance within populations, including the effects on covariances of relatives within populations, were studied for finite monoecious populations. The effects are formulated in terms of three additive partitions, ηb, ηa and ηd of the total ID, each of which increases from zero to a maximum at some generation dependent upon linkage and population size and decreases thereafter. ηd is about four times the magnitude of the other two but none is of any consequence except for tight linkage and very small populations. For single-generation bottleneck populations only d is not zero. With random mating of expanded populations ηb remains constant and ηa and ηd go to zero at a rate dependent upon linkage, very fast with free recombination. The contributions of joint dominance to the genetic components of variance within and between populations are entirely a function of the η's while those of a * a variance to the components are functions mainly of the coancestry coefficient and only modified by the η's. The contributions of both to the covariances of half-sibs, full-sibs and parent-offspring follow the pattern expected from their contributions to the genetic components of variance within populations except for minor terms which most likely are of little importance.

Evolutionary consequences of natural selection, migration, genotype–environment interaction, and random genetic drift on interpopulation variation and covariation of quantitative characters are analysed in terms of a selection model that partitions natural selection into directional and stabilizing components. Without migration, interpopulation variation and covariation depend mainly on the pattern and intensities of selection among populations and the harmonic mean of effective population sizes. Both transient and equilibrium covariance structures are formulated with suitable approximations. Migration reduces the differentiation among populations, but its effect is less with genotype–environment interaction. In some special cases of genotype–environment interaction, the equilibrium interpopulation variation and covariation is independent of migration.

The mutational load of a multigene family with uniform members was studied by computer simulations. Two models of selection, truncation and exponential fitness, were examined, by using a simple model of gene conversion. It was found that the load is much smaller than the Haldane–Muller prediction under the truncation selection, and that it becomes approximately equal to the value calculated by the formula, nv(1 − q)/(m − nq), where n is the copy number, v is the rate of detrimental mutation per gene copy, m is the truncation point in terms of the number of detrimental genes eliminated, and q is the equilibrium frequency of detrimental mutation. However the equilibrium frequency cannot be analytically obtained. For the exponential fitness model, the load is close to the Haldane–Muller value. When there is no gene conversion, the load becomes larger than the cases with conversion both for the truncation and the exponential fitness models. Thus, gene conversion or other mechanisms that are responsible for contraction–expansion of mutants on chromosomes helps eliminating deleterious mutations occurring in multigene families.