Heredity

Wood, J T

doi: 10.1038/hdy.1976.61pmid: 1066339

The method proposed by Hardwick and Wood (1972) for relating genotype-environment interactions to measures of environmental variables is extended and two examples are discussed.

Snaydon, R W; Davies, M S

doi: 10.1038/hdy.1976.62pmid: N/A

Populations of the grass species Anthoxanthum odoratum were collected at intervals across two boundaries between contrasting plots of the Park Grass Experiment, Rothamsted. One boundary (8L/9L) had been in existence for 112 years, and the other (1L/1U) for 60 years. Environmental conditions at both boundaries changed to almost the maximum extent within 0.5 m.

Barlow, B A; Wiens, D

doi: 10.1038/hdy.1976.63pmid: N/A

Male plants of V. fischeri have 2n = 23 and constantly produce seven bivalents and a multivalent chain of nine chromosomes at meiosis. Regular assortment results in transmission of 11- and 12-chromosome genomes via the pollen. Female plants have the chromosome number 2n = 22 and are homozygous for the 11-chromosome genome. The multivalent chain in the males is a consequence of reciprocal translocations, one of which was Robertsonian and one of which involved the chromosome carrying the sex determination factors. There is a constant female-predominant sex ratio of approximately 1:2 in V. fischeri, possibly maintained by gamete selection; the genes involved may have been linked with the sex determination mechanism through the translocation system.

Eaves, Lindon

doi: 10.1038/hdy.1976.64pmid: 1066340

Cultural transmission may depend on the non-genetic transfer of information from parent to offspring. The consequences of such cultural transmission for continuous variation are investigated theoretically for randomly mating populations. Cultural inheritance may act on genetical and environmental differences between individuals. The consequences for cultural inheritance of polygenic variation and variation due to chance environmental factors are considered. An equilibrium may occur in which the population variance and the covariances between relatives can be expressed as functions of estimable parameters of genetical and environmental variation. Whatever the ultimate origin of culturally inherited differences they are expected to lead to environmental differences between families (“E 2” variation). In addition, if cultural transmission maintains differences due ultimately to segregation at many gene loci we may find genotype-environmental covariation is generated.

Nagylaki, Thomas

doi: 10.1038/hdy.1976.65pmid: N/A

Two single-locus deterministic models are constructed for the maintenance of genetic variability in localised plant populations. In the first model, it is supposed that self-pollination is complete, heterozygotes are absent, and only seeds disperse. Then the ploidy is arbitrary. In the second, it is assumed that the plants are diploid, cross-pollination is total, and only pollen disperse. Explicit conditions for a protected polymorphism are derived for each model in the diallelic two-niche case. These conditions are simplified, displayed graphically, and discussed extensively for directional selection for different alleles in the two niches. If the amount of seed dispersal is divided by two, the criteria for a protected polymorphism in the self-pollinating scheme become identical to the criteria in the cross-pollinating situation with no multiplicative dominance. The restrictions that must be placed on the fitnesses for a protected polymorphism always become more stringent as the amount of dispersion increases. If the dispersion rate is not very close to zero and the selection coefficients are very small, the protection conditions reduce to overdominance for the mean fitnesses in the habitat. Unless there is complete dominance, protection always exists with fixed fitnesses for sufficiently low non-zero dispersal. For any amount of dispersion, there is always protection in the second model if, in each niche, the fitness of the deleterious homozygote is less than that of the heterozygote.

Jinks, J L; Towey, Philomena

doi: 10.1038/hdy.1976.66pmid: 1066341

A new method, genotype assay, is described for estimating k the number of genes or more strictly the number of effective factors responsible for variation of a continuous kind. The central feature is the determination of the proportion of individuals in the F n generation of a cross between two pure breeding lines that are heterozygous at, at least, one locus by an assay of their F n+2 grand progeny families. The observed proportion is then equated to a theoretical expectation which is a function of the number of genes involved. Expectations generalised to cover any generation n for experimental designs in which every F n individual is assayed by comparing two F n+2 grand progeny families have been derived for two limiting cases; one in which all genotypic differences are expressed as phenotypic differences and the other where the expression is minimised by imposing the maximum internal and relational balancing out of the contributions of individual gene loci. Equating the observed proportion of heterozygotes to these expectations therefore, leads to an upper and a lower estimate of k corresponding with these two limiting conditions. The reliability and sensitivity of the estimates depends primarily on n the generation chosen for study, the number of individuals (m) assayed from that generation and the number of individuals (l) raised in each F n+2 grand progeny family. The two variables m and l being the principal determinants of the variances of the family means set the lower limit to the size of the gene effects that can be detected.

Wright, A J

doi: 10.1038/hdy.1976.67pmid: 1066342

Methods of regression analysis of genotype-environment interaction are considered in relation to existing theory dealing with the relative efficiencies of selection for general or specific adaptation to the environment, and the choice of environments for assessment. The two alternative models involving regression on to environmental effects (model 2) or genotypic effects (model 3) are equivalent when regression lines are concurrent, but are shown to be mutually exclusive when concurrence is absent. Formulae relating the rates of advance under selection for general and specific adaptation are given, and can be used as a guide to the choice of an effective breeding strategy. When model (3) regression is important, then selection for general adaptation will be an efficient strategy but may be further enhanced by the use of environments with high regression coefficients (γ) for assessment. The advance following assessment in a single environment (kth) is expected to be better than that under n randomly chosen environments if γk>√n−1. If model (2) regression is also important (i.e. regression is concurrent), then the best selector environments can be chosen on the basis of their means. If, on the other hand, model (3) regression does not hold, then selection for general adaptation will be inefficient and it is preferable to group the environments to achieve more homogeneity. When model (2) regression holds, then this grouping can be carried out on the basis of the mean expression of the environments.

Tilney-Bassett, R A E

doi: 10.1038/hdy.1976.68pmid: N/A

Thirty-six G × W crosses and 36 W × G crosses are made between six chimera cultivars, containing mutant white plastids in their germ layers, and the six green clones derived from them. The estimated outputs of green plastids and the percentages of variegated embryos are compared by the analysis of variance. The analysis confirms that changes in the pattern of extra-nuclear plastid inheritance are determined to a major extent by variations in the nuclear genotype of the female cultivar, even when the female contains the mutant plastids, and to a minor extent by changes in the nuclear genotype of the male. The behaviour of mutant plastids closely follows that of normal ones, except that the output of mutant plastids is never as great as for the corresponding green plastids either on the female or the male side. Owing to the effects of their different nuclear genotypes the six cultivars can be arranged in order of decreasing maternal strength as MBC > LG > DV > JCM > FS = FoS. Hence some nuclear genotypes permit a greater output of male than female plastids while others permit fewer male plastids and may even eliminate them altogether. It is suggested that the nuclear genes determine plastid inheritance by the selective control of plastid replication so that the output of plastids need have little resemblance to the input ratio.

Bailey, R J; Rees, H; Jones, L M

doi: 10.1038/hdy.1976.69pmid: N/A

Gill, Ayesha E

doi: 10.1038/hdy.1976.70pmid: 783091

Previously unreported white spotting was found in two subspecies of the California vole, Microtus californicus. The pattern of spots on the ventral coat of the animals differs between the subspecies, and there is variation in the expressivity of the white spots. Expression of white spotting is greatly reduced by the epistatic action of another coat colour gene, the recessive buffy (bf). The incidence of white spotting, its variation in expression, and its inheritance were investigated in this study. The reproductive performance of white spotted voles was also analysed, and effects on fertility and litter size were found associated with the trait.