The effect of subdivision of a population on response to artificial directional selection for abdominal bristle number in Drosophila melanogaster was compared using large, replicated lines. Three different population structures were compared: (i) selection in an Undivided, large population with 50 pairs of parents (treatment U); (ii) selection in each of 10 sublines which were reconstituted every 6th generation by Crossing after Culling the 5 lowest sublines (treatment CC); and (iii) selection in each of 10 sublines which were reconstituted every 6th generation by Crossing after Retaining all 10 sublines (treatment CR). At the end of three cycles of selection and crossing, neither CR nor CC was superior to U; sublining did not increase response to selection. These results agree with the predictions arising from an entirely additive model and provide no evidence for the presence of epistasis.

A comparison of 50-pair lines (U) with several 5-pair lines was made over 31 generations. For the 50-pair lines, there was close agreement between response predicted from the base population (using ih2σp) and observed response throughout all 31 generations of selection. Although the best of the 5-pair lines exceeded the 50-pair lines in the early generations, average response to directional selection in the 5-pair lines soon fell behind that predicted from ih2σp, and soon reached a plateau.

The theory of path analysis is extended by considering a multivariate system of correlations from a dual perspective. Intrinsic factors exert a unidirectional influence on both the variance and covariance of dependent factors. In contrast, extrinsic factors have a bidirectional influence on the covariance structure of both antecedent and dependent factors and do not influence intrinsic variability. The mathematical model assumes a formally complete linear system of unitary factors. A coefficient is defined to quantify the influence of adventitious associations and is called a copath. Copaths are compared to path coefficients and to correlations due to common antecedents. The chain properties of these coefficients are derived along with a general formula and computational algorithm. The method is illustrated for multifactorial inheritance in extended pedigrees in the presence of different types of assortative mating.

A natural population of Atlantic salmon, Salmo salar L. was screened for genetic variation at 59 protein loci, using a sample of parr, the juvenile freshwater stage. The mean heterozygosity per locus, estimated at 0·033 ± 0·014, is similar to that described in other salmonid species but low for fish species in general. Variation was observed at AAT, IDH, MD–ME, GLO, ADA, and SDH loci. The methods described should prove useful in stock discrimination and in producing salmon for restocking and sea-rearing. The observed extent of gene duplication is discussed in relation to the evolution and systematics of the salmonid fishes.

Mutations in the gatB gene as well as mutations in the putative structural gene gatA and the positive acting regulatory gene intA can affect γ-amino-n-butyrate (GABA) transaminase (EC 2.6.1.19) levels in the ascomycete fungus Aspergillus nidulans. Partial or complete loss of function mutations in gatA, gatB and ssuA, which specifies succinic semialdehyde dehydrogenase, can lead to accumulation of ω-amino acids resulting in pseudo-constitutivity and elevated expression of (retained) activities under intA control. These regulatory effects underlie selective methods for gatB−, ssuA− and leaky gatA− mutations. However, all three gatB− alleles which have been selected lead only to partial loss of GABA transaminase activity as judged by both in vivo and in vitro criteria. It has not been established whether the leakiness of these three gatB− mutations is an allele-specific or a locus-specific effect and whether or not the GABA transaminase present in gatB− strains differs from the wild type enzyme. Thus the role of the gatB product remains to be elucidated. The gatB gene is not closely linked to any other gene involved in ω-amino metabolism or related pathways.

Linkage disequilibrium between segregating amino acid sites in repeated genes which form a multigene family was investigated by using the population genetics theory. The degree of non-random association of amino acids is measured by the disequilibrium coefficient which is determined by the balance among various forces; unequal and equal crossing-over, mutation, random genetic drift and recombination which takes place between the two sites in question. Another measure of disequilibrium, ‘standardized identity excess’, represents excess probability of simultaneous identity at the two sites over that expected from random association of amino acids. Sequence data of variable region of immunoglobulins provide most interesting example of multigene family to apply the theory. Statistical analyses on identity excess for various groups and subgroups of variable region sequences of immunoglobulins suggest that a multigene family such as human κ or mouse κ gene family consists of several subfamilies between which recombination is limited. The analyses also indicate that the recombination may take place between any gene members in one subfamily.

The hypothesis is advanced that the evolutionary stability of the unusual sex-determining mechanisms of the Wood Lemming (Myopus schisticolor) and of the Varying Lemming (Dicrostonyx torquatus) is a direct consequence of certain characteristic features of their population dynamics, and that these include phases of unrestrained population growth and of mass dispersal. Computer simulations confirm the feasibility of such an explanation. Predictions of this hypothesis are found to differ in a potentially testable manner from those of the ‘inbreeding’ hypothesis of Stenseth (1978). The demonstration of such a direct link between population ecology and evolutionary genetics would, if substantiated, be exceptional in mammals.

twPa−1 is a new lethal t-haplotype isolated in the South of France. Homozygous embryos die at the time of implantation or shortly there-after following general disorganization. twPa−1 complements all previously known t-haplotypes and has thus to be considered as the first representant of a new complementation group. twPa−2 and twPa−3 are viable, male sterile haplotypes, but twPa−2 is low transmitter.