Genetic exclusion is the ability of a primary infecting phage to prevent a secondary infecting phage from contributing its genetic information to the progeny. The molecular mechanism of the phenomenon is not well understood. The two genes in phage T4 mainly responsible for genetic exclusion are the immunity (imm) gene and the spackle (sp) gene. Evidence is presented that the imm gp enables the host exonuclease V to degrade superinfecting phage DNA. This appears to be accomplished by the imm gp altering gp 2/64, the presumed pilot protein, which protects the 5′ end(s) of the phage DNA. Exonuclease III is also involved in genetic exclusion but its action does not appear to depend upon the imm or sp gene products. Gp sp appears to interfere with the lysozyme activity of gp 5, a component of the central base plug, postulated to aid in tail tube penetration during the injection process. A molecular model of genetic exclusion is proposed. Genes imm and sp are part of a cluster of genes which also includes 42, beta-glucosyltransferase, and uvsX. The genes of this cluster encode proteins apparently adapted for competition and defence at the DNA level. These genes may encode fundamental adaptive strategies found throughout nature.

Phycomyces blakesleeanus wild-type NRRL1555( − ), the standard strain, when crossed with UBC21( + ), another wild type, gives zygospores that germinate in 50–60 days. By backcrossing to UBC21 and selecting for shorter dormancy we have isolated a ( − ) strain, A803, and a ( + ) strain A804, which when crossed give zygospores that germinate in 32 days, the shortest dormancy period found in Phycomyces. The same result was obtained when A803 was crossed with UBC21. Zygospore dormancy decreased as the parental strains became more isogenic with UBC21, but the number of zygospores giving germsporangia with viable germspores also decreased to zero in the third backcross. The existence of germspore-killer alleles in the strain UBC21 is postulated. The strains of shortest dormancy can be used as helper strains (Orejas et al. 1985) in sexual crosses. Tetrad analysis of the cross NRRL1554 × S102, a two-factor cross, showed 90% of reciprocal ditypes plus tetratypes in the progeny, indicating that the ( + ) wild-type strain NRRL1554, when crossed with the standard strain, gives regular meiosis and, contrary to current beliefs, may be used in Phycomyces genetic analysis.

Using two-dimensional polyacrylamide gel electrophoresis (2-D PAGE), the genetic variation of proteins was investigated in three organs (mesocotyl, sheath and blade of second leaf) from two maize lines and their hybrids. Nine hundred and eighty-six spots were numbered over the three organs. One hundred and fifty-five polypeptides were found to be genetically variable, due to allelic polymorphism of structural genes and/or to polymorphism of any genetic elements controlling protein amounts. Of these 155 variants 12% clearly showed complete dominance effects in the hybrid patterns, which could reveal dominance effects in the regulation of the protein biosynthesis. Comparison of the three organs showed that (i) the level of variability between lines depended upon the organ, since it varied from 7·5% for the blade to 12·6% for the mesocotyl and 13·2% for the sheath; (ii) 68 polypeptides displayed different kinds of variation in different organs and (iii) in all cases but one the dominant inheritance was organ-specific.

In this study we genetically analyse a large autosomal region (23 map units) in Caenorhabditis elegans. The region comprises the left half of linkage group V [LGV(left)] and is recombinationally balanced by the translocation eT1(III; V). We have used rearrangement breakpoints to subdivide the region from the left end of LGV to daf-11 into a set of 23 major zones. Twenty of these zones are balanced by eT1. To establish the zones we examined a total of 110 recessive lethal mutations derived from a variety of screening protocols. The mutations identified 12 deficiencies, 1 duplication, as well as 98 mutations that fell into 59 complementation groups, significantly increasing the number of available genetic sites on LGV. Twenty-six of the latter had more than 1 mutant allele. Significant differences were observed among the alleles of only 6 genes, 3 of which have at least one ‘visible’ allele. Several deficiencies and 3 alleles of let-336 were demonstrated to affect recombination. The duplication identified in this study is sDp30(V;X). Lethal mutations covered by sDp30 were not suppressed uniformly in hermaphrodites. The basis for this non-uniformity may be related to the mechanism of X chromosome dosage compensation in C. elegans.

Position-effect variegation is a phenomenon in which cell-autonomous genes, normally expressed in all cells of a tissue, are expressed in some cells but not in others, leading to a mosaic tissue. Variegation occurs when a normally euchromatic gene is re-positioned close to heterochromatin by chromosome rearrangement. The extent of variegation is known to be influenced by a number of environmental and genetic factors. In the courss of investigations of the influence of the pH of larval medium on the extent of eye-colour variegation in In(1)ωm4 Drosophila melanogaster, we have found that the extent of variegation depends on development time. Flies reared at pH 2·6 develop slowly and show more extreme variegation than those reared at higher pH. This effect, as well as variations within the pH treatments, can be accounted for by differences in development time. The observed regression relationship between variegation and development time also appears to accommodate the influences of temperature on both variables. We suggest that development time may account causally for the reported influences of a number of environmental agents (temperature, crowding, chemicals) on variegation. Ways in which this might occur are discussed in the context of models of the molecular basis of differential gene activity.

Twenty autosomal dominant cataract mutations were detected among the offspring of male mice irradiated with γ- or X-rays. The single or fractionated doses ranged from 5.34 to 10.2 Gy. The phenotypic manifestation and penetrance of the mutations as well as fertility and viability of the mutants were studied by extensive breeding. Manifestation of 4 mutations was limited to the lens. Sixteen mutations were characterized by multiple ocular anomalies, of which 4 mutations also affected other organs of the body. Seventy per cent of the mutations caused severe opacity of the lens or lens and cornea. Homologous hereditary diseases in man would be juvenile cataracts with serious impairment of vision or blindness. Expressivity of the lens opacities was almost constant whereas the accompanied defects varied with respect to their severity in different individuals as well as in eyes of an individual. Phenotypic differences dependent on the genetic background could not be observed. Fourteen mutations were classified as mutations with complete penetrance without any effect on viability and fertility of heterozygotes. Four other mutations were shown to be fully penetrant but the viability of the heterozygotes was impaired. Two mutations had incomplete penetrance with no viability or fertility effects. Of the 14 mutations with complete penetrance and normal viability and fertility 6 were shown to be homozygous viable, 1 semi-lethal and 7 lethal.

Two sex-reversed males and eight intersexes have been found in a natural population of the mole species Talpa occidentalis. All individuals of karyotype 34,XY were normal males, while the 34,XX karyotype was found in normal females, intersexes and sex-reversed males. Small testes were present in XX males, and ovotestes in intersexes. Intersexes showed male antigen levels higher than for females and lower than for males (including XX males), as judged by cytotoxicity tests. The X chromosome of sex-reversed males and intersexes and the Y chromosome of males appeared morphologically normal.

Standard linkage testing crosses and ovarian teratoma mapping were used to estimate the length of the chromosomal segment that is marked by galactose-1-phosphate uridyl transferase and soluble aconitase and that has been conserved since divergence of lineages leading to mouse and man. These experiments were also used to determine whether the Rb(4·6)2Bnr Robertsonian translocation suppresses recombination on the proximal portion of mouse Chromosome 4. The estimated length of the conserved segment marked by galactose-1-phosphate uridyl transferase and soluble aconitase in mouse and man was estimated to be 24 cM. It was also shown that Rb(4·6)2Bnr strongly suppressed recombination on the centromeric portion of mouse Chromosome 4.

The extent of accumulation of mouse Y chromosomal repetitive sequences generally correlates with the known phylogenetic relationships in the genus Mus. However, we describe here a M. musculus Y chromosomal repetitive sequence, designated as ACClfl, whose accumulation patterns among eight Mus species do not correspond to their phylogenetic relationships. Although male-specific hybridization bands were present in all the species examined, significant accumulation (> 200 copies) in the Y chromosomes was found in M. minutoides (subgenus Nannomys), M. pahari (subgenus Coelomys) and M. saxicola (subgenus Pyromys) as well as in the three closely related species M. hortulanus, M. musculus and M. spretus that belong to the subgenus Mus. Unexpectedly, the Y chromosomes of M. caroli and M. cookii (both subgenus Mus) had considerably reduced amounts of ACClfl-related sequences. Furthermore, in rats (Rattus norvegicus) the major accumulation sites appear to be autosomal. These observations suggest that caution must be taken in the interpretation of data obtained with repetitive sequences that have evolved quickly.

Age-related reactivation of an X-linked gene which maps close to Xce, the X chromosome inactivation centre, has been observed. In five female mice which carried the X-linked coat colour gene Moblo on the reciprocal translocation T(X;16)16H (Searle's translocation), and the wild-type gene on the normal X chromosome, and therefore expressed the Moblo phenotype due to the non-random inactivation characteristic of Searle's translocation, progressive darkening of the coat was observed as the animals aged. This is due to reactivation of the previously inactivated wild-type gene at the Mo locus on the normal X chromosome. As the Mo locus is located 4 cM distal to Xce, the X chromosome inactivation centre, these observations provide evidence of age-related instability of inactivation of an X-linked gene close to the inactivation centre.