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THE POPULATION GENETICS O F ‘ SEX-RAI’IO ’ IN DROSOPHILA PSEUDOOBSCURd A. W. F. EDWARDS, Department of Genetics, University of Cambridge I n Drosophiia pseudoohsmkru the X-linked gone ‘sex-ratio’ causes males carrying i t to produce few or no males but nearly twice a d many females as are produced by normal males. I n the absence of differential viability one might expect it to increase in a population until there were no males left, and yet equilibria, with gene frequencies up to 30 <yo, have been found in natural populations. Wallace (1948) conducted a series of expsrimcnts to determine the viabilities, and set up some population cages to follow the progress of the gene. In these cages the gene was either eliminated, or was at a very low frequency when the experiment was endod. Bennett (1958) put forward a mathematical model for a sex-linked locus which he applied to Wallace’s findings, but examination of his model reveals an error which makes it inapplicable to this situation. Briefly, he confounded gametic and genotypic viabilities, a course which invalidates some of his findings. The present paper describes a more appropriate model, which has been used to study the polymorphism with tho aid of an electronic digital computer. It is shown that Wallace’s viabilities fit neither the natural nor his experimental populations, and new viabilities are suggested for each case. THE EVOLUTION O F THE DIPLOID-LIKE MEIOSlS O F POLYPLOID W H E A T R. RILEY, V. CHAPMAN and G. KIMBER, Plant Breeding Institute, Cambridge l’revious work has shown that the diploid-like meiotic behaviour of the hexaploid wheat, Triticzm aestivum, is due to the activity of chromosome V . I n the absence of V , corresponding homoeologous chromosomes from the different genomes can pair at meiosis. A series of hybrids between wheat and rye, deficient in turn for each chromosome of the wheat complement, has shown that no other chromosome has a similar activity. Other hybrids between an Aegilop species and wheat, deficient for either the complete chromosome P or its long or short arm separately, have demonstrated that the activity is confined to the long arm. None of the diploid species, from which the chromosome might have been derived in the evolution of polyploitl wheat, produces an effect on meiotic pairing like that caused by V . In appropriate hybrids, one species removes the restriction on pairing imposed by V . The other species do not remove the restriction on homoeologous pairing, nor do they prevent hornoeologous pairing in the absence of chromosome V . Consequently the restriction of pairing to fully homologous partners depends upon functions modified by mutation subsequent to the origin of polyploidy. LIFETIME GROWTH AND PRODUCTlVITY OF LARGE AND SMALL STRAINS O F MICE It. C. ROBERTS, Institute of Animal Genetics, Edinburgh The lifetime growth and rcprodiictive performance of micc selected for high and low 6-week weight, were studied. The experimental material comprised six groups-t)wo large strains, two small strains, a cross between large and small strains and an unselected control strain. Strains differing in weight at 6 wceks attained markedly different mature weights. But two strains with closely similar mature weights differed oonspicunilsly in their rates of growth. Unlike the other four groups, the two large strains shuwetl a sharp decline in weight in laler life, probably throiigh the depletion of accumulated stores of fat. Large strains, as cxpected, had larger litters than the small strains, but they had a much shortcr reproductive life, with the consequence that the small strains event,ually weaned twice as many offspring per mouse as the large strains, over their whole lifetime. The heterosis displayed by the cross-bred mice with respect, to reprodiictivr: capacity was striking. Coin pared with the better parental strain, this group wcarietl three times as many offspring over their lifetime, the tot81 weight of which at weaning time was four times as great. 174 The Genetical Society BREEDING IMPROVED MILKING COWS THROUGH ARTIFICIAL INSEMINATION J. EDWARDS and L. K. O’CONNOR, Production Division, hlilk Marketing Board The widespread use of Artificial Imemination in England and Wales (over 2 million cattle forming 60 % of the National Herd are inseminated annually, some 1-6 million of these inseminations being carried out from the twenty-three centres owned by the Milk Marketing Board) permits the application of scientific principles to the breeding of dairy cattle. The Milk Marketing Board‘s plam for breading improved milking cows through A.I. are briefly described and some of the problem encountered are discussed. THE MECHANISM O F FORMALDEHYDE-INDUCED MUTAGENESIS T. ALDERSON, Genetics Department, Univemity of ’Cambridge It is established by means of a chemically defined and axenic medium, that fprmaldehyde exhibits no mutagenic activity towards Droeophib melanogaster by the larval feeding method, unlese adenylic acid is present in the treatment medium. A reaction product formed between formaldehyde and adenylic acid in the treatment medium is postulated as the metabolite concerned in the mediatisn of the mutagenic activity of formaldehyde. The principal metabolite is assumed to be an adenylate dimer, where two adenylic acid molecules are linked together through their amino-groups by a methylene bridge. The stereochemical properties of this metabolite and its coneequent utilization in deoxyribonucleio acid synthesis, together with mechanism for its mutagenic activity axe discwed. An eaaential role is thus wcribed to the 6-amino group of adenylate for the formation of the adenylate dimer, since the binding of formaldehyde by ribonucleic acid monoor poly-nucleotidee appears to be a specific function of the amino groups of the purine and pyrimidine bases. Whilst adenylate is effective when present in the treatment medium as any of the forms adenosine-3’-phoephate, adenosine-6’-phosphate, or deoxyadenosine-6‘-phoephate, the simple replacement of the 6-amino group of adenosine and adenosine-6’phosphate (to give inosine and inosine-5’-phosphate) completely removes the ability of these compounds to mediate the mutagenic activity of formaldehyde. Although adenosine ia aa effective as the adenine ribonucleotides, adenine shows no ability to mediate formaldehyde-induced mutegeneeis and is actually antimutagenic on a formaldehyde-containing yea&-glucose treatment medium. The patterns of utilization of purine and pyrimidine derivatives by D. mekmogaater are discussed in relation to their ability or inability to mediate the mutagenic wtivity of formaldehyde. POLARIZED REPLICATION AS A POSSIBLE CONDITION FOR RECOMBINATION 0. H. SIDDICJI, Department of Genetics, The University, Glaagow Localized negative interference in Aapergillua nidulaa and other micro-organ* shows that recombination in one zygote is confined to small regions of the chromosome. These regions called, by Pritchard, effective pairing segments, are discontinuously distributed over the chromosomes. Among crosees involving fourteen @a mutante, two by two, the distribution of recombination events in this region hse been studied. The strength of negative interferenoe on either aide of the interval within which a cross-over between two @a mutants is selected, is asymmetrical. With increaaing length of the selected interval ‘negative interference’ on the proximal side falls rapidly while on the distal side it remains unaffected. If, aa generally believed, replication is an essential condition for recombination, the results suggest that replication is polarized. Thie imposes a time sequence on the occurrence of recornbination events within the effectively paired region. The results are consistent with those of Rizet, though in our case the distribution of outside markera permits identification of single and multiple cross-overs. SOME STUDIES OF THE EFFECTS OF ULTRAVIOLET LIGHT ON RECOMBINATION IN N E WROSPORA S. F. H. THELKELD, Botany School, Cambridge Preliminary studies suggest that ultraviolet light may increase both crossing-over and non-reciprocal recombination in Neuroapra. Recombination, aa detected by random spore and tetrad analyses, wm studied in crosses involving a pair of multi-marked stocka of Neuroapwa orama, with markers in Linkage Groups I and VI. Conidia were irradiated with mall doses of ultraviolet light prior to use in a cross. When compared with untreated crosses, two of three treated crosses showed highly significant increases in crowing-over in the region between ~r (crisp) and arg-3 (30300) in Linkage Group I. The Genetical Society 175 Seventy-seven tetrads were analysed from treated crosses, and three were found to show atypical segregations; in each a single locus wm involved. The three loci were hiat-2 (C94), ylo (Y30639y), and tryp-2 (76001). Eighty-nine tetrads isolated from similar crosses not treated with ultraviolet light, showed no atypical segregations. RECOMBINATION AND COMPLEMENTATION BETWEEN SUPPRESSOR GENES IN COPRINUS LABOPUS D. H. MORGAN, Department of Botany, University of Hull Monocaryons of Copinus l agopa canying a gene p r , which affects the synthesis of aromatic compounds and gives rise to purple pigmentation, display instability due to suppression of p r by recessive mutations at other genes. Suppressed cultures are thus white, although not always of wild phenotype. When matings are made between suppressed monocaryons homozygoua for pur but carrying suppressors of independent origin, complementation between suppressors may occur-restoring the purple phenotype. Some forty suppressors have been tested for complementation with each other and can be arranged on a single linear complementation map. Preliminary recombination analysis on the other hand indicates that more than one suppressor locus is involved in this group. Crosses between suppressed cultures and wild type reveal recombination between p r and suppressor loci varying fro

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