Female reproductive performance is reported in mice selected for ten generations for one of three criteria: either appetite (A), fat percentage (F) or total lean mass (P). For each criterion lines were selected for high (H) or low (L) performance, with contemporary unselected controls (C). In the A and P lines, litter size changed in the direction of the selected criterion, the changes being larger and more rapidly established in the A than in the P lines. At generation 10, the differences in litter size between high and low lines were 2·6 live young born in the A lines, and 1·0 live young born in the P lines. The differences in 6-week weight between the high and low lines were 3·5 g in the A lines, 6·5 g in the P lines. Changes in ovulation rate were the primary reason for changes in litter size, the differences between the high and low lines being 3·8 corpora lutea for the A lines, and 3·1 corpora lutea for the P lines. Fitting body weight at mating as a covariate within lines in the analysis of ovulation rate and live foetus number removed the differences between the high and low selected P lines, but not those in the A lines. The high and low selected A and P lines did not differ in prenatal survival. There were no consistent differences in litter size, ovulation rate or pre-natal survival in the F lines.

Genetic and cytological analyses of house-flies collected from 12 pig-breeding farms throughout the British Isles demonstrated that the non-standard sex determination mechanism prevailing in South-East England, involving a dominant female determinant (F) and virtual homozygosity for a male determinant on the X chromosome (Xm, both males and females morphologically XX), was not typical of the country as a whole. Instead there was a gradual decrease in the frequency of F, Xm and a rarer male determinant M III, and a concomitant increase in the standard male determining Y chromosome, on moving north, east and west of this region. Only the Scottish and probably the Irish populations were fully standard (XX females XY males), although one from the East Anglian coast in which non-standard determinants were rare was predominantly of this type. Populations from intermediate areas possessed complex multifactorial mechanisms in which Y, F Xm and M III coexisted. It is hypothesized that this radial cline in sex determinants, like the latitudinal cline known from mainland Europe, represents a transient polymorphism caused by the recent and continuing invasion of non-standard determinants into originally standard populations. The cause(s) of this apparently rapid evolutionary change, however, remain unclear.

Plaque-forming transducing (pt) particles of bacteriophage ø 80 have been used to transduce genes governing tryptophan synthesis. Main interest was devoted to strains which carry deletions extending for varying distances into the tryptophan (trp) operon, covering trp A and ending in trp B, trp C or further. If the deletion does not over-reach trp C, infection by pt particles carrying the trp (CBA)+ segment gives rise to stable partial diploids which express the complete tryptophan independence. If the deletion is further extended, transductants are not entirely complemented and appear only on medium containing indole. If the recipients do not carry any prophage in the attachment site of ø 80 (att80) the indole-requiring transductants are very unstable; if the recipients do carry such prophage, even defective, all transductants are very stable. Stable indole-requiring diploids can be superinfected by other pt phages carrying the complementary genes of the tryptophan operon, giving rise to supertransduced indole-independent bacteria. Transduction to complete independence of tryptophan or any precursor has also been achieved in three steps. Supertransductants yield after u.v. induction pt phages carrying the initial trp segments and segregate rare cells with incomplete ability to synthesize tryptophan. They are therefore polylysogenic polyploid heterogenotes carrying overlapping segments of the trp operon linked to prophage genomes.

When recipients are carrying a prophage in att80, transductants appear at a low frequency. The reduction is particularly pronounced with ø 80 pt transducing segment trp (CBA)+ to immune bacteria.

Alcohol dehydrogenase activity in Drosophila melanogaster may be considered as a quantitative character, since it shows many features typically associated with such traits. Although strains with the electrophoretically fast phenotype generally have activities greater than those with the slow phenotype, presumably reflecting differences in the nucleotide sequences of the structural alleles, within each electrophoretic class there is considerable variation in activity. The expression of the structural gene, in terms of ADH activity, is to some extent regulated by its genetic background. Strains homozygous for particular structural alleles respond to divergent directional selection for ADH activity. Modifiers have been located to the X, second and third chromosomes.

Of 150 wild-type strains of Pseudomonas aeruginosa examined, 48 formed recombinants when mated to P. aeruginosa strain PAO FP− and hence presumably possess sex factors. Three different types of sex factor were distinguished by the pattern of transfer of particular markers in different regions of the chromosome and by the ability to confer resistance to mercury in strain PAO. One new sex factor, FP39, was studied in detail, and while similar to the previously studied FP2 in terms of transfer kinetics, natural stability and resistance to curing by acridines, it differed from FP2 in promoting chromosome transfer from a site 10 min to the left of the FP2 origin and in showing apparently aberrant entry kinetics for a leucine marker situated 48 min from the FP2 origin. This was due to FP39 having a genetic determinant either for a structural gene of leucine biosynthesis or a specific suppressor gene for this locus. PAO strains carrying both FP2 and FP39 were unstable for both sex factors, suggesting a relationship between them.

1. The problem of improving rate of larval development of Drosophila by selecting for this ‘character’ on deficient diets is examined by culturing larvae axenically on low-casein and low-pyridoxine media. Under these conditions it is possible to develop strains which grow faster than the parent population.

2. Selection for fast- and slow-growing larvae on a low-pyridoxine diet proceeds with a realized heritability of about 20%, but progress ceases after eight to nine generations. The selected larvae show no alteration of pyridoxine requirements up to the tenth generation, but the lines develop at different rates under optimal conditions. This difference is exaggerated when the diets are low in pyridoxine. By the fourteenth generation, requirements of the two lines for optimal growth have become distinct, the fast line requiring less pyridoxine than the control. Casein requirements show about the same optimum for the two lines but this is lower than that of the control, foundation population.

3. Selection for fast- and slow-development lines on a low-casein diet continues to be effective throughout the fifteen generations of the test. The realized heritability in this case is about 10%. The optimal requirements of the two lines are the same, and there is little difference in their development rates when reared on this optimal diet. The response is found only under sub-optimal conditions, both of deficiency and of excess casein. Pyridoxine requirements do not seem to be altered in the two lines.

4. Crosses between the selected lines show that each genotype has its own optimal environment, as judged by pyridoxine and casein requirements. Crosses among the lines after fourteen generations show that all the hybrids are superior to the mid-parent and three grew faster than the better parent. Other environments would have given different results.

The available data on heterogeneity in centromere distances for a number of loci in several linkage groups are analysed and interpreted. When the crosses are grouped according to wild-type ancestry, heterogeneity is eliminated in any one group except those which consist of backcrosses or intercrosses. Abbott 4 and 12 are shown to be the source of the heterogeneity while Lindegren and probably Chilton wild strains give consistent, homogeneous distances. In a cross between Abbott 12 and Lindegren wild-types, the centromere distances of mt and asco show heterogeneous values between the spore pairs in an ascus indicating that significantly different distances are genetically determined and that the factors concerned show segregation. The genetic determination differs in the various wild strains; the data suggest that at least three factors are involved. In random spore analyses heterogeneity is present in recombination frequencies between linked markers either proximal or distal to their centromere. The mechanism by which heterogeneity in the data might arise is discussed. To obtain homogeneous data it is suggested that all markers used should be repeatedly backcrossed to the Lindegren wild-type.

The genetic basis of slow growth rate and aberrations in the ciliary pattern was studied in the multi-left-marginal variant of Paraurostyla weissei. The 3:1 segregation in F2 sibling crosses and 1:1 segregation in test crosses indicate that the aberrant phenotype is controlled by a recessive allele at a single gene locus termed mlm. The phenotypic change from wild type to mlm takes place about 5–8 cell cycles after conjugation. The study established that total conjugation in P. weissei is a true sexual process in which meiosis, fertilization and Mendelian segregation occur.

Contrary to opinions expressed by various authors, the phenotype of heterozygotes for mammalian sex-linked genes gives no support for the Lyon hypothesis (L.H.). Evidence, mainly from the mouse, shows that in such heterozygotes, both alleles act together as in autosomal genes.

In the present paper, it is shown that neither the behaviour of double heterozygotes for sex-linked genes nor that of X-autosome translocations provides independent evidence in favour of the L.H.: in each case, the interpretation depends on that of the behaviour of single heterozygotes and hence fails to discriminate. Moreover, new facts from both types of situation are also contrary to the L.H. In particular, a unified interpretation which fits the behaviour of genes in all known types of X-autosome translocations in the mouse requires the assumption that partial inhibition of gene action happens in both X-chromosomes of mouse females, and presumably the females of other mammals. The new hypothesis is consistent with all relevant genetical facts and, like the L.H., it also accounts for dosage compensation.

1. Mass selection for both high- and low-ratio of wing to thorax length has been carried out on a population of Drosophila melanogaster. The response to selection was immediate and sustained. When the experiment was stopped after ten generations, the wing area in the two selected lines differed by about 30%. The heritability estimate worked out at 0·56 ± 0·08.

2. Thorax length remained comparatively unchanged during selection nor was there any change in wing shape. There was some evidence of assymetry of response since there was a relatively greater change in favour of smaller rather than larger size.

3. The tibia length of all pairs of legs showed correlated changes so that the lines with larger or smaller wing sizes had also larger and smaller legs.

4. The normal allometric relation between wing and thorax length, associated with variation in body-size, apparently also changed, so that for a given change in thorax length there was a greater or smaller proportional change in wing size in the high- or low-ratio lines.

5. The changes in relative wing size are due to changes in cell number.

6. It is suggested that the genetic changes due to selection act in the early pupal period when the imaginal discs are undergoing differentiation and proliferation to form imaginal hypoderm and appendages.

7. Tests of genetic behaviour failed to show any departure from additivity in crosses which involved the unselected population and the high-ratio line. But highly significant departures existed in the cross to the low-ratio line. Relatively smaller wing size behaves as largely recessive. Stability of the normal wing/thorax ratio involves dominance and probably also epistasis. The genetic properties of the relative size of the appendage are apparently similar to those which characterize body-size as a whole.

8. It is suggested that selection provides a valuable tool for studying the constancy or lability of the growth patterns which determine morphology.

A viable t-haplotype, t38, has been shown to distort linkage relations in the T−tf region of mouse chromosome 17. Separate experiments were done with two different interstitial markers, Low and qk, which map respectively about 5 and 4 centimorgans (cM) from the locus of T in female T + tf/ + (Low or qk)+ heterozygotes. In females that carried t38, however (i.e. t38+ + / + Low tf or t38+ + / + qk tf), there was virtually no recombination between t38 and either interstitial marker, although the t38−tf distance was normal. These observations suggest that t38 suppresses recombination in the lefthand part of that region and strongly enhances it further to the right.