In samples of sixteen populations of Rattus rattus from southern India, the oldest individuals have less variable molar widths than the younger ones. This is probably due to stabilizing selection by mortality. There is no detectable heterogeneity between sexes or teeth or among populations in this selection. Although there is no average difference between age classes in mean tooth width, the difference between age classes is heterogeneous among populations. This heterogeneity may reflect heterogeneity in directional selection or in direct environmental effects. The selection intensity on the variance is about 0·04.

(i) A computer simulation study has been made of selection on two linked loci in small populations, where both loci were assumed to have additive effects on the character under selection with no interaction between loci. If N is the effective population size, i the intensity of selection in standard units, α and β measure the effects of the two loci on the character under selection as a proportion of the pheno-typic standard deviation and c is the crossover distance between them, it was shown that the selection process can be completely specified by Niα, Niβand Nc and the initial gene frequencies and linkage disequilibrium coefficient. It is then easily possible to generalize from computer runs at only one population size. All computer runs assumed an initial population at linkage equilibrium between the two loci. Analysis of the results was greatly simplified by considering the influence of segregation at the second locus on the chance of fixation at the first (defined as the proportion of replicate lines in which the favoured allele was eventually fixed).

(ii) The effects of linkage are sufficiently described by Nc. The relationship between chance of fixation at the limit and linkage distance (expressed as 2Nc/(2Nc + 1)) was linear in the majority of computer runs.

(iii) When gene frequency changes under independent segregation were small, linkage had no effect on the advance under selection. In general, segregation at the second locus had no detectable influence on the chance of fixation at the first if the gene effects at the second were less than one-half those at the first. With larger gene effects at the second locus, the chance of fixation passed through a minimum and then rose again. For two loci to have a mutual influence on one another, their effects on the character under selection should not differ by a factor of more than two.

(iv) Under conditions of suitable relative gene effects, the influence of segregation at the second locus was very dependent on the initial frequency of the desirable allele. The chance of fixation at the first, plotted against initial frequency of the desirable allele at the second, passed through a minimum when the chance of fixation at the second locus was about 0·8.

(v) A transformation was found which made the influence of segregation at the second locus on the chance of fixation at the first almost independent of initial gene frequency at the first and of gene effects at the first locus when these are small.

(vi) In the population of gametes at final fixation, linkage was not at equilibrium and there was an excess of repulsion gametes.

(vii) The results were extended to a consideration of the effect of linkage on the limits under artificial selection. Linkage proved only to be of importance when the two loci had roughly equal effects on the character under selection. The maximum effect on the advance under selection occurred when the chance of fixation at both of the loci was between 0·7 and 0·8. When the advance under selection is most sensitive to changes in recombination value, a doubling of the latter in no case increased the advance under selection by more than about 6%. The proportion selected to give maximum advance under individual selection (0·5 under independent segregation) was increased, but only very slightly, when linkage is important.

(viii) These phenomena could be satisfactorily accounted for in terms of the time scale of the selection process and the effective size of the population within which changes of gene frequency at the locus with smaller effect must take place.

1. Nude is a new recessive gene causing hairlessness in the mouse. It is linked to rex and trembler in linkage group VII. The order of the three loci and the recombination frequencies are as follows:

2. In addition to hairlessness the new gene causes reduced body growth rate, very low fertility and a liver disease causing death. Nude mice may be classified at birth by the absence of vibrissae.

3. The hairlessness is due to abnormal keratinization of hair in the follicles. The skin histology resembles that of naked mice. The hair follicles were found to be deficient in free sulphydryl groups.

4. The majority of nude mice die of general body weakness within 2 weeks of birth. The survivors grow slowly and may live for a considerable period. But all nude mice eventually die, usually between 3 and 14 weeks of age.

5. The livers of dead or moribund nude mice are covered with lesions and scars. The defect has been traced histologically to its initial stage, namely, necrosis of small areas of tissue.

6. Attempts to relate the deficiency of sulphydryl groups in the hair follicles to abnormal sulphur metabolism in the liver were unsuccessful.

7. Pseudo-cysts of a parasitic protozoan, Toxoplasma gondii, were identified in the liver and brain of nude mice. In one case the free form of the organism was found.

8. The possible relationship between the liver disease and the pathogenic organism is discussed.

1.Complementation between thirteen proline auxotrophs in A. nidulans was studied. Two groups of mutants with different complementation pattern were found. These two groups could also be distinguished on the basis of recombination tests.

2. The spontaneous reversion rate of proline mutants was established. In all cases studied the reversions were due to suppressor mutations. Dominant, semi-dominant and recessive suppressors were distinguished.

3. Complementation between recessive suppressors was studied. Only a few of the suppressors obtained could be located in different complementation groups.

4. Three suppressor loci were mapped, two of them, su-2 and su-6 in chromosome III linked to the phen-2 locus, respectively 22 and 26 map units distant, and the third in chromosome I, linked to the ad-9 locus (1·9 map units). su-2 is a mutant at the Su-4 pro locus already identified by Forbes (1956).

5. The action of these suppressors is thought to consist in affecting the pathway of arginine synthesis by one of three mechanisms: (1) accumulation of an inter mediate (ornithine); (2) increased activity of ornithine δ-transaminase; and (3) a third, as yet, unclear process possibly involving feed-back regulation of arginine synthesis or the regulation of arginine breakdown to ornithine.

In order to develop a system for the study of the mechanism of intragenic recombination in Ustilago, mutants lacking nitrate reductase activity were isolated, and five alleles were combined in pairs in ten vegetative heteroallelic diploids. The diploids have the mutant phenotype, i.e. inability to utilize nitrate as sole source of nitrogen, but they will recombine to produce wild-type cells much more frequently than the back-mutation rates of haploids or homoallelic diploids. The spontaneous rate of recombination can be enormously increased by low doses of UV light, particularly if treatment is during the period of DNA synthesis in the mitotic cycle. By means of half-tetrad analysis it has been shown that this process of intragenic recombination, as in other fungi, is due to gene conversion rather than reciprocal exchange. It has also been shown that the frequency of UV-induced conversion under standard conditions gives a rough measure of the distance between two mutant sites, since it was possible to use these frequencies to make a linear fine structure map of the gene. These results are discussed in relation to a hybrid DNA model for gene conversion slightly modified from that previously suggested for meiotic recombination.

A new recessive gene affecting behaviour and the inner ear in the mouse has been discovered. It was named Snell's waltzer, and assigned the symbol sv. It is in linkage group II, about two map units from short ear (se) on the side opposite to that of luxoid (lu). The behaviour of sv/sv mice closely resembles that of other members of the shaker-waltzer group. The abnormalities of the inner ear consist in degeneration of certain parts following normal morphogenesis. The entire neuro-epithelium —that is, the organ of Corti, the two maculae and the three cristae—is affected, and this feature distinguishes it from other degenerative type mutants of this group.

1. The results of some selection experiments for body weight in the mouse, conducted in the past in this laboratory, have been examined from the point of view of the limits ultimately reached.

2. The limits that are apparently attained do not necessarily remain stable over prolonged periods of time; two large lines showed marked decreases despite continued selection for high body weight.

3. Selection for high body weight reached a limit in the region of 30 g. at 6 weeks of age; small mice reached their limit at around 12 g.

4. The time taken to reach the limit may vary from ten to thirty generations, even for this one trait.

5. The total response for unidirectional selection was between two and six times the phenotypic standard deviation, or three to twelve times the additive genetic standard deviation.

6. Consideration of the half-life of the selection responses excluded the likelihood of the chance fixation of alleles unfavourable to the direction of selection.

7. The loci contributing to the response could each have an effect amounting to anything from one-half to one phenotypic standard deviation in the base population.

8. This indicated that up to twenty loci had contributed to the response.

9. The intensity of selection practised was close to the optimum for obtaining the maximum total response.

10. The rule of parsimony would indicate the exhaustion of the additive genetic variance as an adequate explanation of the limits attained.

1. The effects of long-continued selection for body weight in two lines of mice, one large and one small, are described.

2. The large line showed a sharp increase in weight after remaining at an apparent limit for twenty generations. A rare combinational event is suggested as the most likely explanation.

3. Reversed and relaxed selection from the large line at the limit failed to yield any response. This indicates that effectively, the additive genetic variance in this line had been exhausted.

4. In contrast, the small line at the limit regressed slightly towards the base population when selection was relaxed. Reversed selection yielded a ready response until a new limit was apparently reached. Loci affecting body weight in this line had therefore not been fixed by selection.

5. Natural selection, operating on viability between conception and the time when the selection was made, appears to explain best the lack of fixation in the small line.

6. Attention is drawn to the necessity of more experimental work to elucidate the genetic nature of the limits to artificial selection.

The cataract-producing mutant gene previously described as ‘Shrivelled’ appears to be a recurrence of the mutant described by Paget as cataracta congenita subcapsularis, and should henceforth be referred to as CatFr.