1. An experimental study has been made of recurrent selection to an inbred tester. A suitable inbred line is used as a tester parent, and selection is made within a non-inbred population on the individuals crossing performance with the tester line.

It is concluded that there are two situations in which recurrent selection could be profitably applied. Firstly, recurrent selection should, theoretically, be successful when applied to characters closely related to fitness which have little additive genetic variance and secondly, in cases where a character has already been subjected to individual or family selection and has reached a plateau level in that population.

2. The two experiments—i.e. recurrent selection for large litter size in mice and for low bristle number in Drosophila melanogaster—reported here are respectively an example of each of the above situations. In each experiment selection was made between males within the closed non-inbred population on the basis of the performance of their testcross progeny resulting from matings with inbred line females.

3. Initial generation hybrid performance in both experiments was not intermediate between parental performance levels and the divergence from intermediacy was away from the direction of selection.

4. In both experiments there was no evidence to suspect the presence of over dominance.

5. Response to selection was obtained in each experiment but this was close to or less than the expected response calculated on the assumption that all the variance between sires in crossing performance was additive genetic variance.

6. From these experiments it is not possible to draw any firm conclusions about the effectiveness of recurrent selection for exploiting overdominance. It is, how ever, a very inefficient way of exploiting additive genetic variance. It is suggested that more success might be obtained by careful choice of base population material used in recurrent selection.

Proteins of pollen of Oenothera organensis from heterozygous S2S3, S2S6, S3S6 and S4S6 and homozygous S6S6 genotypes produce characteristic precipitation lines in agar-gel plates against antisera prepared from the pollen. The maximum number of precipitation lines was three, and with some genotypes only one line was formed. The lines were characteristic of the different S alleles, but there was a strong cross-reaction between S4 pollen and S6 antiserum. The S protein diffused out from intact pollen grains as readily as from macerated pollen indicating that the normal site of action of the S protein, when the pollen tubes are growing down the style, is near the surface of the pollen tube. The self-compatible mutant S6′S6 in which the S6′ allele has lost the ability to produce an active pollen S protein, has a protein which cross-reacts strongly with S6 antiserum.

The length of time of development, from oviposition to emergence in Drosophila melanogaster was subjected to stabilizing selection. In each generation only the individuals emerging close to the mean development time were used as parents of the next generation. This line was designated the ‘S’ line. In a parallel line disruptive selection was practised; where in each generation the earliest flies to emerge were mated to the flies last to emerge; those emerging at intermediate times were discarded. This line was designated the ‘D’ line. Two control lines were also carried, where the flies were mated at random with respect to time of emergence. The experiment extended for 40 generations and produced the following results:

(1) The variance of development time decreased in the S line and increased in the D line, relative to the control lines.

(2) The mean development time decreased in the S line and increased in the D line.

(3) The coefficients of variation decreased in the S line and increased in the D line.

(4) The viability, measured as per cent flies emerging, decreased in the D line.

Toward the end of the experiment the amount of additive genetic variance in the selected lines and in the control lines was estimated from the response to directional selection. The estimates showed that (i) the loss of total variance in the S line can be accounted for completely by a loss in additive genetic variance, and (ii) the increase in the total variance of the D line cannot be ascribed to an increase in the additive genetic variance. It was probably due to an increase in the environmental component of variance, i.e. to a loss of ‘buffering capacity’.

Shrivelled, a new mutant in the house mouse, is characterized by degeneration of the fibres of the ocular lens, followed by the development of cataracts. The condition is caused by a mutant autosomal gene showing intermediate dominance.

A method for the experimental estimation of generation interval is presented together with results obtained at 25° C. for D. melanogaster Oregon-R-C maintained in population cages and in population bottles, and for D. simulans v in population bottles. Although there is significant variation between the replicate estimates obtained in population bottles, and although a number of potential sources of error have been discussed, it is suggested that this method provides useful operational estimates of the parameter, which may be taken as 23 days for D. melanogaster Oregon-R-C in population cages, 14 days for the same stock in population bottles, and 16 to 18 days for D. simulans v in population bottles.

The time of action and morphological pattern of damage of eight recessive autosomal lethals in Coelopa frigida (F.) is described. Seven embryonic lethals are phase-specific in their time of action but lethal glassy may cause death at any time from the beginning of the third instar onwards.

A method for estimating the preadult viability of heterozygotes for lethals is described. Heterozygotes for lethal E/L3 show some impairment of viability, whereas, under conditions of larval competition heterozygotes for lethal E/L4 are superior in viability to their wild type sibs.

Results obtained by Young for the expectation of genetic gain in an arbitrary linear function of several traits under selection by independent culling levels, under tandem selection, and under index selection have been obtained in slightly more general form and their dependence on basic genetic and phenotypic parameters exhibited. A warning is given about the effects of selection in modifying the distribution of traits; when the distribution has become appreciably non-normal, any calculation of genetic gains from formulae based on normality will tend to overestimation.

Intersexual individuals occurring among the progeny of a single virgin female Mormoniella are described and compared with similar cases of intersexuality described for Habrobracon. Evidence is presented in support of the hypothesis that the intersexuality resulted from a mutation at a single locus.

Mice of two strains, N and C, were used in studies on body-size, pituitary size, and endocrine potency of the pituitary. Strain N had been selected for large (NL) and small (NS) body-size; strain C had also been selected for large body-size (CL) but had been crossed to an outbred strain segregating pituitary dwarfism.

Pituitary weights and body-weights were highly correlated, the regression lines being common in NL and NS mice. Female pituitaries were considerably heavier than male pituitaries in CL mice. In relation to body-weight, CL pituitaries were consistently heavier than those of NL or NS mice.

No differences were detected in the unit potency of gonadotrophins in the pituitaries of NL and NS mice as estimated by the uterine response of immature outbred mice to subcutaneous injections of pituitary tissue. The uptake of 131I into the thyroid was comparable in NL and NS mice per unit of body-weight, and the thyroid secretion rate was also similar using animals of the same body-weight. Immature mice of both lines responded by increased growth to injections of growth hormone or fresh mouse pituitary, though the response was greater in NS than in NL mice.

The primary response to selection has probably been in the size of the pituitary rather than in its unit potency. The interrelationships between body-size, body components, organ size and endocrine levels are discussed.

Tabby males were irradiated with 500 r and mated to Bn Mobr♀♀. Daughters conceived during the first 3 weeks after irradiation were tested for the presence of sex-linked recessive mutations in their progeny, each daughter representing one irradiated gamete. One visible, but no lethal, was found in the Bn–Mobr segment among 154 tested gametes; one gamete was proved free of lethals or visibles in the Bn–Ta segment, and 21 gametes in the Ta–Mobr segment. Among the whole 176 tested gametes there was no indication of a lethal in the adjoining segments.

Sixty-two mutants of E. coli K-12 resistant to 40 μg./ml. valine were isolated from a sensitive strain. Transduction experiments using phage P1 showed that one group of these mutants, val-r-C, is closely linked to leu, another group, val-r-B is closely linked to thr, and a third mutant, val-r-D57, lies between leu and thr. Conjugation experiments showed that the remainder of the mutants could be divided into three groups, val-r-A, val-r-E and val-r-F on the basis of their different times of transfer from Vhf donors to val-s F− recipients. All the mutants are sensitive to 10,000 μg./ml. valine; val-r-B and val-r-D mutants are resistant up to 80 μg./ml. valine; val-r-C mutants are resistant up to 1000 μg./ml. valine, and val-r-A, val-r-E and val-r-F mutants up to 5000 μg./ml. The functional significance of the genetic locations of these groups is discussed.

The claim that CB 1506 produces an unusually high proportion of visible to lethal mutations could not be confirmed. The ratio of visible to lethal sex-linked mutations was less than 1:10 in both Muller-5 and attached-X tests. In one brood it rose to 1:5 when unconfirmed visible mutants were included. The much higher ratio obtained by Fahmy and Fahmy is interpreted as a methodological artifact, due to the inclusion in their calculations of mosaic visibles but not of mosaic lethals.

Two visible mutations—twisted abdomen and wsatsuma—occurred more than once. This suggests the possibility of a locus or allele specific effect of CB 1506.

Adult Drosophila males were injected with chloroethyl methanesulphonate and late progeny, tracing back to treated spermatogonia, were examined for sex-linked lethals and for translocations involving the X, Y, second and third chromosomes. A dose yielding about 20% sex-linked lethals produced 11 translocations in about 3500 tested chromosome sets (approx. 0·3%). This result is discussed in relation to the problem of chromosome rearrangements in different germ-cell stages.

A method is described which makes it possible to detect induced mutations in the pre-existing gene by determining whether both daughter cells of the first division after mutagenic treatment carry the mutation. The method depends on the use of diploid strains of Ustilago maydis which have biochemical markers in both homozygous and heterozygous condition. Cells which are induced to mutate at a homozygous locus will also occasionally undergo mitotic crossing-over. Since only mutant cells are selected, the reciprocal products of such a cross-over will not be detected if the mutation is transmitted to only one daughter cell after treatment. Mutations induced by UV light were examined in three different diploids, and with each, by use of the appropriate selective media and the replica plating technique, it was possible to detect mutant colonies which did consist of the reciprocal products of the exchange. It is deduced that a high proportion of the mutations must occur in the pre-existing gene; and a hypothesis is suggested which attempts to reconcile this conclusion with the current concept of the structure and replication of the gene.

The inactive-X hypothesis (Lyon, 1961) states that in the normal female mouse only one of the two X chromosomes is genetically active in each cell of the body other than the germ-cells, and that the choice of which X is to be inactivated occurs at an early stage of embryogenesis, and is at random in each cell. The descendants of these cells then abide by the decision so that females are mosaics for two lines of cells and may show a mosaic phenotype if they are heterozygous for a sex-linked gene. One requisite for the validity of this hypothesis is that only one X is necessary for the development of a normal female mouse, and evidence substantiating this supposition is the apparent normality of XO mice (Welshons & Russell, 1959; Cattanach, 1961a, b). On the other hand it has been suggested that XO mice are not fully viable (Russell, Russell & Gower, 1959), but no data on the comparative viability and growth of XO female mice have yet been reported. In the present communication data of this nature, collected in the process of setting up an XO stock, are presented and support the hypothesis that only one X chromosome is necessary for normal female mouse development.