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.

Young birds homozygous for the recessive gene, sc, had a significantly lower requirement than their normally feathered siblings for the sulphur-containing amino-acids, methionine and cystine. When fed a synthetic diet lacking in methionine, feathered birds showed the expected abrupt decrease in weight gain while Scaleless birds grew at a rate equivalent to feathered birds receiving the full diet. Scaleless birds receiving the full diet exceeded feathered birds in weight gain during the first part of the feeding trial but they required more food per gram of gain. Between Scaleless groups those fed the deficient diet dropped behind those on full diet in gain after 3 weeks but even after 6 weeks these did not differ in other respects. On the other hand among normal birds those receiving the deficient diet were higher in heat output and had relatively larger thyroid and adrenal glands than those receiving the full diet.

Differences between the two phenotypes are accounted for by (1) the difference in nutritive requirements determined by feather growth, and (2) the resultant differences in heat loss which accounted for the increase in heat output, packed cell volume and heart, thyroid and adrenal gland size of Scaleless birds.

The use of mutants such as Scaleless in studies concerned with requirements for specific physiological or growth purposes is suggested.

In tetrad data obtained from the Carbondale yeast stock, instances of greater than 50% recombination occur far more frequently than would be expected by chance. In the main, genes exhibiting this effect are located in the vicinity of the centromeres. Difficulties in assigning linear order also persist throughout the total data for certain gene combinations. The affinity hypothesis suffices to account for these two effects; the former resulting from ‘divergent’ combinations of affinity sites in the zygote, the latter, representing nonlinear quasi-linkages resulting from ‘convergent’ combinations.

Chromosome V (ur3-centromere-ch–hi1–is1–an) exhibits quasi-linkage with the gene th and reverse-linkage with the gene cu2, in Family 108. In each case ‘linkage’ is strongest with ch and becomes progressively weaker with the genes hi1, is1 and an, respectively, indicating that the preferentially segregating sites involved lie close to ch. It was impossible to determine whether the two sites were identical, as would be expected on an hypothesis of ‘polar’ rather than ‘mutual’ affinity.

Intra-ascal matings within tetrads NPD for cu2–ch, yielded quasi-linkage for these genes in F2, showing that the attraction is retained by the sites being segregated.

A second case of reverse linkage for cu1 and ar4 in Family 217, gave a similar effect—i.e. reverse linkage in F1, becoming quasi-linkage in the NPD—intra-ascal F2.

An intra-ascal mating between members of the single PD tetrad for cu1 and ar4 in Family 217 indicated that this tetrad did not result from ‘chance’ failure of otherwise active sites to preferentially segregate.

An attempt was made to modify a near-limit of selection response in the egg production of a closed poultry flock which had been under selection for 31 years; to this end the laying hens were subjected to weekly periods of 24-hour feed deprivation, and selected for egg number under these conditions.

In two replicate populations genetic gains of about 20 eggs were made under shock treatment to 40 weeks of age, over 5 and 6 generations of selection. These gains were also manifest under normal management.

Selection, under 24-hour starvation per week, also improved the ability of the selected populations to lay under weekly starvation periods of 33 and 48 hours respectively. Differences between selected and unselected lines increased with the more severe stress treatments. Also it was found that the difference in average egg production between 33-hour and 48-hour treatments was larger in unselected lines than in the selected ones, indicating the existence of genotype-environment interactions.

Correlated responses of reduced egg-weight and body-weight, and somewhat earlier maturity of selected lines correspond with what might be expected under normally effective selection for egg number. These adaptations however varied considerably between replicate lines.

The ancestory of Koller's subline of the C57BL strain is not known. A study of the minor skeletal variants in this subHne showed that it differs from the other ten sublines of C57BL more than they difier from each other. It appears to have separated from. the rest before the genotypes of the sublines had been fixed by inbreeding. There is enough evidence to show that it is indeed a branch of C57BL strain.

The rate of divergence of the sublines of C57BL is surprisingly high. It is reconsidered whether the whole of this divergence can be explained by recent mutations or whether residual heterozygosis has also played a part in it.

A sex-linked recessive anaemia (symbol sla) in the mouse is responsible for a rather uncharacteristic blood picture which persists virtually unchanged throughout life. The red cell count is reduced to about three-quarters of the normal value, the cells produced differ from normal ones by a slight reduction in the mean corpuscular Hb concentration and a slight reduction in the mean cell volume. As the mean cell diameter is more strongly reduced, it is concluded that the cells must be thicker than normal. There is a reduction of haemopoietic tissue both in the liver and in the bone marrow.

The gene ‘sex-linked anaemia’ (sla) appeared first in the progeny of a daughter of a male that had been irradiated with 500 r. of X-rays. The daughter was born one month after the irradiation of her father. Mated to an unrelated wild-type male she produced sixteen sons of which six were anaemic. Her fertility was normal. Experience in subsequent matings showed the anaemia to be rather difficult to classify. In litters examined on the day of birth the anaemic young were usually clearly distinguishable by their pale colour. But in older litters, and even in a few at birth, the classification could not be made with confidence. The haematology of the anaemics is described by Grewal (1962).

Chromosome transfer has been studied in mating pairs formed during a 5-minute interval, under conditions that prevent the formation of new pairs. It has been found that, despite restriction of the period of effective contact formation, there is a considerable spread in the time at which transfer is initiated in the different mating pairs. In some pairs there is a delay of as much as 15 minutes before transfer commences.

The presence of broth in the medium in which transfer takes place leads to a marked reduction in the stability of pairs involving some Hfr strains, but has little or no effect on pairs involving others. The action of broth can be ascribed to the effect of a complete mixture of amino-acids on the metabolic activity of the donor cells.

A segment of chromosome which has penetrated from the Hfr into the F− cell may be subsequently withdrawn. Withdrawal may take place in more than half of the mating pairs, and probably occurs at the time when the donor and recipient cells separate. It is prevented if the chromosome breaks when conjugation is interrupted.

A mathematical treatment has been presented of the ideas of de Haan & Gross concerning transfer delay and chromosomal withdrawal during conjugation In Escherichia coli. The calculations involve three parameters: (i) the maximum delay, λ, which can occur between the formation of effective contact in mating pairs and the initiation of chromosomal transfer, (ii) the probability, b, that mating pairs separate with the withdrawal of that segment of the Hfr chromosome which has entered the female cell, and (iii) the probability, c, that mating pairs separate with the breakage of the Hfr chromosome at the point where it enters the female cell, leaving the injected fragment in the female.

A comparison of the theory with the experimental results of de Haan & Gross obtained when chromosomal transfer occurs either in minimal medium or in broth shows good agreement under the following conditions:

(i) the value of λ is the same under both growth conditions,

(ii) the value of b is the same under both growth conditions,

(iii) the value of c is much greater during transfer in broth than it is in minimal medium.

Fifteen methods of maintaining control populations have been studied over eight generations using the flour beetle, Tribolium castaneum. Populations were reproduced each generation from 50 males and 50 females. The methods were compared as to their ability to establish the level of the environment with respect to the base population and to separate environmental and genetic effects in two directionally selected lines.

It was demonstrated that a foundation stock of these beetles produced pupae which weighed about 10% more when grown in 70% relative humidity than when grown in 40% relative humidity. All lines were grown in one environment for two generations and then in the other environment for the next two generations. The mean pupa weights of two master controls were averaged to give an average master value which served as a standard against which the other controls were compared.

Inbred lines derived from a separate source of stock responded differently to the environments than did the foundation stock from which selected lines were drawn.

Evidence of genetic drift in mass mated lines was presented. No differences among the other methods of maintaining the original population were observed. It was suggested that if a smaller number of families had been used, additional differences might have been detected.

Response to selection in both directions over eight generations was symmetrical. Symmetry was also demonstrated for within environment regressions and it appears that greater progress was made in the dry environment than in the wet.

Initially the foundation stock weighed more in the wet environment than in the dry environment. After selection, the large line weighed more in the dry than in the wet. It was shown that the original population contained the basis for this interaction of directionally selected lines with environments since some ofthe initial famllies weighed more in dry than in the wet environment. It was suggested that the apparent differential response to selection, dependent upon the environment, may have been due to the larger variance in the dry environment. This variance was shown to be about twice as large as that in wet.

Due to the presence of the interaction, the base control populations were ineffective in separating genetic and environmental effects in the later generations of the selected lines. Repeated and relaxed methods of maintaining controls more closely indicated how environmental shifts affected the later generations of the selected lines.

It is concluded that a control line must be closely related to the selected line in origin and time in order for its reactions to environmental shifts to be similar in nature to the selected line. If these conditions are not met, undetected genotype by environment interactions may contribute to faulty comparisons. In addition, some method of reproducing the original population must be followed if interactions with respect to the origin are to be detected.

It is further concluded that it is unnecessary to place restrictions on the method of mating base populations for the purpose of maintaining control stocks, provided that the size of the breeding population is not less than 50 males and 50 females and that mass mating is not used.

Nitrous acid is shown to be a potent killing and mutagenic agent for Aspergillus nidulans. The kinetics of mutation rates induced by nitrous acid are investigated by means of three phenotypically distinguishable suppressors of methionine requirement. The dose-effect curves for two of the suppressors are non-linear. Among w and y mutants produced by nitrous acid there is a high proportion of sectored mutants. Double screening experiments indicate that at least some of the sectored mutations may be due to the presence of more than one genetic strand at the time of treatment. Two of the suppressor genes respond differentially to nitrous acid treatment.

Tetrad data from short gene-marked intervals provide information on the frequency of multiple exchanges within intervals. Non-parental ditype and tetratype frequencies from 58,000 interval-tetrads of Neurospora crassa show that 4-strand double exchanges are far less frequent than would be expected in the absence of chiasma or chromatid interference. These results are in general agreement with meiotic tetrad data from other organisms, except Aspergillus nidulans. They preclude the occurrence of reciprocal meiotic exchanges as clusters unless multiples within each cluster are restricted so as not to involve all four chromatids. If this is not the case, and chromatid interference does not occur, then chiasma interference must be strongly positive within short regions. Known cases of apparent negative interference among random meiotic segregants are probably the result of non-reciprocal conversion of a middle marker, rather than of multiple reciprocal crossing over.