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.

1. Changes in muscles and tendons in sm/sm are confined to hands and feet. They are closely correlated with the changes in the osseous skeleton.

2. In Os/+, muscular changes are much more complex and not confined to hands and feet, but include the muscles of the forearms and of the lower legs. Some of the muscular changes, especially on the postaxial side, are not correlated with the changes in the osseous skeleton.

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A segment of chromosome which has penetrated from the Hfr into the F− cell may be subsequently withdrawn.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.

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.

1. The first known effect of the gene for shaker with syndactylism (sy) in the mouse is a reduction, both preaxial and postaxial, of the foot plates at the 12½-day stage. This reduction forces the blastemata of digits 2, 3 and 4 to be laid down more nearly parallel to each other, with a subsequent tendency for adjacent blastemata to coalesce with each other. The possibility is considered that this essentially quantitative reduction of the foot plate material may itself be the consequence of some more specific and qualitative (but hitherto undiscovered) change in the limb buds.

2. The physiological connexion between syndactylism and the systemic changes in the cartilaginous and osseous skeleton which arise later in sy/sy mice remains obscure.

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.

Until 1960 all the chromosome counts performed on cells of this species were done on histologically prepared specimens of testicular tissue. In January 1960, Chiarelli et al. published the results of their experiments to grow renal tissue in culture. Their specimens were obtained from male and female calves. They confirmed that 2n = 60 and described the chromosomes as being all acrocentric except the X chromosome, which was submetacentric.

The purpose of the work described below was to try to apply the peripheral-blood culture technique of Moorhead to bovine blood, and to attempt to grow bovine muscle cells in a tissue culture.

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.

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.

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.

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.

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.