An allotypic specificity (A1) of sheep serum is described. The antigenic determinant is located on a low molecular weight glycoprotein which has its isoelectric point at pH 5 and is capable of interacting with the lectin concanavalin A.

Family studies showed that the allotype is inherited in a simple Mendelian manner.

Hydroxyurea (HU) effectively inhibits meiosis in Schizophyllum. The predominant cytological stage in inhibited fruit bodies is fusion. The inhibition is reversible and makes possible synchronization of a naturally nonsynchronous system. Microphotometric determinations of the DNA content in prefusion nuclei treated with HU suggest that premeiotic DNA replication occurs in prefusion nuclei. Synaptinemal complexes are not completed in HU-treated nuclei, suggesting that this event is dependent on premeiotic DNA replication.

The influence of sodium octanoate on the polymorphism at the loci G6PD and 6PGD in Drosophila melanogaster was investigated by studying its effect on egg hatchability, larval-to-adult survival and adult survival. The results demonstrate the existence of differences in fitness between the different genotypes of the two loci both for larval-to-adult survival and for adult survival. Furthermore, changes in enzyme activity of both enzymes, brought about by the sodium octanoate treatment, were observed. This makes it highly probable that the observed differences in fitness can be ascribed to the loci themselves and not to linked fitness genes. Finally, this study demonstrates the existence of strong interaction between the two loci with respect to fitness. Epistasis was demonstrated both in the larval-to-adult survival experiment and in the adult survival experiment.

Models of two linked overdominant loci in moderately large, but finite, populations are examined by looking at the variance-covariance matrix of the two gene frequencies and the linkage disequilibrium around stable deterministic equilibrium points. In particular, the effect of genetic drift is examined in cases where, in infinite populations, two stable equilibria with non-zero linkage disequilibrium, D, are maintained. Theoretical formulae are produced and checked by computer simulation. In general, the results show that unless the population size is very large indeed, genetic drift causes the values of D to vary considerably about the equilibrium values and that for many models, where stable equilibria exist at non-zero D values, a wide range of values of D have a high probability. Thus it is very difficult to draw conclusions about the selection regime by measuring Linkage disequilibrium in a finite population.

Homozygotes for recessive visible genes have often been discovered in lines under artificial selection, sometimes many generations from the start. As a help in the interpretation of this phenomenon, the distribution of the time to first detection as a homozygote of a recessive gene occurring only once in the initial generation has been obtained. Alternatively the results may be considered as referring to the time of first appearance as a homozygote of a new mutation occurring in a finite population. For a monoecious random mating population of size N with selfing permitted, the mean time to detection is very close to 2N⅓ over a range of N from 1 to 500 with a coefficient of variation of roughly 2/3 and a 95% upper limit about 2·5 times the mean. If selfing is prohibited, the mean time is increased by a little over 1 generation. The treatment is extended to cover the effects of artificial selection in favour of the heterozygote, of the frequency of occurrence in the initial generation and of the examination of more individuals each generation than are used as parents.

The paper describes a cattle serum antigen (McA2) located on a macroglobulin molecule which has its isoelectric point at pH 5 and is capable of interacting with the wheat germ lectin and concanavalin A.

The specificity is inherited in a simple Mendelian manner and the gene controlling its synthesis is allelic to the one controlling the synthesis of the McAl antigen.

The number of blastoderm cells in Drosophila whose descendants form adult structures has frequently been estimated from genetic mosaics. Data from somatic recombination (method I) and gynandromorph (method II) mosaics both yield very low estimates, e.g. about 10–20 progenitor cells for the eye and antenna, wing or leg.

In gynandromorphs the mosaic dividing line has a random orientation on the blastoderm. In the 6000 cell blastoderm it should be very unlikely that the mosaic dividing line passes through any small patch of only 10–20 cells. Yet it has been reported that 10–25% of eye/antenna, wing or leg disks in gynandromorphs are mosaic. Thus the frequency of mosaicism data seems to be in contradiction to the progenitor population estimates. Similar discrepancies are found in the data for other adult structures.

In this paper we derive a formula for estimating the number of cells in a blastoderm patch from the frequency with which the gynandromorph dividing line passes through it (method III). In a second method (method IV) we use the maximum distances inside the progenitor areas on a fate map to estimate the progenitor patch size. These two estimates agree closely with each other. We find, e.g. that 50–100 cells are in the patches from which the eye/antenna, wing or leg disks derive.

We examine a number of possible explanations for why the first two estimates are so much smaller than the last two. The former estimates refer to the number of progenitor cells which actually have descendants in the adult structure; the latter estimates refer to the total patch area in which the progenitor cells sit. With the present information the most reasonable conclusion is that the progenitor cells for the adult structures are dispersed among other cells which have different developmental fates. If confirmed by experiment, this result has many implications for the process of determination.

Cell numbers in four organs of large, control and small mice were estimated by nuclear counts. Average cell mass was estimated from the cell number and the organ weight. The mice were from the selected Q-strain with six replicate lines in each size-group. The organs were lung, liver, spleen and kidney. At 6 weeks of age the large mice had more cells and larger cells than the controls in all organs; the small mice had fewer and smaller cells than the controls. The regression of log cell-number on log-organ weight provides a measure of how much, proportionately, cell number contributes to the differences in organ weight. In the lung and spleen, cell number contributed about 70% of the strain differences in organ weight, cell mass contributing about 30%; in the liver and kidney the relative contributions were about equal, at 50%.

Cell counts at different ages from 3 to 15 weeks showed that cell number and cell mass contributed to the increases of organ weights during growth in roughly the same proportions as stated above. From this it is concluded that the main effect of selection for body weight has been to speed up or slow down the normal processes of cellular growth.

A study of inbred strains of the mouse using competitive protein-binding techniques revealed significant strain variation in the total plasma thyroxine levels and in the plasma-free thyroxine index. Measurements of minimal metabolic rates also showed strain variation, but the positive correlation which might be expected between the plasma-free thyroxine index and the minimal metabolic rate did not obtain. Possible explanations are discussed.