Computer simulations have been done to study the effects of stabilizing and disruptive selection on a polygenic character. The results are reported in terms of three components of genetic variability which represent changes in gene frequencies, departures from Hardy–Weinberg equilibrium and linkage disequilibrium respectively. Under random mating the first and third components are the most important. The observed changes in gene frequencies are interpreted in the light of previous theoretical work on the stability of equilibria under selection. In addition, large and rapid changes in the genotypic variance result from the generation of linkage disequilibrium under selection; the observed changes are in good agreement with those predicted on theoretical grounds.

Wehrhahn (1975) introduced the method of probability generating function to study the distribution of charge differences between homologous proteins in a population but considered only the special case where the population starts with a single allele. Some of his results, however, contained errors. In this paper, all the formulae are presented in general, correct yet much simpler forms. It is also shown that the method of diffusion equations (Ohta & Kimura, 1973) can produce the same results. Numerical computations show that the difference between the one-step and two-step models of charge changes is practically negligible. The results obtained have also been applied to study Nei's genetic distance. Numerical computations indicate that the genetic distance computed from electrophoretic data is about 10% smaller than the expected number of amino acid substitutions involving charge changes in the early stage of divergence of populations and may give a serious underestimate in comparisons between species.

Eighteen mutations of Saccharomyces cerevisiae, at eight loci, isolated as antisuppressors of SUPQ2, an ochre-suppressing allele of SUP11, were crossed with three other suppressors.

They were found to abolish the ability of SUP2 (inserting tyrosine), to suppress the ochre mutations ade2.1 and can1.100, but not its ability, to suppress his5.2 or lys1.1. When coupled with any antisuppressor, SUPQ5, inserting serine, was also unable to suppress ade2.1, but the suppression of other ochre mutations varied from one asu-SUPQ5 strain to another. No antisuppressor affected the ability of SUP11-am, an amber-suppressing allele of SUP11, to suppress trp1.1, an amber mutation.

Variation in spermatozoal β-glucuronidase levels of inbred strains of mice was found to parallel the known variation usually studied in liver with one outstanding exception: BALB/cJ spermatozoa, but not other tissues, showed an anomalously low level. This difference in spermatozoal β-glucuronidase activity was studied in hybrids and backcrosses but the data did not establish the number of loci affecting the difference. The levels of β-glucuronidase in F1's between C3H/HeJ and A/J depended markedly on the sex direction of the cross.

We have investigated a mathematical model of the process of activation of the X chromosomes in eutherian mammals. The model assumes that the activation is brought about over some definite time interval T by the complete saturation of N receptor sites on an X chromosome by M activating molecules (or multiples of M). The probability λ of a first hit on the receptor site is considered to be very much lower than that of subsequent hits; that is, we assume strong co-operative binding. Assuming further that an incomplete saturation of receptor sites is malfunctional, we can show that for proper activation of X chromosomes in normal diploid males and females, we must have λMT ≥ 3 and 0·96 ≤ N/M ≤ 1. An extension of this analysis for the triploid cases shows that under these conditions, we cannot explain the activation of two X's if the number of activating molecules is fixed at M. This suggests that there must be two classes of triploid embryos differing from each other in a step-wise manner in the number of activating molecules. In other words, triploids with two active X chromosomes would require 2M activating molecules as opposed to M molecules in triploids with a single active X. This interpretation of the two classes of triploids would be consistent with differing imprinting histories of the parental contributions to the triploid zygote.

Four species of Drosophila (melanogaster, simulans, mauritiana and virilis) were tested for their ability to survive on erg strains of Saccharomyces cerevisiae each having a mutant block at a different reaction in the synthesis of ergosterol. Species capable of completing development on a given yeast mutant strain were tested for egg production and viability, and also for their ability to survive on the mutant yeast for five complete generations. Sterol analyses using gas–liquid chromatography show that the sterol composition of flies closely resembles that of the yeasts on which they are grown, confirming that the steric criteria for sterols capable of a structural role in lipoprotein membranes are relatively broad. Even so, different sterols are not equally efficient in this respect and there are differences between species in their tolerance to the sterols available from each of the yeast mutants. The range of sterols capable of satisfying the micronutrient, as distinct from the macronutrient, requirement is narrower. Growth on two of the yeast mutants (erg-2 and erg-6) leads to developmental arrest in the larval stage due, it is suggested, to a block in the synthesis of ecdysone. The four Drosophila species lack an effective level of Δ8−Δ7 isomerase which is necessary for utilization of zymosterol and other Δ8 sterol precursors of ergosterol. Ergosta-5,7-diene-3β-ol appears to be capable of substituting for cholesterol in the metabolism of Drosophila.

By means of rye chromosome CR from Imperial added to Chinese Spring wheat the alien substitutions 4A/CR, 4A/CRS, 4B/CR, 4D/CR, 4D/CRS, 7A/CRL, 7B/CR, 7B/CRL, 7D/CR and 7D/CRL were isolated. Substitutions 4A/CRS and 4D/CRS had a better fertility and vegetative vigour than the corresponding substitutions with the complete chromosome CR. In substitutions the entire chromosome CR did not compensate for the absence of wheat chromosomes of group 7, whereas the substitutions 7A/CRL, 7B/CRL and 7D/CRL were more fertile than the corresponding nullisomics 7A, 7B and 7D. It was shown that the short arm of CR is homoeologous to wheat telosomes 4Aα, 4BL and 4DS, whereas most of the long arm of CR is homoeologous to the wheat arms 7AS, 7BS and 7DS. Rye chromosome CR is designated 4R. Only three substitutions with rye chromosome DR were produced. The fertility of substitutions 7A/DR and 7B/DR were considerably lower than that of nulli-7A and nulli-7B. The 4B/DR substitution was sterile. Rye chromosome DR is believed to be a double interchanged chromosome in comparison to the corresponding Secale montanum chromosomes. Rye chromosome DR is designated 7R. On the basis of substitution ability of CR and DR and several homoeologous gene loci on these chromosomes an evolutionary scheme for the derivation of the Secale cereale genome from Secale montanum is outlined.

Four inbred strains of mice were used, differing in the total percentages of spermatozoa with abnormal heads (KE, 22·1%; C57, 26·4%; KP, 7·7%; CBA, 5·5%) and in the frequency distribution of abnormality types, as divided into four arbitrary classes. The most variable class 2 (narrow heads with canals inside the nuclear material) accounted for 47% of all abnormalities in KE strain, was common in CBA (29%) and almost missing in KP and C57 strains. F1 hybrids from the diallel crosses of these strains exhibited highly significant heterosis effects and significant reciprocal differences in the total percentage of abnormalities. The relative frequency of class 2 ranked in F1 hybrids in a similar order as calculated from the mid-parental values. After seven generations of backcrosses performed to introduce the Y chromosome from CBA to the genetical background of the KE strain, the total percentage of abnormalities was significantly reduced, although the relative proportion of class 2 was similar to that in KE strain. Also the Y chromosome from C57 strain, introduced into the genetical background of KE strain, caused a significant reduction of total abnormalities, but again the relative frequency of class 2 was not affected. It is concluded that the Y chromosome plays an important role in determining the total percentage of sperm head abnormalities, but does not seem to be involved in influencing specific abnormality types.

A metric character determined by a large number of loci without epistasis is normally distributed. In the absence of linkage the joint distribution in two or more relatives is multivariate normal, so that all regressions are linear and have constant residual variance. In the presence of linkage this is no longer true except in the case of parent and child; for all other types of relatives the regression line is unaffected by linkage but the residual variance about this line is no longer constant but increases away from the mean.