Introducing a new genetic model called the discrete allelic-state model, the evolutionary change of genetic variation of quantitative characters within and between populations is studied under the assumption of no selection. This model allows us to study the effects of mutation and random genetic drift in detail. It is shown that when the allelic effects on phenotype are additive, the rate of approach of the genetic variance within populations to the equilibrium value depends only on the effective population size. It is also shown that the distribution of genotypic value often deviates from normality particularly when the effective population size and the number of loci concerned are small. On the other hand, the interpopulational variance increases linearly with time, if the intrapopu-lational variance remains constant. Therefore, the ratio of interpopulational variance to intrapopulational variance can be used for testing the hypothesis of neutral evolution of quantitative characters.

Monosomic and disomic substitutions of Imperial rye chromosome C for 4A of Chinese Spring wheat were analysed for two enzyme markers, alcohol dehydrogenase (ADH) and 6-phosphogluconate dehydrogenase (6-PGD). The gene(s) coding for ADH and 6-PGD were located on the short and long arms of rye chromosome C respectively.

Isozyme analyses revealed that the substitute chromosome was a translocation involving alpha arm of wheat chromosome 4A and long arm of rye chromosome C. Pairing behaviour of the substitute chromosome with rye and wheat chromosomes in hybrids with rye addition line C and ditelosomic 4A (alpha) confirmed this.

The gene conversion parameters which affect allele frequencies in populations are defined, and their ranges and typical values are given for several genera of fungi, where meiotic octads and tetrads provide the best information on conversion. Both gene conversion and disparity in direction of conversion are common. Data from Ascobolus immersus show that conversion properties are largely stable with time, but can be changed environmentally and by genetic conversion control factors. Equations are given for the interactions of selection, mutation and gene conversion in determining equilibrium frequencies. Numerical examples, using typical values of conversion parameters from the fungal data, show that for alleles which are selectively neutral or have very low selection coefficients, conversion will often have very large effects on their equilibrium frequencies and may lead to fixation. Where selection coefficients are higher, conversion has major effects on the frequencies of deleterious recessive alleles, but lesser effects on deleterious dominant alleles: a critical comparison is that of s with 2y. The available estimates for conversion parameters (at least in fungi) are of a magnitude to make gene conversion an important factor in evolution.

A mutation to an apparently new allele at the W-locus of the mouse arose spontaneously in a cross between two inbred strains. Heterozygotes have a broad white sash, leading to the name and symbol sash, Wsh. Homozygotes are black-eyed whites which are viable, fertile and not anaemic, although the gene does cause mild haematopoietic defects. The original mutant animal also carried a spontaneous recessive lethal mutation on chromosome 5, mapping at 2 cM distal to the W-locus.