The evidence for complementation maps being linear is examined by analysis of all known complementation maps in micro-organisms, and by constructing maps from mutants randomly sampled from amongst those at the leu-2 locus in Neurospora with known complementing properties. Eleven loci out of thirty-five examined in six micro-organisms have non-linear complementation maps. Two linear maps, his-3 and ad-3b (having 25 and 35 complementation groups respectively) have a sufficiently large number of groups for it to be likely that if they do not remain linear on testing further mutants, they will at least have a lower frequency of mutants exceptional to linearity than known non-linear loci. On the basis of maps made from mutants sampled from the leu-2 data, it seemed unlikely that non-linearity would be observed with less than 24 complementing mutants or 13 complementing groups in the sample, and therefore many loci with linear maps are likely to be found to have non-linear maps when larger samples of mutants are tested. This conclusion is important in attempting to correlate the structure of complementation maps with recombination maps and with functional data concerning enzyme activities.

The relationship between the number of complementing mutants, number of groups and number of units at the leu-2 locus is described and a statistical method of determining the total number of groups at a locus is discussed.

Known complex complementation maps have been replotted according to consistent rules, and are illustrated in a shorthand form. The form of the complex maps is discussed in relation to current hypotheses concerning the interpretation of complementation maps. In particular an interpretation of the ‘circular’ leu-2 map is given in terms of the theory of complementation proposed by Crick & Orgel (1964).

1. The results of some selection experiments for body weight in the mouse, conducted in the past in this laboratory, have been examined from the point of view of the limits ultimately reached.

2. The limits that are apparently attained do not necessarily remain stable over prolonged periods of time; two large lines showed marked decreases despite continued selection for high body weight.

3. Selection for high body weight reached a limit in the region of 30 g. at 6 weeks of age; small mice reached their limit at around 12 g.

4. The time taken to reach the limit may vary from ten to thirty generations, even for this one trait.

5. The total response for unidirectional selection was between two and six times the phenotypic standard deviation, or three to twelve times the additive genetic standard deviation.

6. Consideration of the half-life of the selection responses excluded the likelihood of the chance fixation of alleles unfavourable to the direction of selection.

7. The loci contributing to the response could each have an effect amounting to anything from one-half to one phenotypic standard deviation in the base population.

8. This indicated that up to twenty loci had contributed to the response.

9. The intensity of selection practised was close to the optimum for obtaining the maximum total response.

10. The rule of parsimony would indicate the exhaustion of the additive genetic variance as an adequate explanation of the limits attained.

X/Y male mice carrying the sex reversal factor, Sxr, on their Y chromosomes typically produce 4 classes of progeny (recombinant X/X Sxr ♂♂ and X/Y non-Sxr ♂♂, and non-recombinant X/X ♀♀ and X/Y Sxr ♂♂) in equal frequencies, these deriving from obligatory crossing over between the chromatids of the X and Y during meiosis. Here we show that X/Y males that, exceptionally, carry Sxr on their X chromosome, rather than their Y, produce fewer recombinants than expected. Cytological studies confirmed that X-Y univalence is frequent (58%) at diakinesis as in X/Y Sxr males, but among those cells with X-Y bivalents only 38% showed normal X-Y pseudo-autosomal pairing. The majority of such cells (62%) instead showed an illegitimate pairing between the short arms of the Y and the Sxr region located at the distal end of the X, and this can be understood in terms of the known homology between the testis-determining region of the Y short arm and that of the Sxr region. This pairing was sufficiently tenacious to suggest that crossing over took place between the 2 regions, and misalignment and unequal exchange were suggested by indications of bivalent asymmetry. Metaphase II cells deriving from meiosis I divisions in which the normal X-Y exchange had not occurred were also found. The cytological data are therefore consistent with the breeding results and suggest that normal pseudo-autosomal pairing and crossing over is not a prerequisite for functional germ cell formation. The data support the concept that Y short arm-Sxr pairing and crossing over may be the mechanism responsible for the occurrence of the Sxr variants reported in the literature.

Lethal frequencies on the second and third chromosomes were estimated three times in six replicate lines of Drosophila melanogaster selected for increased abdominal bristle number, at G 14–16, G 37–44 and G 79. Ten lethals were detected at a frequency of about 5% or higher at G 14–16, of which only one recurred in subsequent tests. Another ten lethals which had not been detected previously were found at G 37–44, and the 5 most frequent ones recurred at G 79. In the last test, 15 presumably new lethals were detected, of which at least 4 appeared well established. In addition, six reversions (from sc to sc+), a new mutant at the scute locus and sca were discovered. The effects on the selected character of some lethals and visible mutants were large and variable, but not always sufficient to explain the observed frequencies. The major lethals detected at G 37–44 and G 79 for the first time were most probably ‘mutations’ (in the broad sense) which occurred during selection. The likely origins of such ‘mutations’ were discussed, with a suggestion that the known mutation rate for recessive lethals would not be incompatible with the observed frequency of occurrence of the ‘mutations’.

In 1983, an extensive survey of populations of D. melanogaster was started in a southern French region (Languedoc) in two non-Mendelian systems: the P–M system of transposable elements and the hereditary Rhabdovirus sigma. Unexpectedly fast-evolving phenomena were observed and interesting correlations were noted, giving similar geographical pattern to the region in both systems. For these reasons, the analysis was continued and extended towards the north (Rhône Valley) and the south (Spain). In the P–M system, all the Languedoc populations evolved from 1983 to 1991 towards the Q type which is characteristic of the Rhône Valley populations. In contrast, M′ strains are currently observed in the southernmost French populations and in all Spanish ones, so that there is a clear pattern in their geographical distribution.

The frequency of flies infected by the sigma virus dramatically increased from 1983 to 1988 in Languedoc; this increase was clearly correlated with some viral characteristics. But, in northern France, similar characteristics did not trigger any increase in the frequency of infected flies. The data presented here show that the distinctive features of Languedoc extend northwards through the Rhone Valley up to Lyon and disappears southwards before the Spanish border.

Within-line heterogeneity has been found in the mitochondrial DNA (mtDNA) in two isofemale lines of D. simulans. The co-existing types, S and M, were typical of the mtDNA in D. simulans and in D. mauritiana, respectively, their nucleotide divergence per site being ca. 2·1%. Segregation analysis confirmed that some individuals in these lines were heteroplasmic and suggested incomplete maternal inheritance of mtDNA in Drosophila. Examination of other lines of D. simulans revealed that the M type of D. mauritiana occurs at 71% in Réunion, 38% in Madagascar and 0% in Kenya. This finding and interspecific sequence comparisons of both M types indicate that D. mauritiana diverged from D. simulans probably less than 240000 years ago.

The regression of progeny on mid-parent value is often used in progeny tests to estimate the heritability of a quantitative character. The statistical precision of such an estimate can be considerably increased without increasing the size of the test, by using assortative mating or selection of parents (or both together) so as to increase the mid-parent variance; but the danger arises that this may introduce bias into the estimate through correlation between non-additive gene effects.

It is shown by a mathematical argument that such bias will be negligible provided that all individual gene substitution effects are small compared with the phenotypic standard deviation of the character. Under this condition, deviations from additive effects either within or between loci will not appreciably affect the expected value of the regression on mid-parent, compared with its expected value in a test using random mating.

Correlation between the non-additive gene effects is likely to cause more serious bias to the correlation between sibs, when non-random mating is used.

Regardless of their origins, functions, and base compositions, all DNAs are scriptures written following the same grammatical rule. At the level of syllables, two, CG and TA are seldom used, while three, TG, CT and CA are utilized with abundance. Accordingly, at the level of three-letter words, two complementary base trimers, CTG and CAG, invariably enjoy frequent usage. Inasmuch as two of the three frequently used syllables, TG and CA are complementary to each other, while two seldom used syllables, CG and TA, are both palindromes, two complementary strands of DNA are inherently symmetrical with each other. Consequently, palindromic sequences as favourite targets of DNA-binding proteins occur at unsuspectedly high frequencies, if they contain TG and CA or CTG and CAG. Nevertheless, there are grammatical rules operating among these high frequency palindromes as well; e.g. the palindromic tetramer TGCA occurs nearly two times more often than its reciprocal; CATG. Thus, DNA-binding proteins are provided with a wealth of abundant targets whose densities are influenced by a regional difference in GC/AT ratios to variable degrees. One palindromic heptamer CAGNCTG is an ideal target of one DNA-binding protein engaged in chromosome packaging and in generation of banding patterns. This heptamer occurs once every 1000 bases in moderately GC-rich sequences, while its incidence is reduced to once every 3000 bases in extremely AT-rich sequences. The above must be the very reason that a solitary human X-chromosome DNA coated with mouse DNA-binding proteins in mouse-man somatic hybrids still maintains the original banding pattern and that the inactive X remains inactive, while the active X remains active.

Using the moments of gene frequencies, the drift variances of heterozygosity and genetic distance in transient states have been studied under the assumption that all mutations are selectively neutral. Interestingly, this approach provides a simple derivation of Stewart's formula for the variance of heterozygosity at steady state. The results obtained indicate that if all alleles in the initial population are equally frequent, the standard derivation of heterozygosity is very small and increases linearly with time in the early generations. On the other hand, if the initial allele frequencies deviate appreciably from equality, then the standard deviation in the early generations is much larger but increases linearly with the square root of time. Under certain conditions, the standard deviation of genetic distance also increases linearly with time. Numerical computations have shown that the standard deviations of heterozygosity and genetic distance relative to their means are so large that a large number of loci must be used in estimating the average heterozygosity and genetic distance per locus.

The glucose analogue 2-deoxy-D-glucose seriously inhibits the growth of Coprimts lagopus. Following treatment with N-methyl-N′-nitro-N-nitrosoguanidine 388 resistant mutants were isolated. It is shown that the mutants isolated are probably allelic; they were phenotypically similar and no complementation was observed. The mutants were pleiotropic in the sense that although they were initially selected only for resistance to 2-deoxy-D-glucose they were found to be cross-resistant to both of the related analogues, sorbose and glucosamine. Furthermore, the mutants were unable to utilize fructose as a sole carbon source. It is demonstrated that the inability to utilize fructose results from a defect in sugar transport. The gene symbol ftr is proposed for this cistron and it is shown that though the gene is quite closely linked to its own centromere, it is unlinked to centromere markers of the six known linkage groups.

In a random sample of 2263 cloned genomic DNA fragments of the wild potato species Solanum spegazzinii six related, highly repetitive fragments (SPG repeat family) were identified that were present in much higher copy numbers in S. spegazzinii when compared with the closely related cultivated potato S. tuberosum. The SPG repeat family was organized in long arrays of multiple copies. Cross hybridization experiments with 29 wild and cultivated Solanum species and with the related tomato showed specificity of the SPG repeat family for tuber-bearing Solanum species. Among tuber bearing Solanum species a high degree of variation was observed for restriction fragment length and copy number. The variation in copy number was not correlated with established taxonomic relationships between tuber-bearing Solanum species. DNA sequence analysis revealed a subrepeat structure of 120–140 base pairs embedded in longer repeat units of variable length. Length polymorphisms between highly repeated restriction fragments detected by the SPG probes were used for segregation- and linkage analysis in four mapping populations of potato, for which RFLP maps had been constructed. Twelve loci were identified, eleven of which mapped to the distal ends of nine linkage groups. All the evidence suggested that the SPG repeat family represents a satellite repeat members of which are localized in the subtelomeric region of potato chromosomes. The SPG repeat family could be used, therefore, for completing the genetic map of potato.

It is shown that, in a finite population, by ensuring an equal contribution of offspring from each family we achieve a higher selection limit than by using mass selection, given that the correlation between selection criterion and additive genetic value is sufficiently high. The difference between the selection limits of the two schemes increases with more intense selection. The theoretical results were verified by Monte Carlo Simulation, and the influences of several factors were investigated. It is shown that such a breeding scheme might be useful in dairy cattle.