Selection was carried out in mice for concentration of thyroxine hormone (T4) in plasma of males at 11 weeks of age over seven generations. Selection was practised for high level in two replicate lines and for low level in two replicate lines, and there was an unselected control. There was a response in both directions, and the divergence of 12·4 ng/ml observed in generation seven was equivalent to about 20% of the base population mean or nearly one phenotypic standard deviation. The realised heritability was 9%.

Plasma thyroxine level had a repeatability of 0·54 when two measurements were made 24 h apart. The responses made at 11 weeks in males were also evident in both males and females at 5 weeks. Plasma tri-iodo thyronine (T3) concentrations showed a correlated response almost as large, relative to the mean level, as that in T4.

Positive correlated responses were observed in total weights of the litter at 12 days, and in individual weights at 3, 6 and 9 weeks, the responses in the early weights being greater relative to their mean. The results suggest that the correlated weight changes were due to genetic responses in maternal characteristics, probably milk production, rather than individual growth.

Thirty-four populations of Aedes aegypti representing the world-wide distribution of the species, were analysed for genetic variation at 19–22 isozyme loci. The species has an average expected heterozygosity of 0·129±0·045 based on 19 loci analysed in every population. Based on this genetic information, two major groups can be defined: the dark, often sylvan, African subspecies formosus and the light domestic subspecies aegypti in Africa and the New World. Asian populations do not fall easily into either group. These results are related to models which have been proposed for the evolution of this species. Although A. aegypti was introduced into the New World c. 350 years ago and has recently recolonized many areas following eradication programmes, no signs of founder effects are evident in this region. Asian populations, on the other hand, do show a significantly lower level of genetic variation compared to other populations. This may be related to the time of introduction of A. aegypti into Asia and historical absence of yellow fever on the Asian continent.

1. A random bred population of mice was subjected to inbreeding and the changes of litter size, measured as the number of live young in first litters, were followed.

2. The mean litter size declined at a rate of 0·56 young per 10% increase of the inbreeding coefficient.

3. Selection for large litters within the lines during the inbreeding did not effectively reduce the rate of decline.

4. Out of twenty lines at the beginning of the inbreeding seventeen were lost by the time the inbreeding coefficient reached 76%. Two more were lost later and one survived indefinitely. The three lines that survived longest started at a level below the mean and did not decline in litter size. The one that survived indefinitely reached 99% inbreeding without dropping below the non-inbred control.

5. The three lines surviving at 81% inbreeding were crossed and the litters produced by the crossbred progeny were larger than the non-inbred control by about two young per litter. This gain from heterosis is attributable to selection among the lines on their performance as inbreds. A second and third cycle of inbreeding and crossing yielded no further progress, and the level of the first cross was never regained. This is attributable to the ineffectiveness of the selection applied and to the previous restrictions of the population size.

6. The behaviour of the lines in the inbreeding and crossing point to simple dominance rather than over-dominance at the loci causing variation of litter size.

7. This experiment suggests that, as a means of improvement of farm animals, cyclical inbreeding and crossing does not look very hopeful.

I develop a mathematical model which can account for a distribution of the number of repeated genes per chromosome under the joint effects of sister chromatid exchange (SCE), inter-chromosomal crossing-over (ICC), and selection. The model can be applied not only to the cases of small gene clusters but also to multigene families. Based on this model, an appropriate mathematical formula is derived and used to obtain the equilibrium distribution. Assuming stabilizing selection and two simple schemes concerning SCE and ICC, I numerically calculate the equilibrium distribution and compare the result with observations on frequencies of single and triple α-haemoglobin genes in primates. It is also shown that if SCE and ICC occur according to the same probabilistic law, the distinction between them does not make much sense in the equilibrium distribution.

It was demonstrated that the number of generations until fixation or loss of an overdominant alleles is influenced by random fluctuation of selection coefficients. When 2 < Vs, where is the mean selection coefficient against either homozygote and Vs is the between -generation variance of the selection coefficient, overdominance generally accelerates rather than retards fixation of segregating alleles. This finding should have important bearing on our consideration of the behaviour of polymorphic variants which are nearly neutral but have very slight overdominance. When the population size (Ne) is extremely large, not only Ne but also /Vs have to be considered in discussing the effectiveness of overdominance.

tsd1–1 strains of Ustilago maydis are blocked in DNA synthesis under restrictive conditions. On recovery from the block, both intergenic and allelic recombination are enhanced, and aneuploids are generated. UV is synergistic in the induction of recombination. Direct assays of allelic recombination have shown that gene conversion occurs during the block in DNA synthesis.

DNA reassociation kinetics were used to determine the amounts and organization of repetitive and unique DNA in four mosquito species: Anopheles quadrimaculatus (Say), Culex pipiens (L.), Aedes albopictus (Skuse) and Ae. triseriatus (Say). Intraspecific variation in repetitive DNA amounts was examined in two geographic strains of Ae. albopictus fom Calcutta, India and the island of Mauritius. Repetitive and unique sequences in An. quadrimaculatus were distributed in a pattern of long period interspersion. Repetitive DNA in all other mosquito species exhibited a pattern of short period interspersion. The amounts of fold-back, middle repetitive, and highly repetitive sequences increased with genome size. The amount of foldback DNA increased at a much slower rate than the middle and highly repetitive sequences. Intraspecific variation in genome size in Ae. albopictus was due primarily to the amounts of highly repetitive DNA. S1 nuclease digestion of repetitive DNA in all species revealed a positive correlation between genome size and the proportion of the repetitive DNA consisting of short repeats. The amounts of long and short repeats increased with genome size but short repeats increased at a higher rate. The repetitive DNA of the Mauritius strain contained approximately 15% more short repeats than the Calcutta strain. These findings suggest that genome evolution in mosquitoes has resulted from changes in both the amounts and organization of repetitive elements.

White-spored mutants of A. immersus were used in experiments carried out to study the relationship between conversions occurring simultaneously in linked genes, and to check whether conversion occurs at random in the two chromatids of a chromosome.

Conversions in two loosely linked loci were shown to be independent of each other and randomly distributed between sister chromatids. On the other hand, conversions in two closely linked loci showed a high correlation of occurrence.

Several of the B–A translocations of maize produce a small-kernel phenotype which is associated with hypoploid endosperms from fertilizations by sperm which result from nondisjunctional events of the B centromere at the second microspore division. Lin (1975) demonstrated that the small-kernel phenotype was not a consequence of the deficient nature of said endosperms, but involved a differential effect of male and female chromosomal regions.

The TB-A effect has been examined for evidence of autonomy in mosaics for the most extreme such case known, i.e. the small-kernel effect produced by the compound B–A translocation, 1La-5S8041. Although reduced-sized kernels are formed when the paternal contribution of the 1L-5S element is totally lacking from the endosperm, sectorial loss has no detrimental effect on dry weight. This phenomenon is therefore considered to be nonautonomous.

Ten glycosidases were measured in suspensions of spermatozoa from the vasa deferentia of two inbred mouse strains and their intercrosses. Eight of these glycosidases were associated with the sperm cells and all of these showed genetical variation between the strains except α-l-fucosidase with optimal activity at pH 5·4. In contrast liver enzyme activities showed no significant variation except α-l-fucosidase. Genetic studies indicated that the variation of spermatozoal β-d-hexosaminidase, α-d-mannosidase, α-l-fucosidase and β-d-galactosidase are inherited at autosomal loci and α-d-galactosidase variation shows X-linked inheritance. We propose a new provisional gene symbol (Afuc-2) for a spermatozoal variant of α-l-fucosidase.

The possibility that primary CRM− isolates at the td locus of Neurospora crassa are multisite lesions has been examined. By comparing the behavior of primary CRM− mutants to known multisite CRM− mutants in the genetic tests of recombination and reversion, it can be concluded that primary CRM− mutants are not multisite. The genetic implications of a CRM− and CRM+ mutant have been discussed.

Rates of transposition and excision of the Drosophila melanogaster retrotransposon elements mdg3, 297, Doc, roo and copia were estimated directly, by in situ hybridization analysis of their cytological insertion sites in 31 replicates of a highly inbred line that had accumulated spontaneous mutations for approximately 160generations. Estimated transposition rates of Doc, roo and copia were, respectively, 4·2 × 10−5, 3·1 × 10−3 and 1·3 − 10−3; no transpositions of 297 nor mdg3 were observed. Rates of transposition of copia varied significantly among sublines. Excisions were only observed for roo elements, at a rate of 9·0 × 10−6 per element per generation. Copy number averaged over these element families increased 5·9 %; therefore, in these lines the magnitude of the forces opposing transposable element multiplication were weaker than transposition rates.

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.

Data from foetal and weanling studies suggest that Oxford race metacentrics kq and no, but not pr, are transmitted preferentially from Robertsonian heterozygote common shrews collected from the Oxford area. These results suggest a possible mechanism for the maintenance of the Robertsonian polymorphism in that area.

Genet. Res., Camb. (1986), 48, pp. 175–178

A chromosomal segment conserved since divergence of lineages leading to man and mouse: the gene order of aminoacylase-1, transferrin and beta-galactosidase on mouse chromosome 9

In the summary on page 175 and in the last paragraph on page 176, the second aminoacylase locus, Acy-2, was inadvertently described as requiring MgC12. The correct compound is MnC12.

(a) Coprinus lagopus has in nature both a monokaryon with one haploid nucleus per cell and a dikaryon with two haploid nuclei per cell. It also has in experiment both a heterokaryon with two types of nuclei distributed irregularly in the mycelium and a diploid monokaryon with one diploid nucleus per cell.

(b) Two pairs of linked recessive auxotrophic mutants, me-5 ad-8 and adhis-1 chol-1 have been combined in two different sets of multiple heterogenotes so that in the dikaryon and heterokaryon the + (active) alleles of the four genes are distributed equally in the two component nuclei. In one combination the linked genes are in trans and in the other they are in cis arrangement in diploid, dikaryon and heterokaryon.

(c) Complementation between the + genes, as measured by growth on minimal and complete media, was complete in the diploid and dikaryon but was only partial in the heterokaryon.

(d) A similar comparison of a recessive suppressor of the mutant me-1 revealed that the recessive gene was not expressed and therefore was fully recessive in the diploid and dikaryon but was only partially recessive in the heterokaryon.

(e) The exact similarity of complementation of the structural genes of the auxotrophic mutants and the recessiveness of the me-1 suppressor in diploids and dikaryons exclude a localization of gene interaction in the nucleus. In a heterokaryon cytoplasmic dilution of the product of a dominant gene can occur and has the effect of producing a phenotypic difference between heterokaryons and diploids.