The serum transferrin and post-albumin phenotype distributions of 228 Sahiwal, 138 Nganda, 265 Boran, 114 Tanganyika Shorthorned Zebu, 72 Teso and 267 Ankole cattle from East Africa were determined.

Five transferrin alleles, TfA, TfB, TfD, TfE, and TfF were present in all the breeds examined, and a sixth allele, TfG, was present in three of the Borans. TfE and TfF were the most frequent alleles, except in the Nganda cattle where TfA was the most frequent. TfB had a frequency of less than 0·1 in each breed. Two post-albumin alleles, PaF and Pas, were present in each breed. In each breed except Sahiwal PaF was two to three times more frequent than Pas. In the Sahiwal PaF and Pas had about the same frequency. It was concluded that both transferrin and post-albumin gene frequencies in East African cattle differ significantly from the corresponding frequencies in European cattle.

There was no evidence of an excess of heterozygotes in the post-albumin system other than that expected from the use of relatively small numbers of bulls in these herds. However, allowing for the same factor in the transferrin system, there appeared to be an excess of transferrin heterozygotes in the cattle populations sampled although the extent of this excess could not be calculated accurately.

I investigate models of the spread of transposable elements, such as the Drosophila melanogaster P elements, that can exist in autonomous and non-autonomous forms. Elements which have their major impact on host fitness in the process of transposition can, under certain conditions, come to a stable balance between transposition and selection. This stable balance for autonomous elements can be disrupted by the invasion of further elements, which do not produce a transposase enzyme, and may produce a repressor of transposition. I examine this secondary invasion process, and show that a stable equilibrium copy number for intact elements is neither a necessary nor a sufficient condition for non-autonomous elements to invade. Nevertheless, invasion occurs under a broad range of models and conditions. This requires neither that the non-autonomous elements produce a trans-acting repressor of transposition, nor that they titrate transposase. The elimination of autonomous elements follows the increase in non-autonomous elements unless the latter encode powerful repressors of transposition. Approximate solutions for the equilibrium copy number of autonomous elements and rate of invasion of non-autonomous elements can be found under some models for transposition and selection. The predictions of the model are compared with recent empirical studies of the D. melanogaster P system

1. Auxotrophs were sought in a slow-growing reversion of the tryA47 strain of S. typhimurium. This reversion differs from tryA47 by a genetic change that is inseparable from the 47 site and has been designated 47S. Out of thirty-nine auxotrophs that derived from independent mutations, twenty-two grew on minimal medium supplemented with anthranilic acid. Eight of these auxotrophs were examined and each was shown to have the 47S site unchanged and to carry a further mutation in the tryA gene. These further mutations were shown to be at different sites in different auxotrophs.

2. Auxotrophs were sought in the wild-type LT-2 strain. None out of thirty-six were mutant in the tryA gene.

3. The 47S site in tryA47S was replaced by 47+ transduced from wild-type LT-2. Auxotrophs were sought in this strain and only one out of nineteen was mutant in the tryA gene.

4. Auxotrophs were sought in a wild-type reversion of tryA47. Out of twenty-six none were mutant in the tryA gene.

5. The 47+ site in wild-type LT-2 was replaced by 47S transduced from tryA47S. Auxotrophs were sought in this strain and twenty-two out of fifty-five were mutant in the tryA gene.

6. We conclude that tryA auxotrophs are only recovered at a high frequency when the 47S site is present in the tryA gene.

7. In strains with the 47S site in the tryA gene the frequency of auxotrophs that will grow on minimal medium supplemented with indole but not on minimal medium supplemented with anthranilic acid is appreciably higher than in those strains without the 47S site in the tryA gene. These auxotrophs are mutant in genes that are in the same operon as tryA.

Derepressed mutants of F-like transfer factors, isolated by mutagenesis, were characterized as repressor-minus (i−) or operator-constitutive (oc). Mutants of the i− class are derepressed in K12 but repressed in Salmonella typhimurium. They are derepressed in S. typhimurium by a kanamycin resistance determinant carrying a locus der, described previously. Most oc mutants of F-like factors are derepressed in both K 12 and S. typhimurium. However, one mutant of F-lac was oc in K12 but was repressed in S. typhimurium. It was derepressed by der. Repression by S. typhimurium is different from that by fi+ factors, since der reverses the former but does not affect the latter. Possible interpretations of these findings are discussed.

This paper is concerned with three related aspects of the behaviour of populations under artificial selection for increased scutellar bristle number: (i) the pattern of response on the probit scale; (ii) the homeostatic behaviour of the selection lines on relaxation of artificial selection; and (iii) correlated responses in generation interval, reproductive capacity and competitive ability. The study was designed so that linkage would be a comparatively unimportant factor in promoting correlated responses to selection, and the effects of genetic sampling from generation to generation were also reduced to a low level.

Progress from the base mean of 4·05 bristles in females to a level of almost 8 bristles has been shown to involve two distinct phases with realized heritabilities of 0·34 and 0·10 respectively, the zone of transition corresponding closely to the position of the 6/7 threshold on the underlying scale. In addition to an apparent average reduction of about 25% in the additive genetic standard deviation in phase II by comparison with phase I, the loss in response due to the opposition of natural selection has been shown to reach a maximum near the zone of separation of the two phases.

The pattern of behaviour of the populations under artificial and natural selection has suggested the presence in the base population of genes of large effect on both bristle number and reproductive fitness. There is also evidence of additional genetic variation in bristle number which is effectively neutral with respect to fitness. Continued selection for increased scutellar bristle number in large populations has been shown to reduce mean competitive ability by more than 80%.

The ability of restricted selection indices to prevent genetic change in a restricted trait over several generations of selection was studied using deterministic computer models. Four loci, two affecting each trait independently, and two pleiotropic loci, one affecting each trait in the same direction, and one with opposite effects, were modelled. In general, continued effectiveness of the restriction was achieved only when the restricted trait was affected by only one locus. In some conditions (equal gene frequencies), an independent locus and one pleiotropic locus affecting the restricted trait allowed maintenance of the restriction. The results suggest that long-term restriction may be very difficult without re-estimation of parameters.

The effects of tight linkage on the total response due to pairs of identical additive loci, segregating in a population initially in linkage equilibrium, have been studied both algebraically and by means of computer simulation. Particular attention has been given to the effects of finite population size on the probabilities of (a) the elimination from the population of the gamete carrying both ‘plus’ alleles; (b) the joint preservation of the two types of repulsion gametes; (c) the recovery of the desired combination of plus alleles through crossing-over; and (d) the fixation of the gamete in the population following its recovery.

The study is restricted to situations in which linkage is known to have an appreciable effect on total selection response, i.e. to the case of genes of large effect initially at low frequency. A comparison of regimes with the same expected response under free recombination has shown the probability of (a) to be high, and the probability of (b) to be very nearly the same for all regimes tested. Provided that the recovery of the gamete carrying both plus alleles is an unlikely event at any given point in time, the probability of the fixation of the gamete, once reconstituted, is expected to be independent of population size for genes of large effect. In this context, approximate algebraic expressions have been derived for the probability of effective recovery of the required gamete, and for the mean waiting time involved.

The production of MSHA+ (type-1 fimbriate) recombinants was observed in transductional crosses between different pairs of naturally occurring strains of Salmonella paratyphi B. MSHA+ recombinants were readily produced in transductions from MSHA+ donor strains to MSHA− (type-2 fimbriate or non-fimbriate) recipient strains, and less frequently between some pairs of MSHA− strains. The genetic evidence suggests that there are at least three different clones among MSHA− strains of S. paratyphi B. Relationships between the different strains and their possible origins are discussed.

A diploid heteroallelic at the gal1 locus and producing little residual galactokinase activity was chosen. Following either γ- or UV-irradiation, recombination occurred leading to the formation of wild-type GAL1 genes and an increase in detectable galactokinase. An enhanced level of enzyme was first detectable 2 h after irradiation and reached a maximum within 12–14 h. With increasing doses of irradiation, more enzyme was produced and this increase superficially resembled that obtained for plated recombinants. However, far more enzyme was actually synthesized than expected from the number of viable recombinants in the cells assayed. This suggests that recombination must have occurred in cells without colony-forming ability, indeed it may have occurred more frequently in these cells than in viable cells.

Multilocus recombination frequencies are expressed in terms of pairwise recombination frequencies for paracentric and pericentric intervals. Applications of this theory to mapping and genetic counselling are discussed.

Complement testing of a Thp line revealed a failure of survival of Thp/t0 embryos. The genetic factor responsible for this lethality maps between Brachyury (T) and tufted (tf) on the murine seventeenth chromosome. This lethal factor permits recombination between T and tf and does not affect the transmission of its seventeenth chromosome. Its effect upon embryonic development is similar to that of the t0 haplotype. It would appear to represent a point mutation of a single gene, t-complex lethal zero (tcl0).

Asexual populations experiencing random genetic drift can accumulate an increasing number of deleterious mutations, a process called Muller's ratchet. We present here diffusion approximations for the rate at which Muller's ratchet advances in asexual haploid populations. The most important parameter of this process is n0 = N e−U/s, where N is population size, U the genomic mutation rate and s the selection coefficient. In a very large population, n0 is the equilibrium size of the mutation-free class. We examined the case n0 > 1 and developed one approximation for intermediate values of N and s and one for large values of N and s. For intermediate values, the expected time at which the ratchet advances increases linearly with n0. For large values, the time increases in a more or less exponential fashion with n0. In addition to n0, s is also an important determinant of the speed of the ratchet. If N and s are intermediate and n0 is fixed, we find that increasing s accelerates the ratchet. In contrast, for a given n0, but large N and s, increasing s slows the ratchet. Except when s is small, results based on our approximations fit well those from computer simulations.