If natural selection chose where new mutations occur it might well favour placing them near existing polymorphisms, thereby avoiding disruption of areas that work while adding novelty to regions where variation is tolerated or even beneficial. Such a system could operate if heterozygous sites are recognised and ‘repaired’ during the initial stages of crossing over. Such repairs involve an extra round of DNA replication, providing an opportunity for further mutations, thereby raising the local mutation rate. If so, the changes in (...) heterozygosity that occur when populations grow or shrink could feed back to modulate both the rate and the distribution of mutations. Here, I review evidence from isozymes, microsatellites and single nucleotide polymorphisms that this potential is realised in real populations. I then consider the likely implications, focusing particularly on how these processes might affect microsatellites, concluding that heterozygosity does impact on the rate and distribution of mutations. (shrink)

It is hypothesized that human faces judged to be attractive by people possess two features—averageness and symmetry—that promoted adaptive mate selection in human evolutionary history by way of production of offspring with parasite resistance. Facial composites made by combining individual faces are judged to be attractive, and more attractive than the majority of individual faces. The composites possess both symmetry and averageness of features. Facial averageness may reflect high individual protein heterozygosity and thus an array of proteins to which (...) parasites must adapt. Heterozygosity may be an important defense of long-lived hosts against parasites when it occurs in portions of the genome that do not code for the essential features of complex adaptations. In this case heterozygosity can create a hostile microenvironment for parasites without disrupting adaptation. Facial bilateral symmetry is hypothesized to affect positive beauty judgments because symmetry is a certification of overall phenotypic quality and developmental health, which may be importantly influenced by parasites. Certain secondary sexual traits are influenced by testosterone, a hormone that reduces immunocompetence. Symmetry and size of the secondary sexual traits of the face (e.g., cheek bones) are expected to correlate positively and advertise immunocompetence honestly and therefore to affect positive beauty judgments. Facial attractiveness is predicted to correlate with attractive, nonfacial secondary sexual traits; other predictions from the view that parasite-driven selection led to the evolution of psychological adaptations of human beauty perception are discussed. The view that human physical attractiveness and judgments about human physical attractiveness evolved in the context of parasite-driven selection leads to the hypothesis that both adults and children have a species-typical adaptation to the problem of identifying and favoring healthy individuals and avoiding parasite-susceptible individuals. It is proposed that this adaptation guides human decisions about nepotism and reciprocity in relation to physical attractiveness. (shrink)

It is illegitimate to read any ontology about "race" off of biological theory or data. Indeed, the technical meaning of "genetic variation" is fluid, and there is no single theoretical agreed-upon criterion for defining and distinguishing populations (or groups or clusters) given a particular set of genetic variation data. Thus, by analyzing three formal senses of "genetic variation"—diversity, differentiation, and heterozygosity—we argue that the use of biological theory for making epistemic claims about "race" can only seem plausible when it (...) relies on the user’s own assumptions about race; the move from biological measures to claims about “race” inevitably amounts to a pernicious reification. We also excavate assumptions in the history of the technical discourse over the concept of "race" (e.g., Livingstone's and Dobzhansky's 1962 exchange, Edwards' 2003 response to Lewontin 1972, as well as contemporary discussions of cladistic "race", and "races" as clusters). We show that claims about the existence (or non-existence) of "race" are underdetermined by biological facts, methods, and theories. Biological theory does not force the concept of "race" upon us; our social discourse, social ontology, and social expectations do. We become prisoners of our abstractions at our own hands, and at our own expense. (shrink)

The possibility of performing germline modifications on currently living individuals targets future generations’ health and well-being by reducing the diversity of the human gene pool. This can have two negative repercussions: reduction of heterozygosity, the latter being associated with a health or performance advantage; uniformization of the genes involved in reproductive recombination, which may lead to the health risks involved in asexual reproduction. I argue that germline interventions aimed at modifying the genomes of future people cannot be ethically justifiable (...) if there is no possibility of controlling the intervention either by reversing or altering it, whenever need demands it. This argument is challenged on six different grounds: safety, population versus individual focus, spontaneous mutations, exceptionalism, the intentional pursuit of genetic diversity through germline interventions, and harm reduction potential. (shrink)

Self-resemblance has been found to have a context-dependent effect when expressing preferences for faces. Whereas dissimilarity preference during mate choice in animals is often explained as an evolutionary adaptation to increase heterozygosity of offspring, self-resemblance can be also favored in humans, reflecting, e.g., preference for kinship cues. We performed two studies, using transformations of facial photographs to manipulate levels of resemblance with the rater, to examine the influence of self-resemblance in single vs. coupled individuals. Raters assessed facial attractiveness of (...) other-sex and same-sex photographs according to both short-term and long-term relationship contexts. We found a preference for dissimilarity of other-sex and same-sex faces in single individuals, but no effect of self-resemblance in coupled raters. No effect of sex of participant or short-term vs. long-term attractiveness rating was observed. The results support the evolutionary interpretation that dissimilarity of other-sex faces is preferred by uncoupled individuals as an adaptive mechanism to avoid inbreeding. In contrast, lower dissimilarity preference of other-sex faces in coupled individuals may reflect suppressed attention to attractiveness cues in potential alternative partners as a relationship maintenance mechanism, and its substitution by attention to cues of kinship and psychological similarity connected with greater likelihood of prosocial behavior acquisition from such persons. (shrink)

Perhaps 20% of known animal species are haplodiploid: unfertilized haploid eggs developinto males and fertilized diploid eggs into females. Sex determination in such haplodiploid species does not rely on a difference in heteromorphic sex chromosome composition but the genetic basis has been elucidated in some hymenopteran insects (wasps, sawflies, ants, bees). In these species, the development into one sex or the others depends on an initial signal whether there is only one allele or two different alleles of a single gene, (...) thecomplementary sex determiner(csd), in the zygotic genome. The gene has been most‐recently identified in the honey bee and has been found to encode an arginine serine‐rich (SR) type protein. Heterozygosity generates an active protein that initiates female development while hemizygosity/homozygosity results in a non‐active CSD protein and default male development. I will discuss plausible models of how the molecular decision of male and female is made and implemented. Comparison to hierarchies of dipteran insects suggests that SR‐type protein has facilitated the differentiation of sex‐determining systems and hierarchies. BioEssays 26:1131–1139, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

The majority of higher plants (including almost all important crops) demonstrate strict uniparental maternal inheritance of plasmagenes in the process of conventional sexual crossing; it is therefore impossible to generate heterozygosity for these genes with standard crossing procedures. However, recent experiments have shown that hybrid plants can be produced by somatic cell fusion and that these contain the cytoplasmic genes of both parents. The phenotypic and genetic properties of these hybrid plants are described here.

Classification of the boys in 48 families of three or more brothers according to their Moray House Test (VIQ) scores awarded at age 11±1[fraction one-half] years in 1947–75 confirms the postulated existence of a total of nine male phenotypes for this X-linked trait in the score range <70 to 140 points. The phenotypic means lie close to the sequence 69(8)133. Recombination is shown to occur. An additive effect of three alleles at each of two X-chromosome loci is the most likely (...) simple scheme. An iterative approach matching the observed mixtures of proposed genotypes in the families with probabilities for variable levels of homozygosity, non-crossover/crossover heterozygosity, and maternal meiotic recombination yields a best goodness of fit where this last is 20%. (shrink)

The term ‘breeding system’ is used to describe the morphological and behavioural aspects of the sexual life cycle of a species. The yeast breeding system provides three alternatives that enable hapoids to return to the diploid state that is necessary for meiosis: mating of unrelated haploids (amphimixis), mating between spores from the same tetrad (intratetrad mating, automixis) and mother daughter mating upon mating type switching (haplo‐selfing). The frequency of specific mating events affects the level of heterozygosity present in individuals (...) and the genetic diversity of populations. This review discuses the reproductive strategies of yeasts, in particular S. cerevisiae (Bakers' or budding yeast). Emphasis is put on intratetrad mating, its implication for diversity, and how the particular genome structure could have evolved to ensure the preservation of a high degree of heterozygosity in conjunction with frequent intratetrad matings. I also discuss how the ability of yeast to control the number of spores that are formed accounts for high intratetrad mating rates and for enhanced transmission of genomic variation. I extend the discussion to natural genetic variation and propose that a high level of plasticity is inherent in the yeast breeding system, which may allow variation of the breeding behaviour in accordance with the needs imposed by the environment. BioEssays 28: 696–708, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

Current theories of breast cancer progression have been greatly influenced by the development and refinement of mouse transgenic and gene targeting technologies. Early transgenic mouse models confirmed the involvement of oncogenes, previously implicated in human breast cancer, by establishing a causal relationship between overexpression or activation of these genes and mammary tumorigenesis. More recently, the importance of genes located at sites of loss of heterozygosity in human breast cancer have been examined in mice by their targeted disruption via homologous (...) recombination. The union of these two approaches allows the generation of complex animal models that more accurately reflect the multistep nature of human breast cancer. This review will examine how the study of transgenic mice has increased our understanding of the molecular events responsible for oncogenic transformation of the mammary gland. BioEssays 22:554–563, 2000. © 2000 John Wiley & Sons, Inc. (shrink)

The problem of heterosis has been scrutinized from two points of view: the general experimental methodology and the interpretation of the facts in existing theories.Concerning the general experimental methodology, the relativity of the concept of heterosis is shown with respect to age and time of development of the parental strains, to the variations of the environmental conditions and to the direction of the reciprocal crosses. Therefore, in a large number of cases, experiments have taught not to be exhaustive.Concerning the existing (...) theories, heterosis is considered to be the result of a heterozygosity which can express itself at three levels, which are defined as heteroallely, heterogenomy and heterocytomy. Some works, which are well-representative for one of these three levels, are analysed, and within the results and theories of these works is looked for a point of agreement, in order to render a general hypothesis of heterosis possible. This point of agreement appears to be the cytoplasm, or more precisely, a nucleo-cytoplasmic equilibrium. In this way alone it is possible to disclose the existing continuity from crosses, as well as in the expression of the ovula-spermatozoid compatibility, which varies from lethality to hybrid vigour. From these evidences a general theory of heterosis is presented, by which heterosis is explained as the result of a disturbed nucleocytoplasmic equilibrium: it considers heterosis as a particular case of a more general law,i.e. the ovula-spermatozoid compatibility.This hypothesis is also discussed with respect to the diverse existing theories of cytoplasmic heredity. Some unsolved questions bearing on these problems are put forward, and the way to their solution has been indicated. (shrink)