Academic journal article Mankind Quarterly

Heterosis versus Inbreeding Depression Effects: An Investigation of Personality, Digit Ratio (2D:4D), and Laterality Traits

Academic journal article Mankind Quarterly

Heterosis versus Inbreeding Depression Effects: An Investigation of Personality, Digit Ratio (2D:4D), and Laterality Traits

Article excerpt

The opposed genetic effects of heterosis and inbreeding depression consist of beneficial augmentations (vs. detrimental reductions) of phenotypic trait values in offspring of genetically distant (vs. related) parents. This is due to increased offspring heterozygosity for heterosis vs. increased homozygosity for inbreeding depression, with the consequence of escape from (vs. falling prey to) the cumulative effects of deleterious recessive alleles. Heterosis/inbreeding depression is well documented for human health outcomes and psychometric intelligence, but has only sporadically been investigated for heritable personality traits. This study of 129 adults from a geographically isolated region in westernmost Austria examined heterosis/inbreeding depression in differentially heritable individual difference variables: personality dimensions (Big Five, sensation seeking), digit ratio (2D:4D), and laterality traits. Consistent with heterosis/inbreeding depression effects, 2D:4D (strongly genetically influenced) of both hands was expressed more sex-typically with farther birthplace distance of relatives (approximating genetic distance) among women (but not men), whereas no effects emerged for directional asymmetry in 2D:4D (almost non-heritable) or laterality traits (only weakly heritable). However, not replicating recent suggestive findings, effects were also largely absent for the (strongly heritable) Big Five and sensation seeking personality dimensions. Future directions for exploring heterosis and inbreeding depression in the personality and individual differences domain are discussed.

Key words: Heterosis; Hybrid vigor; Inbreeding depression; Big Five; Sensation seeking; Digit ratio (2D:4D); Lateral preferences

Introduction

Heterosis (synonyms: hybrid vigor, outbreeding enhancement) and its opposite, inbreeding depression, are well known, entirely genetic effects. They have been observed in many species, ranging from plants and animals to humans. Both phenomena affect individuals' (genetically influenced) phenotypic trait values through redistribution of existing genetic configurations in a population, depending on specific mating and reproduction habits therein (Charlesworth & Willis, 2009). Heterosis (vs. inbreeding depression) consists in beneficial augmentations (vs. detrimental reductions) of phenotypic trait values in the offspring of genetically distant (vs. related) parents by decreasing (vs. increasing) the proportion of genetic homozygosity within individuals. The affected traits include fitness-related characters related to offspring survival, fertility, and health.

The genetic basis of both effects is the perennial existence of mutational load in the genome (Charlesworth & Willis, 2009). New deleterious alleles arise constantly through spontaneous mutation, but those that are expressed recessively are asymptomatic in heterozygotes and thus cannot easily be removed by natural selection. The probability of receiving a double dose of a recessive allele on any genetic locus increases with increasing parental genetic similarity. Hence, offspring of closely related, genetically more similar parents have higher degrees of homozygosity across genetic loci. In consequence, they are more likely than the children of unrelated parents to show the detrimental effects of recessive mutations. That is, inbreeding depression consists in a loss of individual genetic diversity, whereas heterosis is due to an increased heterozygote-homozygote ratio at every polymorphic genetic locus. Homozygosity by common descent, which technically is the inbreeding coefficient, amounts to 25% of the genome for children of incestuous matings (brother-sister, parentchild) and 6.25% for children of first cousins. Recent research suggests that even outbred modern populations show genome-wide homozygosity ranging from 1% to 5% of the genome, depending on the length of regions of homozygosity considered in genetic analyses (Nalls et al. …

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