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No genetic contribution to variation in human offspring sex ratio: a total population study of 4.7 million births.

ABSTRACT: The ratio of males to females among an individual's offspring at birth (offspring sex ratio) has long been of great interest to evolutionary biologists. The human offspring sex ratio is around 1 : 1 and is understood primarily in terms of Fisher's principle (R. A. Fisher, The genetical theory of natural selection, 1930), which is based on the insight that in a population with an unequal sex ratio, each individual of the rarer sex will on average have greater reproductive value than each individual of the more common sex. Accordingly, individuals genetically predisposed to produce the rarer sex will tend to have greater fitness and thus genes predisposing to bearing that sex will increase in frequency until the population sex ratio approaches 1 : 1. An assumption of this perspective is that individuals' offspring sex ratio is heritable. However, the heritability in humans remains remarkably uncertain, with inconsistent findings and important power limitations of existing studies. To address this persistent uncertainty, we used data from the entire Swedish-born population born 1932 or later, including 3 543 243 individuals and their 4 753 269 children. To investigate whether offspring sex ratio is influenced by genetic variation, we tested the association between individuals' offspring's sex and their siblings' offspring's sex (n pairs = 14 015 421). We estimated that the heritability for offspring sex ratio was zero, with an upper 95% confidence interval of 0.002, rendering Fisher's principle and several other existing hypotheses untenable as frameworks for understanding human offspring sex ratio.

SUBMITTER: Zietsch BP

PROVIDER: S-EPMC7062014 | biostudies-literature | 2020 Feb

REPOSITORIES: biostudies-literature

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No genetic contribution to variation in human offspring sex ratio: a total population study of 4.7 million births.

Zietsch Brendan P BP Walum Hasse H Lichtenstein Paul P Verweij Karin J H KJH Kuja-Halkola Ralf R

Proceedings. Biological sciences 20200219 1921

The ratio of males to females among an individual's offspring at birth (offspring sex ratio) has long been of great interest to evolutionary biologists. The human offspring sex ratio is around 1 : 1 and is understood primarily in terms of Fisher's principle (R. A. Fisher, <i>The genetical theory of natural selection</i>, 1930), which is based on the insight that in a population with an unequal sex ratio, each individual of the rarer sex will on average have greater reproductive value than each i ...[more]

PMID: 32070249

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Project description:BACKGROUND: Relative deprivation has previously been discussed as a possible mechanism underlying the income-health relation. The idea is that income matters to the individual's health, over and above the increased command over resources, as the basis of social comparisons between a person and his or her reference group. The following study aimed to analyze the role of individual-level relative deprivation for all-cause mortality in the Swedish population. The Swedish context, characterized by relatively small income inequalities and promoting values as egalitarianism and equality, together with a large data material provide unique possibilities for analyzing the hypothesized mechanism. METHODS: The data used are prospective longitudinal data from the Swedish population and based on a linkage of registers. Restricting selection to individuals 25-64?years, alive January 1st 1990, gave 4.7 million individuals, for whom a mortality follow-up was done over a 16-year period. The individual level relative deprivation was measured using the Yitzhaki index, calculating the accumulated shortfall between the individual's income and the income of all other's in the person's reference group. All-cause mortality was used as the outcome measure. RESULTS: Relative deprivation, generated through social comparisons, is one possible mechanism within the income and health relation. The present study analyzed different types of objectively defined reference groups, all based on the idea that people compare themselves to similar others. Results show relative deprivation, when measured by the Yitzhaki index, to be significantly associated with mortality. Also, we found a stronger effect among men than among women. Analyzing the association within different income strata, the effect was shown to be weak among the poorest. Revealing the importance of relative deprivation for premature mortality, over and above the effect of absolute income, these results resemble previous findings. CONCLUSION: Relative deprivation, based on social comparisons of income, is significantly associated with premature mortality in Sweden, over and above the effect of absolute income. Also, it was found to be more important among men, but weak among the poorest.

Project description:A major component of sex-allocation theory, the Trivers-Willard model (TWM), posits that sons and daughters are differentially affected by variation in the rearing environment. In many species, the amount of parental care received is expected to have differing effects on the fitness of males and females. When this occurs, the TWM predicts that selection should favour adjustment of the offspring sex ratio in relation to the expected fitness return from offspring. However, evidence for sex-by-environment effects is mixed, and little is known about the adaptive significance of producing either sex. Here, we test whether offspring sex ratios vary according to predictions of the TWM in the house wren (Troglodytes aedon, Vieillot). We also test the assumption of a sex-by-environment effect on offspring using two experiments, one in which we manipulated age differences among nestlings within broods, and another in which we held nestling age constant but manipulated brood size. As predicted, females with high investment ability overproduced sons relative to those with lower ability. Males were also overproduced early within breeding seasons. In our experiments, the body mass of sons was more strongly affected by the sibling-competitive environment and resource availability than that of daughters: males grew heavier than females when reared in good conditions but were lighter than females when in poor conditions. Parents rearing broods with 1:1 sex ratios were more productive than parents rearing broods biased more strongly towards sons or daughters, suggesting that selection favours the production of mixed-sex broods. However, differences in the condition of offspring as neonates persisted to adulthood, and their reproductive success as adults varied with the body mass of sons, but not daughters, prior to independence from parental care. Thus, selection should favour slight but predictable variations in the sex ratio in relation to the quality of offspring that parents are able to produce. Offspring sex interacts with the neonatal environment to influence offspring fitness, thus favouring sex-ratio adjustment by parents. However, increased sensitivity of males to environmental conditions, such as sibling rivalry and resource availability, reduces the fitness returns from highly male-biased broods.

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No genetic contribution to variation in human offspring sex ratio: a total population study of 4.7 million births.

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No genetic contribution to variation in human offspring sex ratio: a total population study of 4.7 million births.

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