Grandma plays favourites: X-chromosome relatedness and sex-specific childhood mortality

Molly Fox, Rebecca Sear, Jan Beise, Gillian Ragsdale, Eckart Voland, Leslie A. Knapp

Abstract

Biologists use genetic relatedness between family members to explain the evolution of many behavioural and developmental traits in humans, including altruism, kin investment and longevity. Women's post-menopausal longevity in particular is linked to genetic relatedness between family members. According to the ‘grandmother hypothesis’, post-menopausal women can increase their genetic contribution to future generations by increasing the survivorship of their grandchildren. While some demographic studies have found evidence for this, others have found little support for it. Here, we re-model the predictions of the grandmother hypothesis by examining the genetic relatedness between grandmothers and grandchildren. We use this new model to re-evaluate the grandmother effect in seven previously studied human populations. Boys and girls differ in the per cent of genes they share with maternal versus paternal grandmothers because of differences in X-chromosome inheritance. Here, we demonstrate a relationship between X-chromosome inheritance and grandchild mortality in the presence of a grandmother. With this sex-specific and X-chromosome approach to interpreting mortality rates, we provide a new perspective on the prevailing theory for the evolution of human female longevity. This approach yields more consistent support for the grandmother hypothesis, and has implications for the study of human evolution.

Footnotes

  • The views expressed in this paper are those of the author and do not imply the expression of any opinion on the part of the United Nations Secretariat.

  • These two authors contributed equally to the study.

  • 1 The Y-chromosome contains only 89 genes, accounting for only approximately 0.4 per cent of all human genes (NCBI 2009b), and so it has been left out of this analysis.

    • Received September 11, 2009.
    • Accepted October 2, 2009.
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