Proceedings of the Royal Society B doi: 10.1098/rspb.2009.1660
Grandsons and granddaughters differ in the proportion of their X-chromosomes shared with MGMs and PGMs (figure 1). According to our proposed X-linked grandmother hypothesis, if grandmothers invest in grandchildren because of their genetic relatedness with them, then their adaptive incentive to invest may vary in a way that mirrors this variation in genetic relatedness. As a consequence, grandmothers’ differential investment in grandchildren could cause differential survivorship of those grandchildren.
Therefore, MGMs and grandchildren are likely to share 25 per cent of their genomes, while PGM and granddaughter may share a total of approximately 31 per cent of their genes, with a likelihood of 27 per cent inheritance, while a PGM and grandson may share only approximately 23 per cent.
Genetic relatedness between kin is often used to explain behavioural phenomena associated with altruism and caretaking, as well as biological traits and developmental trends. The grandmother hypothesis is the prevailing theory to explain why human female longevity extends beyond menopause. It suggests that elderly women are able to contribute to their grandchildren's survivorship through nutritional provisioning, which would increase a woman's inclusive fitness because she shares one-quarter of her genes with a grandchild. In seven previously studied populations, separating grandchild survivorship rates by sex reveals that X-chromosome relatedness correlates with grandchild survival in the presences of MGMs and PGMs. In all seven populations, boys survive better in the presence of their MGM than PGM. In all bar one population, the PGM has a more beneficial effect on girls than on boys. Our X-linked grandmother hypothesis demonstrates how the effects of grandmothers could be sex-specific because of the unusual inheritance pattern of the X-chromosome. This provides a more universally applicable model to explain differential survival of grandchildren in the presence of their grandmothers.
Grandma plays favourites: X-chromosome relatedness and sex-specific childhood mortality
Molly Fox et al.
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.