It was discovered that the effect of a grandmother's presence on grandchild survivorship corresponds relatively with her X-relatedness to the grandchild, which is not equivalent in boys and girls.
A child mortality in the first three years of life it was found that a grandmothers' effect on grandchildren varies according to their X-chromosome relatedness.
Maternal grandmothers have 25% X relatedness with both grandsons and granddaughters and both grandchildren are equally likely to inherit any one of her X-linked genes but paternal grandmothers have no X relatedness with grandsons and a 25% relatedness with granddaughters. The influence of the grandmothers parallels the relatedness to the grandchildren.
Daryl Dawkins and his 'selfish gene' hypothesis has some support here.
While the Elephant Man may not have been an 'animal', apparently grandmothers are.
Contrastingly, paternal grandmothers will pass on one of her X chromosomes to their granddaughters (making them 50% X-related) but she will not pass this chromosome on to her grandson (making them 0% X-related).
"The presence of a paternal grandmother in all seven of the populations had a harmful effect on grandsons because her presence was linked with an increase in mortality.
"Meanwhile, in six out of seven populations, the paternal grandmother's presence in her granddaughter's early life had a beneficial effect in terms of the risk of mortality. This difference between paternal grandsons and granddaughters would explain a lot of the inconsistencies in previous studies, where the sex of the grandchild was not considered.
"We've only looked at child mortality, and the mechanism itself remains mysterious. Other studies have given evidence against conscious favouritism towards one grandchild or another".
The importance of grandmothers in the lives of their grandchildren