Boys or girls don’t run in families

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  Last updated February 20, 2020 at 11:46 am

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Old myths that having girls or boys ‘runs in the family’ have been debunked – according to research the gender ratio is all up to chance.


gender ratio_children_kids

There’s no way to predict whether you’ll have more boys or girls – it’s all up to chance. Credit: Liderina




Why This Matters: The findings suggest we need to rethink of offspring sex ratio theory.




It’s been long thought that the odds of having a boy or a girl is a heritable trait that is passed down through generations.


However, a study by the University of Queensland has upended the theory, proving parents’ genes do not determine their child’s gender.


Gender ratio is all up to chance


Brendan Zietsch from UQ’s School of Psychology says the study was the largest conducted on the often-debated question, and concluded the sex of offspring is essentially random.


“We found individuals don’t have an innate tendency to have offspring of one or the other gender,” he says.


“The chances are more like 51 to 49 of having a boy, but the genes of the mother and father don’t play any role.”




Also: Husband’s genes can affect the age women become mothers




The study used data from Swedish population registers, which includes every Swede born since 1932, equating to 3,543,243 individuals and their 4,753,269 children.


The research team linked all family members and tested whether the sex of a person’s children was associated with the sex of their brother or sister’s children.


The findings are crucial for evolutionary theories


Zietsch explains the large body of scientific theory around what influences whether someone has boys or girls had been proven to be wrong, and it has implications for evolutionary theories.


“These findings have crucial implications for biological and evolutionary theories of offspring sex ratios.”


“It was thought that rich or tall parents should have more boys and beautiful parents should have more girls.


“It was also thought that parents’ hormone levels at the time of conception were important.


“Our results rule out all these possibilities and suggest a rethink of offspring sex ratio theory is necessary to properly understand why offspring sex ratios appear to vary, for example, across countries.”


The study is published in the scientific journal Proceedings of the Royal Society B.


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