The age at which boys begin puberty is linked to when their mothers started having periods, a study suggests.
Mothers who started earlier than their peers had sons who had armpit hair two and a half months earlier, acne and voices breaking two months earlier.
Their daughters, meanwhile, developed breasts six months earlier.
The study, in the Human Reproduction journal, analysed data from nearly 16,000 Danish mothers and children.
One of the authors, Dr Nis Brix, of Aarhus University, Denmark, said:
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"Whenever a doctor meets a patient with delayed or early onset of puberty, the doctor obtains a family history.
"The relationship between the mother's pubertal age and the son's pubertal age has been taken as common knowledge but now our data from a large study confirms it."
The age boys and girls start puberty has been gradually decreasing around the world.
In the UK, it is currently starting about one month earlier every decade. And the current average age is 11 for girls and 12 for boys
Experts put this down to improved health and nutrition in the industrialised world - but studies have also shown a link between obesity and the early onset of puberty.
In 2015, a study indicated early or late onset of puberty were linked to a increased risk of diabetes, obesity, early menopause, cardiovascular disease
In girls, an early puberty was defined as starting between eight and 11, while a late puberty started between 15 and 19. In boys, a normal puberty started between nine and 14.
An American study in January suggested early puberty in girls was also linked to increased likelihood of mental health problems during adolescence and into adulthood.
"Both genetic and environmental factors undoubtedly influence puberty timing," said Dr Christine Wohlfahrt-Veje, a researcher at the University of Copenhagen and one of the lead authors on a study in 2016 looking at puberty and timing.
"Boys and girls inherit from both mothers and fathers - but early pubertal markers, onset of breasts and pubic hair, in girls are less dependent on genetic and hence more on environmental factors such as childhood growth patterns and possibly other environmental exposures."