Over 4cm of men's height is due to the combination of just two genes, according to a new study undertaken at the University of Melbourne.
The research, published in the Journal of Clinical Endocrinology and Metabolism this week, identifies one gene on chromosome 15 and one on the Y chromosome as contributors to height in men. These genes may also help explain why men are taller than women.
The genes operate independently but have an additive effect, said Dr Justine Ellis, who authored the study with colleagues Professor Stephen Harrap and Dr Margaret Stebbing from the Department of Physiology at the University of Melbourne.
"If you have the tall version of both of these genes you will be taller than a person who only has the tall version of one of them," she said.
The Y chromosome is the male sex chromosome, absent in women. The gene on chromosome 15, called CYP19, codes for aromatase - an enzyme that converts testosterone into oestrogen in both sexes. Oestrogen influences height because it is responsible for bones fusing over at the ends, which stops people growing.
Aromatase seems to have its effect mainly on long bones, like those in the legs, said Dr Ellis. Men and women generally have similar-sized bodies, but men tend to have longer legs.
"That's why there is a difference in height between men and women - because the legs are so important in determining height," she explained.
A person's height depends on both genetic and environmental influences. Previous studies have identified the aromatase gene and the Y chromosome gene as height genes in men who are very tall or very short, but the Melbourne study shows they also help determine normal adult height in the general population.
"We didn't recruit people on the basis of their height in any way," said Dr Ellis.
The aromatase gene accounted for 2.0cm in height, but the difference between genetically different individuals was much stronger in men (2.3cm) than in women (0.2cm), the researchers found. Variability in the Y chromosome accounted for 1.9cm.
"We have tracked down a couple of genetic components which together determine about 4cm of height in males," Dr Ellis said.
The next step is to look at whether tall men have lower bone density, which may put them at risk of osteoporosis.
"One of the outcomes of work in this area is the design of drugs which encourage aromatase," said Dr Ellis. Such drugs could assist people who have a mutation in the aromatase gene, so cannot make oestrogen in large amounts. This leads to long, thin bones that are vulnerable to fracture.
"In this next study we want to look at the more extreme height difference," said Dr Ellis.
Her team would work with medical researchers who are bone specialists in the next stage of the research, she said. The team will include Associate Professor Ego Seeman from Austin & Repatriation Medical Centre and Associate Professor Peter Ebeling from Royal Melbourne Hospital.

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