Scientists say they have begun to crack the genetic code that helps determine when a girl becomes a woman.

A UK-led team located two genes on chromosomes six and nine that appear to strongly influence the age at which menstruation starts.

The Nature Genetics study also provides a clue for why girls who are shorter and fatter tend to get their periods months earlier than classmates.

The genes sit right next to DNA controlling height and weight.

A second paper, published in the same journal, also concludes that one of the two genes highlighted by the first study plays a key role in the timing of puberty in both girls and boys.

Reproductive lifespan is closely linked to the risk of developing conditions such as heart disease, breast cancer and osteoporosis.

It is thought that the female sex hormone oestrogen - produced at higher rates during a woman's reproductive life - raises the risk of these diseases.

Therefore, the earlier a woman goes through puberty, the more risk she may be at.

So the researchers say their work not only improves our understanding of the genetics underpinning development, it may ultimately aid the fight against disease.

However, they also accept that the onset of puberty is influenced by factors such as nutrition and exercise, and the effect of a single gene is likely to be relatively small.

Developing earlier

In the western world children are reaching puberty at younger and younger ages - some girls at the age of seven.

This study provides the first evidence that common genetic variants influence the time at which women reach sexual maturation
Researcher Dr Anna Murray from the Peninsula Medical School

Many blame rising obesity rates because, generally, girls who achieve menstruation earlier in life tend to have greater body mass index (BMI) and a higher ratio of fat compared to those who begin menstruation later.

From its analysis, a team led by Exeter's Peninsula Medical School predict one in 20 females carry two copies of each of the gene variations which result in menstruation starting earlier - approximately four and half months earlier than those with no copies of the gene variants.

In collaboration with research institutions across Europe and the US, they studied 17,510 women from across the world, including women of European descent who reported reaching menstruation of between nine and 17 years of age.

When they split the women up according to the age they began menstruating, certain gene patterns appeared.

Scanning the whole genome enabled them to hone in on these differences and pinpoint the exact genes most likely accountable.

Researcher Dr Anna Murray said: "This study provides the first evidence that common genetic variants influence the time at which women reach sexual maturation.

"Our findings also indicate a genetic basis for the associations between early menstruation and both height and BMI."

Biological mechanisms

Co-worker John Perry said: "Understanding the biological mechanisms behind reproductive lifespan may also help inform us about associated diseases that affect a lot of women as they get older, including diabetes, heart disease and breast cancer."

The second paper, led by the MRC Epidemiology Unit at Cambridge, analysed genetic information from thousands of people.

It linked a specific variant one of the two genes highlighted by the Exeter team - LIN28B - with earlier breast development in girls, and earlier voice breaking and pubic hair development in boys.

Lead researcher Dr Ken Ong said: "LIN28B works by controlling whether or not other genes are active.

"There are a number of such 'master switch' genes known, but this is the first evidence linking such a gene to growth and physical maturation."

Dr Aric Sigman, psychologist and fellow of the Royal Society of Medicine, said: "Early menstruation is a health issue because beyond being an inconvenient surprise for a girl and her parents, it's also associated with a higher risk of a variety of diseases and psychological problems.

"Girls maturing earlier are more likely to become depressed, delinquent, aggressive, socially withdrawn, suffer sleep problems drinking, smoking, drug abuse, lower self-esteem and suicide attempts.

"They're also more likely to exhibit poor academic performance in high school than on-time or later maturing peers.

"It is important that we understand why early menstruation occurs and these findings bring us closer to explaining this in some girls."

Three other papers, also published in Nature Genetics, throw up other candidate genes which appear to be involved in the onset of puberty.

科学家宣称已经开始破解有助于确定女孩成年的基因密码。

一个由英国领导的科研小组确定了染色体中编号为六号和九号的两条基因似乎强有力地影响了女性月经开始的年龄。

《自然遗传学》的研究也提供了一条线索，那就是矮胖的女孩为什么比班上的同学月经初潮早几个月。

基因与DNA一起控制身高和体重。

发表于该杂志的另一篇论文也断定，由第一项研究强调的两条基因中的一条对于女孩和男孩进入青春期的时间具有重要作用。

人的生殖期限与产生心脏病、乳腺癌和骨质疏松症的条件的危险紧密相连。

据说女性在生殖周期内分泌更多雌性激素使得她们更易于染上这些疾病。

因此，女性经历青春期越早，就会越危险。

研究者认为他们的工作不仅提高了我们对基因基础发展的了解，还会最终有助于抵御疾病。

然而，他们也认为青春期的开始也受到营养和运动等因素影响，单个基因的影响相对而言还是很小的。

发育更早

西方世界的小孩进入青春期的年龄越来越小——有些女孩七岁就进入青春期了。

许多人把此归咎为越来越高的肥胖率，因为在通常情况下，与那些月经初潮晚的女孩相比，月经初潮早的女孩倾向于有更高的身体重量指数（BMI）和肥胖率。

据埃克塞特市的半岛医学院研究小组分析，20名女性中有一人拥有每个基因变异的两个复制体，这两个复制体导致了月经初潮的提前到来，与那些没有基因变异复制体的女孩相比，她们的月经提前了四个半月。

他们还和欧洲、美国的研究机构合作，研究了全世界17，510名女性，包括具有欧洲血统女子，据说她们在9至17岁有了月经。

当他们根据月经初潮的年龄将女性分类，某些基因图谱就显现了。

审视整个基因组使他们消除了这些不同并且确定了真正起作用的基因。

研究者安娜·墨雷博士宣称：“这项研究为基因变异影响女性性成熟的时间提供了第一个证据。

“我们的发现也显示了月经提前与身高和身体重量指数关系的基因基础。”

生物机制

合作者约翰·佩里指出：“了解隐藏于生殖期限后的生物机制也可以帮助我们认识影响某些女性早衰的疾病，包括糖尿病，心脏病和乳腺癌等。”

由位于剑桥大学的英国医学研究理事会流行病学中心发表的第二篇论文分析了数千人的基因信息。

这篇论文把由埃克塞特市研究小组强调的两条基因中的一个变异——LIN28B——与女孩乳房的提早发育、男孩提前变声和阴毛提前生长进行联合研究。

首席研究员肯·奥博士指出：“LIN28B通过控制其它是否活跃的基因而起作用。

“有许多已知的这样的‘主开关’基因，但是基因与人的生长和身体成熟有联系，这还是第一项证据。

心理学家、皇家医学会成员艾日克·希格曼博士说：“月经初潮提前是一个健康问题，因为对于女孩和她的父母来说，这都是一个令人措手不及的意外，同时也与容易产生多种疾病和心理问题有关。

“女孩发育越早就更容易变得忧郁、容易违法、好斗、孤僻、失眠、喝酒、抽烟、滥用药物、缺乏自尊并有自杀企图。

“与发育合适和晚发育的同龄人相比，她们也更容易在中学表现得学习成绩很差。

“我们了解了为什么月经初潮提前，这些发现也可以进一步解释发生在某些女孩身上的现象，这些都很重要。”

发表于《自然遗传学》的其余三篇论文，提出了另外的三个基因选项似乎也与青春期开始有关。