新一期的Nature新闻板块有这样一则新闻，标题是：

No ‘gay gene’: Massive study homes in on genetic basis of human sexuality

我第一个反应是，哦天，是说有实锤同性恋不是由遗传决定的吗? 

副标题是这样写的：Nearly half a million genomes reveal five DNA markers associated with sexual behaviour — but none with the power to predict the sexuality of an individual. 
分析了近50万个基因组，发现了5个跟性行为有关的DNA标记，但是没有哪一个能预测一个人的性取向。

OK， 延续了标题给我的印象，性取向不是由基因决定的，找到一些标记，但无法预测的标记有啥用呢？

再继续往下看，

“The largest study1 to date on the genetic basis of sexuality has revealed five spots on the human genome that are linked to same-sex sexual behaviour — but none of the markers are reliable enough to predict someone’s sexuality.”

这段基本是副标题的一个扩展，没有多出什么内容，再看下一段

“The findings, which are published on 29 August in Science and based on the genomes of nearly 500,000 people, shore up the results of earlier, smaller studies and confirm the suspicions of many scientists: while sexual preferences have a genetic component, no single gene has a large effect on sexual behaviours.”

好嘛，刚发表的，还是Science, 近50万人的全基因组分析，与之前的、更小的研究，许多科学家的猜测一致，够有说服力了吧! 那他们究竟得出了什么结论呢？我尝试翻译一下：尽管性取向有遗传因素在里面，但并没有某一个基因在性行为上有很大的决定作用。

原来说了半天只是说性取向不是单基因决定的啊。

这有什么大惊小怪的吗？这个世上单基因决定的性状委实不多阿！看到这里我已经开始怀疑自己一开始关于标题的“性取向不是由基因决定的”反应。

继续下一段：“There is no ‘gay gene’,” says lead study author Andrea Ganna, a geneticist at the Broad Institute of MIT and Harvard in Cambridge, Massachusetts.

原来不是Nature自己起的题目阿，是一作自己说出了“没有所谓的‘同性恋基因’”这样的话。好吧看看他还说了说么。

Ganna and his colleagues also used the analysis to estimate that up to 25% of sexual behaviour can be explained by genetics, with the rest influenced by environmental and cultural factors — a figure similar to the findings of smaller studies.

看到这里我有点理解作者，25%，确实是小于75%的。但是25%的解释度真的很低吗？我们来看看Science小编是如何总结这篇文章的：

The genetics of sexual orientation

Twin studies and other analyses of inheritance of sexual orientation in humans has indicated that same-sex sexual behavior has a genetic component. Previous searches for the specific genes involved have been underpowered and thus unable to detect genetic signals. Ganna et al. perform a genome-wide association study on 493,001 participants from the United States, the United Kingdom, and Sweden to study genes associated with sexual orientation (see the Perspective by Mills). They find multiple loci implicated in same-sex sexual behavior indicating that, like other behavioral traits, nonheterosexual behavior is polygenic.

感觉立场很不一样，这里强调的是大规模和多基因性，但这句“like other behavioral traits”不知道是臧是否。这是一句事实性的描述，看读者怎么理解。你可以说同性行为跟其他许多行为一样，是由多基因决定的，我们不应该把它作为一个什么特殊的行为。也可以理解为同性行为跟其他许多行为一样，并没有什么决定基因，所以是可以后天怎样怎样的。

于是问题回到那个关键的25%。

原文的结构式摘要（Structured Abstract）的结果部分是这样写的：

In the discovery samples (UK Biobank and 23andMe), five autosomal loci were significantly associated with same-sex sexual behavior. 
在我们所分析的来自UK Biobank 和 23andMe样本里，五个来自常染色体的位点与同性行为有显著关联。

Follow-up of these loci suggested links to biological pathways that involve sex hormone regulation and olfaction. 
对这五个位点的进一步研究显示他们在与性激素的调节和嗅觉相关的生物学通路上。（从生物学角度来讲能说得通）

Three of the loci were significant in a meta-analysis of smaller, independent replication samples. 
其中有三个位点在另一个更小的、独立的荟萃分析样本集中仍然显著。（～5000人，Wald test P值三者为0.027, 0.003, 0.006）

Although only a few loci passed the stringent statistical corrections for genome-wide multiple testing and were replicated in other samples, our analyses show that many loci underlie same-sex sexual behavior in both sexes. 
虽然只有几个位点通过了严格的对全基因组多次检验进行的统计修正，并在其他样本集依然显著，我们的分析显示很多其他位点都与无论是在男性还是女性的同性行为有关。

In aggregate, all tested genetic variants accounted for 8 to 25% of variation in male and female same-sex sexual behavior, and the genetic influences were positively but imperfectly correlated between the sexes [genetic correlation coefficient (rg)= 0.63; 95% confidence intervals, 0.48 to 0.78]. 
这句话前半句有些模糊，不好翻译，我找到了正文中的原文，说的更清楚一些：
Nevertheless, the contribution of all measured common SNPs in aggregate (SNP- based heritability) was estimated to be 8 to 25% (95% CIs, 5 to 30%) of variation in female and male same-sex sexual behavior, in which the range reflects differing estimates by using different analysis methods or prevalence assumptions (table S11) (14) 
将所有经过检验的，两个数据集（UK Biobank和23andMe）里共有的单核苷酸多样性（SNPs）数据的效用加起来，共能解释8-25%的与同性行为（无论男女），这个范围反映了基于不同流行率假定下的不同计算方法所得出的结果。（看来25%还是最高估计啊）

后半句是说：遗传因素在不同性别中呈不完美的正相关（即一个SNP在男性中与同性行为呈正相关的话，这个SNP在女性中也与同性行为呈正相关的概率也更大），遗传相关系数为0.63（95%置信区间为0.48-0.78）。（即男女之间有共同之处，但差异也不可忽略。）

These aggregate genetic influences partly overlapped with those on a variety of other traits, including externalizing behaviors such as smoking, cannabis use, risk-taking, and the personality trait “openness to experience.” 
这些影响同性行为的遗传变异与影响其他人类特征，如抽烟、吸食大麻和冒险行为等外显行，以及“乐于接受新的体验”的个性的遗传变异有部分重叠。

Additional analyses suggested that sexual behavior, attraction, identity, and fantasies are influenced by a similar set of genetic variants (rg > 0.83); 
进一步的分析表明，影响性行为、性吸引、性身份以及性幻想的遗传变异组成相似（rg > 0.83）；

however, the genetic effects that differentiate heterosexual from same-sex sexual behavior are not the same as those that differ among nonheterosexuals with lower versus higher proportions of same-sex partners, which suggests that there is no single continuum from opposite-sex to same-sex preference. 
但是，区分异性和同性行为的遗传变异与区分有同性行为的人群中同性伴侣占比高低的遗传变异并不一致，这说明从遗传上讲从异性倾向到同性倾向并不呈连续分布。

最后来看看根据这些分析结果，作者得出了什么结论吧：

CONCLUSION: 

Same-sex sexual behavior is influenced by not one or a few genes but many. 
同性行为不止受到一个或几个基因的影响，而是很多个（no "gay gene").

Overlap with genetic influences on other traits provides insights into the underlying biology of same-sex sexual behavior, 
这些（很多个）基因也同时影响人类其他行为或特征，为同性行为的生物学研究带来启示。

and analysis of different aspects of sexual preference underscore its complexity and call into question the validity of bipolar continuum measures such as the Kinsey scale. 
同时，对性倾向的其他（非生物学）方面的分析也突出了它的复杂性，让我们有理由质疑现行的一些两极化连续评价体系，如金赛量表。

Nevertheless, many uncertainties remain to be explored, including how sociocultural influences on sexual preference might interact with genetic influences. 
然而，很多未知，包括社会文化对性偏好的影响及其与遗传变异的互作关系，还有待探索。

To help communicate our study to the broader public, we organized workshops in which representatives of the public, activists, and researchers discussed the rationale, results, and implications of our study. 
为了帮助我们向大众沟通这项研究，我们还举办了研讨会并邀请了一些民间代表、社会活动家和科学家来一起讨论这项研究的原理、结果和启示。

翻译到这里我觉得原文正文里的这句话最适合用来总结，
The discrepancy between the variance captured by the significant SNPs and all common SNPs suggests that same-sex sexual behavior, like most complex human traits, is influenced by the small, additive effects of very many genetic variants, most of which cannot be detected at the current sample size (22). 
统计结果显著的SNPs所能解释的同性行为变异(非常小)与两个数据集共有的SNPs所能解释的变异（8-25%）之间的差异表明，同性行为如大多数复杂的人类特征一样，受到到很多各自效力很小的遗传变异的加性效应的影响，但就当前的样本量，这些遗传变异大多数无法被检测到（500k的样本量还是敌不过人类基因组3亿对碱基，文中涉及的SNPs与样本量也只差一个数量级）。

面对这样的结果，我实在无法赞同Nature新闻标题里的“homes in”这个说法。感觉未知还是大于已知。如果想了解更多该实验的背景，推荐哈佛大学杂志的这篇报道。