一项加州大学和耶鲁大学的共同研究表明，朋友之间虽然没有血缘关系，但是在基因的其它方面却有不同程度的相似性。这项研究成果于14日发表在了PNAS上。
“纵观人类整个基因组，不难发现，我们与我们的朋友在基因上是相似的，同陌生人相比，我们之间有更多相同的DNA”，参与研究的加州大学圣迭戈分校教授詹姆斯•福勒说。
这是一项针对1500万个变异标记基因的基因组学分析，数据是来源于弗雷明翰心脏研究所，这个资料库包含了大量的遗传信息以及这个个体之间是否为朋友关系。
研究人员的目标是1932个独特个体，两两比较无血缘关系的朋友之间和无血缘关系的陌生者之间的基因组信息，除血缘关系之外，也排除了配偶的关系。因此，两组之间唯一不同的就是他们的社会关系。
研究人员说，研究中排除了人们因为种族背景而导致的交朋友的倾向性，因为数据来源都是欧洲人。当然，这也是研究的一个缺点，但是在此项研究中是必须的。研究还通过现有技术排除了比较者具有共同祖先的问题。
究竟相似度有多少
Fowler and Christakis说，平均来看，朋友之间约有1%的基因相似度，即有如同第四代表兄弟表姐妹的相关性，或者是相当于拥有共同曾曾曾祖父母的基因相似度。
1%可能听起来很小，但是在遗传学中是一个意义重大的数字。这样来比较，有很多人甚至都不知道他们的第四代表兄弟姐妹是谁，因此，这个相似度的卓越性就可想而知了。
此外，Fowler and Christakis在研究中还发明了一种他们叫做“友谊分数”的项目，可以用来预测谁和谁会成为朋友，他们对这一预测准确度充满信心，称堪比现如今通过基因预测肥大症和精神分裂症的水平。
究竟哪些基因相似
在之前的研究中，研究人员发现，对于一个称为DDR2的基因上的某一位点，朋友之间倾向于拥有相同的SNP；而对于一个叫CYP2A6的基因上的某一位点，朋友之间的SNP差别反而比其他人大。
目前人们还不清楚上述两个基因的具体作用，但以前有研究显示，DDR2基因与酗酒的习惯有关，而CYP2A6影响人体的药物代谢和激素合成，与烟瘾和思维开放的程度有关。有趣的是，福勒教授承认，他自己就具有DDR2基因。
而在本次研究中，研究人员又发现朋友之间最相似的基因是控制味觉的基因，而差异最大的是控制免疫力的基因，因此朋友们面对不同疾病时具有不同的天生抵抗力，这是一个相对简单的进化优势现象。Fowler and Christakis说，抵挡不同病原体的人们之间有更多的联系，这就降低了病原体的传播概率，但是其中的作用机制仍然未知。
这一发现也支持了最近刊登在PNAS上的有关配偶的研究，研究人员分别对 800对已婚夫妻的DNA 和800对随机选取的陌生人的DNA进行了对比。结果显示，夫妻之间的DNA相似度要高于任意两个陌生人之间的DNA相似度。基因对人类婚姻的影响更容易理解，它可能是潜移默化的，比如，基因会影响人的长相、身高等外貌上的特征，在择偶时这些都是“硬性”条件。
这是具有进化优势的
朋友之间有共同的属性，或者说有“功能性的亲属关系”，是具有一系列的进化优势的。
只有你的朋友也具有某些特性，那么你的这些特性才能发挥出来。
研究中最有趣的发现在于，朋友之间最相似的基因似乎比其它基因进化得更快。他们说这可能是人类在过去的三万年间进化如此之快的原因所在，而社会环境本身就是进化的力量来源。
这项新发现同时也表明了人类的宏基因组学不仅仅与微生物有关，与周围的人类也有关，我们的健康不仅依赖于自身的遗传，与我们朋友的基因构成也有某种程度的相关性。
Genetic and educational assortative mating among US adults
It is well established that individuals are more similar to their spouses than other individuals on important traits, such as education level. The genetic similarity, or lack thereof, between spouses is less well understood. We estimate the genome-wide genetic similarity of spouses and compare the magnitude of this value to a comparable measure of educational similarity. We find that spouses are more genetically similar than two individuals chosen at random but this similarity is at most one-third the magnitude of educational similarity. Furthermore, social sorting processes in the marriage market are largely independent of genetic dynamics of sexual selection.
Correlated genotypes in friendship networks
It is well known that humans tend to associate with other humans who have similar characteristics, but it is unclear whether this tendency has consequences for the distribution of genotypes in a population. Although geneticists have shown that populations tend to stratify genetically, this process results from geographic sorting or assortative mating, and it is unknown whether genotypes may be correlated as a consequence of nonreproductive associations or other processes. Here, we study six available genotypes from the National Longitudinal Study of Adolescent Health to test for genetic similarity between friends. Maps of the friendship networks show clustering of genotypes and, after we apply strict controls for population stratification, the results show that one genotype is positively correlated (homophily) and one genotype is negatively correlated (heterophily). A replication study in an independent sample from the Framingham Heart Study verifies that DRD2 exhibits significant homophily and that CYP2A6 exhibits significant heterophily. These unique results show that homophily and heterophily obtain on a genetic (indeed, an allelic) level, which has implications for the study of population genetics and social behavior. In particular, the results suggest that association tests should include friends’ genes and that theories of evolution should take into account the fact that humans might, in some sense, be metagenomic with respect to the humans around them.
Friendship and natural selection
More than any other species, humans form social ties to individuals who are neither kin nor mates, and these ties tend to be with similar people. Here, we show that this similarity extends to genotypes. Across the whole genome, friends’ genotypes at the single nucleotide polymorphism level tend to be positively correlated (homophilic). In fact, the increase in similarity relative to strangers is at the level of fourth cousins. However, certain genotypes are also negatively correlated (heterophilic) in friends. And the degree of correlation in genotypes can be used to create a “friendship score” that predicts the existence of friendship ties in a hold-out sample. A focused gene-set analysis indicates that some of the overall correlation in genotypes can be explained by specific systems; for example, an olfactory gene set is homophilic and an immune system gene set is heterophilic, suggesting that these systems may play a role in the formation or maintenance of friendship ties. Friends may be a kind of “functional kin.” Finally, homophilic genotypes exhibit significantly higher measures of positive selection, suggesting that, on average, they may yield a synergistic fitness advantage that has been helping to drive recent human evolution.
