一項(xiàng)加州大學(xué)和耶魯大學(xué)的共同研究表明，朋友之間雖然沒有血緣關(guān)系，但是在基因的其它方面卻有不同程度的相似性。這項(xiàng)研究成果于14日發(fā)表在了PNAS上。
“縱觀人類整個(gè)基因組，不難發(fā)現(xiàn)，我們與我們的朋友在基因上是相似的，同陌生人相比，我們之間有更多相同的DNA”，參與研究的加州大學(xué)圣迭戈分校教授詹姆斯•福勒說。
這是一項(xiàng)針對(duì)1500萬個(gè)變異標(biāo)記基因的基因組學(xué)分析，數(shù)據(jù)是來源于弗雷明翰心臟研究所，這個(gè)資料庫包含了大量的遺傳信息以及這個(gè)個(gè)體之間是否為朋友關(guān)系。
研究人員的目標(biāo)是1932個(gè)獨(dú)特個(gè)體，兩兩比較無血緣關(guān)系的朋友之間和無血緣關(guān)系的陌生者之間的基因組信息，除血緣關(guān)系之外，也排除了配偶的關(guān)系。因此，兩組之間唯一不同的就是他們的社會(huì)關(guān)系。
研究人員說，研究中排除了人們因?yàn)榉N族背景而導(dǎo)致的交朋友的傾向性，因?yàn)閿?shù)據(jù)來源都是歐洲人。當(dāng)然，這也是研究的一個(gè)缺點(diǎn)，但是在此項(xiàng)研究中是必須的。研究還通過現(xiàn)有技術(shù)排除了比較者具有共同祖先的問題。
究竟相似度有多少
Fowler and Christakis說，平均來看，朋友之間約有1%的基因相似度，即有如同第四代表兄弟表姐妹的相關(guān)性，或者是相當(dāng)于擁有共同曾曾曾祖父母的基因相似度。
1%可能聽起來很小，但是在遺傳學(xué)中是一個(gè)意義重大的數(shù)字。這樣來比較，有很多人甚至都不知道他們的第四代表兄弟姐妹是誰，因此，這個(gè)相似度的卓越性就可想而知了。
此外，F(xiàn)owler and Christakis在研究中還發(fā)明了一種他們叫做“友誼分?jǐn)?shù)”的項(xiàng)目，可以用來預(yù)測誰和誰會(huì)成為朋友，他們對(duì)這一預(yù)測準(zhǔn)確度充滿信心，稱堪比現(xiàn)如今通過基因預(yù)測肥大癥和精神分裂癥的水平。
究竟哪些基因相似
在之前的研究中，研究人員發(fā)現(xiàn)，對(duì)于一個(gè)稱為DDR2的基因上的某一位點(diǎn)，朋友之間傾向于擁有相同的SNP；而對(duì)于一個(gè)叫CYP2A6的基因上的某一位點(diǎn)，朋友之間的SNP差別反而比其他人大。
目前人們還不清楚上述兩個(gè)基因的具體作用，但以前有研究顯示，DDR2基因與酗酒的習(xí)慣有關(guān)，而CYP2A6影響人體的藥物代謝和激素合成，與煙癮和思維開放的程度有關(guān)。有趣的是，福勒教授承認(rèn)，他自己就具有DDR2基因。
而在本次研究中，研究人員又發(fā)現(xiàn)朋友之間最相似的基因是控制味覺的基因，而差異最大的是控制免疫力的基因，因此朋友們面對(duì)不同疾病時(shí)具有不同的天生抵抗力，這是一個(gè)相對(duì)簡單的進(jìn)化優(yōu)勢現(xiàn)象。Fowler and Christakis說，抵擋不同病原體的人們之間有更多的聯(lián)系，這就降低了病原體的傳播概率，但是其中的作用機(jī)制仍然未知。
這一發(fā)現(xiàn)也支持了最近刊登在PNAS上的有關(guān)配偶的研究，研究人員分別對(duì) 800對(duì)已婚夫妻的DNA 和800對(duì)隨機(jī)選取的陌生人的DNA進(jìn)行了對(duì)比。結(jié)果顯示，夫妻之間的DNA相似度要高于任意兩個(gè)陌生人之間的DNA相似度。基因?qū)θ祟惢橐龅挠绊懜菀桌斫?，它可能是潛移默化的，比如，基因?huì)影響人的長相、身高等外貌上的特征，在擇偶時(shí)這些都是“硬性”條件。
這是具有進(jìn)化優(yōu)勢的
朋友之間有共同的屬性，或者說有“功能性的親屬關(guān)系”，是具有一系列的進(jìn)化優(yōu)勢的。
只有你的朋友也具有某些特性，那么你的這些特性才能發(fā)揮出來。
研究中最有趣的發(fā)現(xiàn)在于，朋友之間最相似的基因似乎比其它基因進(jìn)化得更快。他們說這可能是人類在過去的三萬年間進(jìn)化如此之快的原因所在，而社會(huì)環(huán)境本身就是進(jìn)化的力量來源。
這項(xiàng)新發(fā)現(xiàn)同時(shí)也表明了人類的宏基因組學(xué)不僅僅與微生物有關(guān)，與周圍的人類也有關(guān)，我們的健康不僅依賴于自身的遺傳，與我們朋友的基因構(gòu)成也有某種程度的相關(guān)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.
