两个夏尔巴儿童

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人口孤立株的遗传学

“人口隔离”是指族裔群体，族裔群体与周围人群分开，并具有最小的基因流。这些障碍可以是地理，文化或语言。此外，孤立的人群还充当旨在理解常见疾病基础的研究的催化剂。因此，研究人群孤立株一直对人类学家引起极大的兴趣。

Two new papers published in Genome Biology focus on two such isolates and explore their demographic history and genomic selection and while I pen down my thoughts about them, I recall two individuals from these populations who shared their thoughts during a journey. Little did I know then, that the information shared by them would be a connecting thread to write on these population isolates.

帕尔西: like sugar in milk

帕尔西是世界上最小的民族宗教社区之一，根据2011年印度人口普查，约有57,264名成员（从2001年的69,601和1940年的114,000名成员下降）。他们的出生：死亡率在过去一个世纪稳步下降，从而使他们更加脆弱。

为了控制这一下降，2013年，印度政府创建了“ Jiyo Parsi”（Live Parsi的字面意义）运动。

This population traces its origin to Persia (present day Iran), from where they fled during 8-10^Th世纪伊斯兰征服。A group of Zoroastrian first landed in Sanjan (Gujarat, India), and were granted asylum by the local King named Jadi Rana with a promise to mix culturally with native Hindus “like sugar in milk”.

The Parsi make up less than 0.005% of India’s population, but as described by the famous Indian writer Amitav Ghosh, “they have essentially created modern India”.

Genetic history of Parsi populations

Previous studies on Parsi populations have relied mainly on small sample sizes and low resolution genetic markers, limiting the power of the analysis.此外，主要的Parsi集团（居住在印度）的代表性不足，同样，在这场辩论中也没有考虑过高分辨率的常染色体证据。

两个印度人and Pakistani Parsi populations are genetically closer to each other and Iranians than they are to the neighboring Indian and Pakistani populations.

Using a combination of high resolution uniparental and biparental genetic markers on modern Parsi samples as well as mtDNA markers on ancient Parsi samples,最近的研究specifically looked at whether modern Parsi people living in India and Pakistan have a common genetic basis, shared among them and also the Iranian population, or if their genomes had been affected by admixture with their Indian and Pakistani neighbors? Also, to know that at what extent current Parsi populations of India assimilated local females in the past? First successful ancient DNA data from remains excavated in Sanjan (Gujarat, India) were analyzed.

与印度的其他历史移民相比，与当地南亚人口的混合量很少（例如犹太人and西迪分析了在我们以前的工作)。两个印度人and Pakistani Parsi populations are genetically closer to each other and Iranians than they are to the neighboring Indian and Pakistani populations. Neolithic Iranian samples shared higher number of alleles with Parsis than modern day Iranians.

Of course, the modern Iranian population exhibited marked difference from Parsis, in that they carry an additional European ancestry, and significantly lower South Asian specific ancestries. This genomic difference was likely due to Islamic conquest in Persia.

Indian and Pakistani Parsi populations showed high level of homozygosity in their genome compared to their putative parental and present neighbors. This is likely due to small population size and high level of inbreeding.

Maternal ancestry was examined via analysis of mtDNA haplogroup composition in the ancient remains excavated from Sanjan. With this information, the authors observed that there seems to have been some assimilation of local South Asian women during initial settlement, indicating a single wave of population admixture that corresponds with the historical date of arrival of Paris to South Asia. On the other hand, paternal heritage aligns between the Iranian and Pakistani populations.

Sherpa and Tibetan: the fine scaled differentiation

People living at high altitude, such as the Sherpa in Nepal, have been exposed to low pressure and decreased oxygen levels for several thousand years, facilitating natural selection to live efficiently under high altitude hypoxia.

科学家建议，全世界的高海拔适应不一定是共享的通过identity-by-descent. For example, Andean highlanders’ high-altitude adaptation is enabled by increased oxygen-carrying blood hemoglobin, whileless血红蛋白是藏族和夏尔巴高海拔适应的标志。降低的血液血红蛋白有助于这些种群避免血液凝块和由血液增厚引起的中风。

一系列关于夏尔巴人和藏人的研究揭示了高海拔适应的奥秘层经过层。

夏尔巴斯和藏人的遗传历史

夏尔巴人和藏人的遗传起源和相互关系最近一直是热门辩论的话题。使用性别特异性标记物（MTDNA和Y染色体）的研究表明，夏尔巴人种群是藏族的最新分支，而基于常染色体数据和高海拔适应性的研究发现，藏族种群是在祖先Sherpa和Han Centern之间的混合物之后起源的。

To resolve this complexity,Zhanget al.研究了一个更广泛的地理区域，分析了来自藏族的全基因组序列和基因分型数据（来自西藏和青海）和Sherpas（来自西藏 - Zhangmu和Nepal-Khumbu）。重叠区域中两个人群的抽样覆盖范围是排除等位基因共享的有希望的步骤通过The isolation by distance model.

Although a substructure exists among both Tibetans and Sherpa, they are two genetically distinct populations. Sherpas have significant South Asian ancestry compared with Tibetans.

有趣的是，与他们的地理位置相反，藏族（Zhangmu）夏尔巴人的南亚血统高于尼泊尔（Khumbu）Sherpa，这可能是由于尼泊尔人（Khumbu）Sherpa的基因流量较低而引起的，他们的生活在较高的情况下。高度。该人群的血缘气质也很高，有效的人口规模较低 - 进一步反映了其地理隔离。

In fact, the authors suggest that other components (e.g. East Asian and Siberian/Central Asian) in Sherpa genomes are likely caused by indirect gene flow from Tibetans.

值得注意的是，这项研究推断了夏尔巴人在320-360世代前的大瓶颈，这在藏族的遗传史上找不到。

The authors estimate that Nepalese Sherpa diverged from Han Chinese 9,500-18,000 years ago, and from Tibetans 6,100-13,300 years ago. Meanwhile, Nepalese Sherpa and Tibetan Sherpa show a more recent divergence time of 1,200-7,800 years ago.

这项研究还确定了新型Sherpa特异性基因（例如aldh3a1，，，，ANGPT1），这可能有助于高空适应。