<img loading="lazy" decoding="async" class="alignright size-full wp-image-2135" title="Peruvian family" src="https://blog-api.23andme.com/wp-content/uploads/2008/12/mestizo-mestiza_peru.jpg" alt="" width="350" height="266" srcset="https://blogcms.23andme.com/wp-content/uploads/2008/12/mestizo-mestiza_peru.jpg 350w, https://blogcms.23andme.com/wp-content/uploads/2008/12/mestizo-mestiza_peru-300x228.jpg 300w" sizes="auto, (max-width: 350px) 100vw, 350px" />
People sometimes think of ancestry as the &#8220;fun&#8221; part of the 23andMe Personal Genome Serviceâ„¢, an intriguing glimpse at the past that is completely unrelated to our more serious Health and Traits features.
But a new paper shows how ancestry can actually reveal quite a bit about the genetics of disease. In the December issue of <a href="http://cancerres.aacrjournals.org/cgi/content/abstract/68/23/9723" target="_blank" rel="noopener noreferrer">Cancer Research</a>, Laura Fejerman of the University of California San Francisco and several colleagues show that among Latinas, a woman&#8217;s breast cancer risk increases with her proportion of European ancestry.

Most Latinos are descended from mix of Native American ancestors and European immigrants. In countries such as Brazil, where slavery was practiced until the late 19th century, many people have substantial African ancestry as well.
Fejerman and her colleagues used 106 <a href="https://en.wikipedia.org/wiki/Ancestry-informative_marker" target="_blank" rel="noopener noreferrer">ancestry informative markers</a> (AIMs) to estimate the relative European and Native American ancestry of 440 Latina women from the San Francisco Bay area with breast cancer and 597 control subjects without the disease. With every 25 percent increase in European ancestry, the researchers found, a woman&#8217;s chances of having breast cancer rose 79 percent.
The finding is no surprise – Latinas have lower breast cancer rates compared to European women, as do Native Americans. But the study does demonstrate that ancestry and population history can be valuable tools for understanding the genetics of disease.
And researchers still have a lot more to learn about population differences in rates of breast cancer. Though environmental factors are known to play a role, the new study strongly suggests that genetics is worth investigating as well.
There&#8217;s also an intriguing puzzle in breast cancer mortality differences among different populations. Though Latinas, Native Americans and African Americans are less likely to develop the disease, when they do their chances of dying from it are greater. Researchers would like to know whether those differences are due to genetics, differences in access to health care, or other factors.
Image: Peruvian &#8220;<a href="http://www.artnet.com/magazine_pre2000/features/ramirez/ramirez12-02-96.asp" target="_blank" rel="noopener noreferrer">casta</a>&#8221; painting, late 18th c.

23andMe Blog

23andMe Blog Team

Study of Breast Cancer among Latinas

By Chris Gignoux and Mike Macpherson
It should be no surprise that in general, we are more genetically similar to our neighbors than to people living far away. The reason is fairly simple – until recently in human history it was fairly rare for people from widely separated geographic regions to even meet, much less reproduce.
This pattern, known as isolation-by-distance, has been observed in a number of studies over the past several decades. This week, it has been confirmed in Europe by the largest study of its kind to date.
The researchers produced a two-dimensional map, like the one below, that preserves the genetic similarities between individuals as far as possible; in other words, the closer two dots (people) are on the map, the more closely related they are genetically.
<img decoding="async" class="alignright size-full wp-image-815" title="PCA figure modified from Tian et al. PLOS Genet. 2008." src="https://blog-api.23andme.com/wp-content/uploads/2008/08/seldin-cleaned-up-pca.png" alt="" width="620" />A two-dimensional genetic similarity map of Europeans shows the northern and southern clusters. Each colored symbol in the plot on the left represents a person&#8217;s genotype. Note the similar placement of symbols on the plot to the left and the geographic legend to the right. Adapted from Tian et al., Plos Genetics, (2008).
In the figure above, each individual was labeled with their country of origin after the mapmaking procedure was run. If Europe were genetically homogeneous, you would expect the different nationalities to appear in a jumble. Instead, they separate into clusters that remarkably recapitulate the geography of Europe.

<h2 class="p1">Northern vs. Southern Europe</h2>
Even though Europe has been occupied for only a relatively short time compared to other parts of the world, different populations within the continent have had time to differentiate from one another. Scientists have known for a long time that certain traits, like lactase persistence and light-colored eyes and hair are more common in northern than in southern Europe. Likewise, there are certain diseases such as sickle cell anemia that, although rare across Europe, are found more in the south than in the north. Height and skin color also vary from northern to southern Europe: both vary gradually with latitude rather than in quick jumps. 
Early genetic studies (such as those in the landmark population genetics text History and Geography of Human Genes) showed that this north-south cline was also a genetic one: even though Europeans of different nationalities did not fit into simple clusters, there was an overarching north-south difference. Newer studies have increased the number of people typed, and the number of markers, to approach the genome-wide level of hundreds of thousands of SNPs we use here at 23andMe – which brings us to this week&#8217;s paper.
<h2 class="p1">A summary of genome-wide findings</h2>
The Lao et al. study out this week obtained genotypes from more than 2,500 individuals of known European ancestry. Each of the genotypes consists of about half a million SNPs typed on the Affymetrix 500K, a chip similar in size to the Illumina 550K used here at 23andMe. They confirm the findings of several recent but smaller European studies (<a href="http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.0020143">Seldin et al, PLoS Genetics (2006)</a>; Bauchet et al, AJHG (2007); <a href="http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.0040004">Tian et al, PLoS Genetics (2008)</a>; <a href="http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.0030236">Price et al, PLoS Genetics (2008)</a>; <a href="http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1000114">Paschou et al, PLoS Genetics (2008)</a>), namely:
<ul>
<li>Overall, all SNPs and Europeans are very genetically similar.</li>
<li>A small set of SNPs does allow European populations to be distinguished—at least when used among people whose ancestors are all from the same part of Europe—and they are surprisingly effective.</li>
<li>Most of the genetic variation in Europe is found along the north-south axis, which is consistent with archaeological knowledge. The next most prominent axis of genetic variation runs roughly east-west.</li>
<li>More isolated populations tend to exist at the extremes of these plots. In this current paper, the Finns are the only nationality utterly distinct from the rest of the European samples. The Finns speak a different language from much of the rest of Europe.</li>
</ul>
There&#8217;s plenty of action in the blogosphere on this one. For more discussion, check out <a href="http://dienekes.blogspot.com/2008/08/500k-snp-europe-wide-study-of-genetic.html">dienekes&#8217; anthropology blog</a>, <a href="http://anthropology.net/2008/08/11/genetic-geographic-and-linguistic-structure-of-european-populations/">anthropology.net</a>, <a href="http://www.gnxp.com/blog/2008/08/european-population-substructurefinns.php">gene expression</a>, and <a href="http://www.genetic-future.com/2008/08/how-well-does-your-genome-predict-your.html">genetic future</a>.

The Genetic and Geographic Structure in Europe

What can we learn from studying how variations of human genes are spread out around the world?
A lot, said population geneticist and Harvard junior fellow Sohini Ramachandran, who spoke at 23andMe this week.
Ramachandran focused on how genes spread from one continent to another, and how they vary within each region as well.
As an example, she referenced some of the research she did as a graduate student at Stanford, tracing how genetic diversity decreases as the populations sampled get geographically farther away from Africa, long considered the cradle of humanity. For the purposes of measuring genetic diversity, distance from Africa is measured along the same paths that people presumably took at they migrated around the world — over land, without crossing major oceans.
<img decoding="async" class="alignright size-full wp-image-283" style="float: right;" title="rsplosg07fig3b" src="https://blog-api.23andme.com/wp-content/uploads/2008/06/rsplosg07fig3b.jpg" alt="" width="401" srcset="https://blogcms.23andme.com/wp-content/uploads/2008/06/rsplosg07fig3b.jpg 866w, https://blogcms.23andme.com/wp-content/uploads/2008/06/rsplosg07fig3b-300x279.jpg 300w, https://blogcms.23andme.com/wp-content/uploads/2008/06/rsplosg07fig3b-768x713.jpg 768w" sizes="(max-width: 866px) 100vw, 866px" />Genetic diversity in the Americas (the darker the color, the more diverse the populations).
<h6>Genetic Diversity</h6>
As people migrated, Ramachandran said, genetic diversity decreased due to what she called “serial bottlenecks.” In these cases, though a fairly large number of people might reach one spot, only part of that group would move farther on. By chance, some of the genes in the population would not make it into the new territory.



For this reason, Ramachandran said, the researchers find more diversity in DNA samples from African populations than they do in samples from American populations.

Because genetic diversity varies based on who got how far, Ramachandran doesn’t just study the genetic variations across continents but between continents as well.
<h6 class="MsoNormal">Guns, Germs Steel</h6>
One of the topics she looks at is how genetics could support the theories outlined by evolutionary biologist Jared Diamond in his 1998 book, <a href="https://www.pbs.org/gunsgermssteel/" target="_blank" rel="noopener noreferrer">Guns, Germs and Steel</a>. One section of the book she focused on — and which she called her favorite part — was Chapter 10, &#8220;Spacious Skies and Tilted Axes.&#8221;

Diamond proposed that continental expansion was partly dependent on how the landmasses are oriented, and that humans brought crops and technology with them as they did so. For example, because Eurasia is on an east-west axis, it was easier for humans to move plants and animals along that axis because the climate was similar. Ramachandran noted that a 2005 study found that humans and domesticated crops for farming spread from east to west about 3,000-5,000 years ago in Europe.


On the other hand, Diamond said, the Americas and Africa are oriented on a north-south axis and as people traveled north and south, they would have found that plants and animals they brought along couldn’t always adapt to the seasonal changes. For example, Diamond noted, the Indians in North and Central America had no pack animals because the llamas and alpacas never moved out of the Andes. And the corn grown in Mexico needed thousands of years to adapt to the changes in growing season and day-length to make its way up north.

<h6 class="MsoNormal">Genetic Differences in the Americas</h6>
If this holds true on a larger scale as Diamond suggested, then there should be population bottlenecks and more distinct genetic differences between populations across the Americas latitudinally (north-south) than there are longitudinally (east-west). Ramachandran also said that the distinct genetic differences between populations across the Americas latitudinally should be greater than the distinct genetic diversity seen longitudinally (east-west) across Eurasia.

Ramachandran and her colleagues <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.0030185" target="_blank" rel="noopener noreferrer">reported</a> on the genetic diversity of the Americas last year, but she says more data is needed for this region. She also thinks that Diamond’s proposed north-south axis of orientation isn’t quite right, arguing that it should run northwest-southeast. To better study the genetic diversities of the populations across this tilted axis, she and her colleagues change the way they view the map of the world.

If the planet were a ball, the point at which it would be spinning on someone’s finger might be the South Pole. Using points such as the southernmost area of South America, South Korea or Sri Lanka as the axis of rotation on which to turn the map, Ramachandran demonstrated how she and her colleagues can look at the genetic diversities of populations in the Americas and Asia and find patterns in the genetic variations that might not be so easily seen from other perspectives.

Ramachandran said she hopes to publish her research on the populations in the Americas soon. In the meantime, her talk illustrated how studying genetics can support theories raised about human origins in other areas of study such as archeology, anthropology and history.

Image from <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.0030185">PLoS Genetics: Wang et al, 2007</a>

A Genetic Look at “Guns, Germs and Steel”

Editor&#8217;s Note: This post does not reflect our current product offering. 23andMe&#8217;s ancestry product looks at the autosomes (chromosomes 1-22), the sex chromosomes (XY), and mitochondrial DNA (mtDNA). For more information about how results are different for men and women go here.
One of the most exciting parts of 23andMe’s <img decoding="async" class="right alignright" src="https://blog-api.23andme.com/wp-content/uploads/2008/05/gus.jpg" alt="gus.jpg">
<h6>Paternal vs. Maternal</h6>
At present, 23andMe customers can trace two branches of their genetic family tree — one that follows the all-female line on the maternal side (through mitochondrial DNA) and another the all-male line on the paternal side (through the Y chromosome).
Not all DNA is created equal, however: males have both mitochondrial DNA AND a Y chromosome, so they can trace both their maternal and paternal ancestry. Females, who have mitochondrial DNA but no Y chromosome, can trace only their maternal ancestry.
So how can females discover their paternal history? One solution is to ‘borrow’ the Y chromosome of her most immediate paternal ancestor — her father. A female can have her father send his own DNA sample to 23andMe, then examine his Y chromosome as a way of understanding his paternal ancestry and her own.
<h6>Discovering Paternal Ancestry</h6>
But what if a woman&#8217;s father can&#8217;t or won&#8217;t share his DNA? By sharing 23andMe accounts with the right male relative, a woman can still discover both her maternal and paternal ancestry.
So whose DNA can a female customer use, besides her father&#8217;s? It could be anyone who shares his Y chromosome — her brother, paternal uncle (father’s brother), or even paternal grandfather (father’s father). The chart below illustrates some of the possibilities in one woman&#8217;s family tree; male relatives who share her father&#8217;s Y chromosome are depicted in blue.
You may look at the chart and ask: Why not her son’s Y? After all, he&#8217;s a male relative, too. But even though a mother shares 50% of her genes with each of her children, only fathers pass Y chromosomes to their sons. So any examination of her son’s Y chromosome would yield not her father&#8217;s paternal history, but her husband&#8217;s.
<img decoding="async" src="https://blog-api.23andme.com/wp-content/uploads/2008/05/erinpaternal33.png" alt="erinpaternal33.png"> 
The woman whose family tree is shown here (&#8220;Me&#8221;) could determine her paternal ancestry using the Y chromosomes of males who are colored blue.

Ancestry Tools — Page 3

Study of Breast Cancer among Latinas

The Genetic and Geographic Structure in Europe

A Genetic Look at “Guns, Germs and Steel”

Whose Y to Use? Paternal Ancestry for Ladies

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