Tag Archives: mtDNA

DNA and Race

I just finished reading DNA USA: A Genetic Portrait of America by Bryan Sykes.  This is a very odd book.  I was expecting to read a major genetic analysis of population diversity in the US.  Instead, it is more a travel log of Sykes’ tour of American landmarks with a few, essentially random, meetings with individuals where their DNA was collected for analysis. This analysis is discussed in one, relatively brief, concluding chapter. The topic of the book was more genealogical than genetic.

It seems that Sykes may have been hoping to write a book about the US similar to those Bill Bryson has written about Britain and Australia.  DNA USA somewhat resembles Bryson’s book on Australia: In a Sunburned Country, but Sykes does not have Bryson’s comedic flare nor verbal virtuosity.

Having said that, once I got past the fact that the book was not what I expected, I did enjoy reading it, perhaps because I have been to most of the places Sykes visited. In addition, I am interested in the ways in which genetics can inform, but also misinform (or, more precisely, under-inform) genealogy.

Sykes is a geneticist who uses mtDNA (passed through the maternal line) and Y chromosome (passed through the paternal line) to tie genetic information to the past. Soon after he began this research, he began to be inundated with requests from the general public to have their DNA analyzed.  Sykes made the decision to create a business, Oxford Ancestors, designed to meet this need. A similar business model, African Ancestry, was set up in the US by Rick Kittles and Gina Paige.

While some interesting genetic information can be obtained from these methods, vast amounts of information are unavailable.  To simplify this, think about a woman who has one or more sons, but no daughters.  Her mtDNA will not show up in her grandchildren since the only material passed from the sperm to the egg is the nuclear DNA (nDNA), not any mtDNA.  If her granddaughter has her mtDNA analyzed, the granddaughter will learn about her mother’s genetic line, but nothing about her paternal grandmother’s line.  The grandson can learn about his paternal grandfather’s line (along with his maternal line), but, again, nothing about his paternal grandmother’s line.  A huge chunk of genetic knowledge is unavailable by these methods. Not to mention that the actual amount of genetic information in mtDNA and the Y chromosome is extremely tiny compared to nDNA. Making broad statements about anyone’s ancestry when so much information is missing is, at the least, highly problematic.  Yet, that is exactly what genetics researchers using these two methods claim.  These claims even extend to human origins. I don’t wish to get into that topic more deeply in this blog post.  However, given what I’ve just written, I hope readers will apply great caution towards accepting claims about human origins made on such limited mtDNA and Y chromosome data.

For his American odyssey, Sykes decided to use a new, more informative genetic analysis developed by the company 23andMe. As described by Sykes, 23andMe uses nDNA and creates a colored portrait of an individual’s 22 autosomal chromosomes.  Prior nDNA researchers who analyzed the DNA of individuals from many different countries found genetic variants that are associated with particular groups.  For ease of analysis, these variants were lumped into three continental groups: Asian, European, and African.  For the purposes of analysis in the US, Asian is a proxy for Native American since genetic research has shown that these groups have a common origin. This method accesses information from both parents while also giving information on specific genes that have been identified on each chromosome.  In these respects, tying genetics to genealogy is more effective and complete than is the case with mtDNA or Y chromosome analyses. However, it is still incomplete.

The image below shows the process of genetic recombination during meiosis.  The orange and green represent one chromosome pair from the man while the pink and blue represent the same chromosome pair from the woman.  During meiosis, the chromosomes make a copy of themselves.  These copies line up close enough that chunks of DNA can be exchanged between the chromosomes.  Upon completion of meiosis, one chromosome each ends up in the sperm and egg.  These chromosomes passed on to their child represent only a small fraction of the DNA diversity in the parents.  As this process occurs in each generation, huge amounts of genetic information are lost over the generations.  If solid-color chromosomes were the ones in the egg and sperm, all genetic information for that chromosome from one paternal and one maternal grandparent would be lost in the child. Therefore, while nDNA is better for analyzing genetic history, it is by no means a complete picture of an individual’s genealogy.

Given these caveats, the method used by 23andMe does provide a great deal of useful information that is presented in the visually appealing format of chromosome painting. It is in the final chapter describing the genetic ‘portraits’ of the few individuals from whom Sykes obtained DNA that he makes observations that are particularly relevant to the subject of whether or not race is biological.  You might think that Sykes would support the idea of biological races given that these genetic methods divide the world into three groups: Asian/Native American, European, and African.  But Sykes recognizes that these are over-simplifications of actual diversity and views them more as geographical, rather than biological entities.

Americans are especially revealing in that most of them display genetic diversity rather than uniformity.  The only individuals Sykes analyzed that did not display diversity were the members of a genealogical society in Boston who could trace their ancestry in America back to the 17th and 18th centuries.  He found this quite surprising and concluded that any of their ancestors who inter-married with Native Americans became part of those cultural groups rather than the European-descent cultural group.  This is supported by the genetic analysis of individuals of Northeast Native American ancestry whose chromosome analyses show their genes to be almost entirely European derived.  European Americans with Southern ancestry showed some genetic evidence of African ancestry, while all African Americans showed European and Native American ancestry, although the percentages differed widely.  Sykes concluded that “…many whites with deep roots in the South have some black ancestors.” (p.313)  He mentions that he would like to have analyzed the DNA of a Ku Klux Klan member because he is pretty sure it would have sections indicating genes with African ancestry.  It would have been interesting to find out how that individual reacted to this knowledge.

Sykes notes that assuming because of someone’s appearance and/or culture that you can draw any conclusions about their genetics and health concerns demonstrates a lack of knowledge of the complexity of genetic inheritance.  As an example, Sykes points out that he has African ancestry for the tip of chromosome 11 while one of the African-American men he analyzed has European ancestry for that same region.  As this region includes the genes for beta-globin, Sykes states, contrary to what most physicians would conclude, that he, Sykes, could be a carrier for sickle cell anemia while the other man could not.

Another gene that showed diversity was P450 cytochromes found on chromosome 10.  This gene produces proteins which help to clear drugs and toxins from the liver.  Medical researchers have found that an African-derived form of the gene is less effective.  This led to different, lower dosing recommendations of drugs such as beta-blockers for African Americans.  However, since Americans have diverse genetic ancestry, simply assuming an individual African American should have a lower dose than an individual European American can lead to major errors.  Sykes states, “…that of my nine African American volunteers, only three have both copies of their P450 gene from African ancestors, three have one European and one African copy, and the genes of the remaining three are completely European.”  On the other hand, one of his southern European-American volunteers had the African form of the gene.  Racially categorizing these individuals would lead to medical errors.

The conclusion I draw from this book is one I have long held. Racial categories have little meaning whether they are assumed to be cultural or biological because genetics and culture have no necessary overlap.