Genetic genealogy or genetic ancestry testing is the use of DNA to study ancient origins and relatedness between individuals. When combined with traditional paper genealogy, it is a powerful tool for the genealogist’s toolbox. There are four types of genetic ancestry testing: Y-chromosome, X-chromosome, mtDNA, and autosomal DNA test.
The Y chromosome is only found in males, who have one Y chromosome and one X chromosome. The Y chromosome is composed of 58 million base pairs and contains 83 genes which code for only 23 proteins.
During meiosis (preparation of cells for reproductive purposes) the Y chromosome and the X chromosome only recombine at the very ends of the chromosomes (roughly 5%) in a region called the telomeres. Most of the Y chromosomes are located in this region, meaning that they are shared between both sex chromosomes.
The lack of recombination means that the Y chromosome is passed down from father to son without significant change. Over long periods of time the chromosome begins to accumulate mutations that are typically silent and have no impact on the carrier. These mutations, however, are useful for genealogical purposes – they can be used to analyze the relationships between populations and individuals.
When a Y chromosome is submitted for analysis, short segments of DNA known as short tandem repeats (STRs) are sequenced. STRs are also known by their DYS number (DNA Y-chromosome Segment number). The number of repeats can change over time and are passed on from father and son, giving scientists the ability to trace paternal lineages. STR analysis typically provides a person’s haplotype, although they are often used to suggest which haplogroup an individual matches.
Some genetics tests also analyze SNPs (Single Nucleotide Polymorphisms), single nucleotide changes in the DNA sequence. SNPs are typically identified using something called “SNP chips” which can analyze hundreds of thousands of different SNPs at the same time. SNP analysis typically provides a person’s haplogroup.
Most Y chromosome tests examine between 12 and 67 STR markers, but many more are known and could eventually be used for relationship analysis. Each STR has a numerical value. DYS392, for example, has a value of 11, 12, 13, 14, 15, of 16, with 13 being the most common. The numerical values for all the STRs tested can then be compared to the thousands of other numerical sets available in DNA databases. If two people have identical numerical values, it is more likely that they have a recent common ancestor. Statistical analysis can be used to calculate the number of generations to the most common recent ancestor (MRCA).
Mitochondrial DNA (mtDNA) is a small circle of DNA that is located inside a small organelle found inside our cells, the mitochondria. mtDNA is only 16,569 base pairs long and contains only 37 genes. Every human cell contains between 100 and 10,000 copies of mitochondrial DNA.
Unlike nuclear DNA, mitochondrial DNA does not recombine and thus there is no change between parent and child. Most importantly, mtDNA is only passed on from mother to child; although males inherit mtDNA from their mothers, they do not pass it on to their children. This unique feature of mtDNA allows it to be used for tracing matrilineage, the inheritance of mtDNA from mother to child.
When mtDNA is tested for genealogical purposes, a region of the DNA is sequenced for SNP mutations. mtDNA is divided into three regions – the coding region and two hyper-variable regions (HVR1 and HVR2). Most companies sequence the HVR1 (16001-16569), with HVR2 (073-577) and full-length (1-16569) sequencing becoming more and more useful for comparison. The DNA sequence is then compared to a single (randomly-chose) mtDNA sequence, the Cambridge Reference Sequence. The differences are listed as mutations that can be compared to the thousands of other mtDNA mutation lists that are stored in publicly-available databases. The results can also be used to determine the amount of time in which two individuals shared a most recent common ancestor (MRCA). The results of the mtDNA test can also be used to determine a person’s mtDNA haplogroup and haplotype.
Autosomal DNA is the 22 pairs of non-sex chromosomes found within the nucleus of every cell. Autosomal DNA tests examine SNPs, or alleles, located throughout all of the DNA.
One of the most popular uses of autosomal DNA testing is to determine an individual’s ethnic heritage. According to recent research, ethnic groups can contain distinctive alleles that are different from all other ethnic groups. The presence of that allele in an individual’s DNA suggests that they are descended from that ethnic group.
Some companies disclose the results as what percentage of each ethnic group the individual is, such as Native American, European, East Asian, and African. Other tests only look for certain markers to reveal a certain type of ancestry, such as Native American, African, Cohanim, Hindu, or European, among others.
The X chromosome, found in both males and females, is more than 153 million base pairs and contains roughly 1000 genes. Females have two X chromosomes while males have just one.
The use of X chromosomes to study genealogical relationships is still relatively new. The X chromosome, just like the Y, contains STRs, called X-STRs. The problem with studying X-STRs is that the entire X chromosome undergoes recombination during meiosis. In other words, in females the two X chromosomes randomly swap information and genes. Family Tree DNA, one of the leaders of X-STR research (using methods developed by DNA-Fingerprint), uses “haplotype blocks”, or regions of X-STRs that are inherited intact over several generations.
A male’s X chromosome is inherited from his mother and is a mixture of her two X chromosomes, one from her mother and one from her father. It is therefore a mixture of the maternal grandparent’s X chromosomes.
A female inherits one X chromosome from each of her two parents. The X chromosome from her father is passed on from his mother is a mixture of her parent’s (the paternal great-grandparent’s) DNA, while the X chromosome from the female’s mother is a mixture of her parent’s (the maternal grandparent’s) DNA.