More Distant Relationships

Using DNA to Measure Relatedness

A New SNP Approach to Measuring Relatedness

Recently developed SNP technology together with the development of bioinformatics have enabled a new method that can determine a measure of relatedness between people.

It is available to anyone who wants to research their DNA family tree not just by looking back through the unbroken mitochondrial and Y-chromosome lineages but by utilising information present in the wider chromosomal DNA in our genetic make-up (see Family Tree and DNA Inheritance). It is informative about most relationship types even those as distant as 3rd or 4th  cousins and it can provide convincing evidence to support or deny the existence of close relationships such as siblings.

It works on the basis that if you are related to someone, and hence share a recent common ancestor, it is likely that you will both have inherited some identical segments of DNA from that ancestor. The closer the relationship the larger the number of segments you are expected to share and, on average, the bigger those shared segments are likely to be. Larger segments are more likely in close relatives due to the lower amount of DNA shuffling (called recombination) that will have taken place between them and their common ancestor.

Recombination is a process that occurs between the DNA in the paired maternal and paternal chromosomes during the formation of sex cells (egg and sperm). It’s purpose is to shuffle the DNA and its genes between chromosome pairs to ensure that sex cells contain an even mixture of genes from both parents. The more distant the relationship the more recombination or gene shuffling will have taken place through the generations to the common ancestor and the less likely it is that long segments will be identically passed on to the relatives. The scientific test is based on a huge SNP micro-array that simultaneously types many hundreds of thousands of nucleotide points within the 22 autosomal chromosomes (autosomal chromosomes are the non-sex chromosomes – all chromosomes except X and Y).

Results are compared from different people and predict the size and number of identically inherited DNA segments which is then translated into a measure of relatedness. The test isn’t approved by accreditation bodies such as AABB, or bodies that uphold the ISO17025 standard, because it is based on a new method and a more complex statistical approach. As a result it is not recognised in legal situations and there is some risk that the inferences produced aren’t totally robust. However, the scientific basis is sound and you might feel it is useful in circumstances where legal issues aren’t the main motivation for getting tested. It does offer a new dimension and added power in many situations particularly when the relationship being tested isn’t a direct parent child type of relationship.


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