Consanguinity is the condition of kinship with someone else; a blood relationship in historical terms.  Someone you share an ancestor with (that is, you share a MRCA). In legal and religious circles, they look to define a measure of closeness of the relationship to then enable the specification of laws governing prohibited behavior with close blood relations.

There is a measure of this closeness of kinship called the degree of relationship. This distance of a relationship is based on the number of generations or meiosis events from each side to the (MRCA). But in genetic genealogy, we use a slightly different measure than has traditionally been determined.  The genetic genealogy measure reflects the fact that full siblings are inheriting DNA from both parents and thus are still only a single generation apart.  The diagram to the right, created originally on Wikimedia by Citynoise, better illustrates this DNA-based measure showing a distance of 1 for parents, children and siblings. And then expanding out in waves from there.

Wikimedia also has a Table of Consanguinity by Sg647112c which utilizes a more traditional genealogical chart to express this degree of relationship in a path form. This table is expanded here and changed to show the genetic genealogy degree of relationship measure (or generation count).  A dotted line shortcut is added between siblings in the chart and the distance to them is thus short-cutted and reduced by one.  Subsequent generations below are then one closer as well.  For Half siblings / cousins / etc add one to the degree of relationship as they only share a single parent and thus the shortcut in the diagram does not exist for them. The grayed boxes represent areas where the matching of autosomal DNA between cousins is more unlikely than likely to occur. Generally, below 0.1% matching across the autosomal DNA or less than a 7cM length of a matching, longest segment length.  This corresponds to roughly 10 generation events and just beyond 4th cousins.  The light yellow boxes represent generations greater than 3 away and thus ones you likely are not able to test and compare using the recently available DNA test technology.  So it is, for the most part, the white boxes that represent those you might match with using autosomal DNA testing technology.

Click image to download

The traditional Table of Consanguinity with the degree of relationship modified to for Genetic Genealogy illustration purposes.

Average versus Actual shared DNA

The chart estimate above is based on a predicted, Mendel-style “perfect” dilution of the autosomes.  It ignores the fact that there are 44 discrete autosomes of greatly varying length which disrupts this perfect estimate. The perfect dilution is based on a new, outside individual contributing 1/2 their DNA each generation. Not just a single, depicted ancestor that is in the relationship line. Remember that Chromosome 1 is five times larger than 22 and thus which chromosome passes down from which ancestor contributes to a wider variance in matching strength as well.  Finally, there is something called cross-overs or recombination that occurs during meiosis and causes a mixing of the DNA strands one inherits from the grand-parents. This actually tempers or lessens the variance seen with the discrete chromosome strand inheritance by making chromosome strands in the child be more of a mix of the strands from the grandparents.

Although matching after 10 generations is more unlikely, discernible matching may still be found up to 20 generation events out. This is due to some matching segments of a chromosome being less susceptible to breaking up during recombination in meiosis.  The shorter the matching segment, the less chance it has of being split.  Once a segment gets down to 20 cM or so in length, it can be passed down intact for many generations.  The farther beyond 10 generations, the rarer the occurrence just simply due to statistical chance the segment is included, but it is still possible.  Up until a 10 generation count, you are more likely than not to detect a match.  Shared DNA has a lot of statistical variance through the generations of inheritence.  Although a parent and child will always share (roughly) 50%, there are no guarantees beyond that.  Although highly, highly improbable, realize that two siblings can possibly NOT share ANY matching DNA segments in their chromosomes (1-23).

When adding up the length of all autosomal matching segments (defined by at least 500 SNPs and a minimum length of 5cM) to form a total matched length, one can estimate the degree of relationship from the resultant value.  For roughly 50cM and more (corresponding to roughly 3rd cousins and closer or a degree of relationship of 7 or smaller), you can almost exactly determine the degree of relationship with a pretty strong certainty.  This is because the standard deviation from the expected average is fairly small compared to the overall separation of the averages.  Below 50cm, roughly, it is much more of a guess.  Statistically, the nominal value is known.  But the distribution is much flatter with a very distant standard deviation leading to a wide variance.  The variances overlap with the other degree of relationship average values. And thus there are many possibilities for the degree of relationship based on the amount of DNA shared; often all with the same probability.  That is to say, for 7cM of total matching segments, the degree of relationship is almost as likely to be anywhere from 10 to 16 with no one value anymore more likely than the other.  This is why you are seeing the range of possible relationships reported by the testing companies.

The table above uses a rough base of 7,200cM (or 3,600cM for half identical matching) to represent all of the autosomes. This is approximately what is reported with GEDMatch and 23andMe. FTDNA’s value is about 3,400cM (roughly). The centimorgan (cM) is not an exact measure like the count of nucleotides or base-pairs and hence different sources compute the value in different ways. If one is to add the X chromosome into the total shared measure (which 23andMe does), then this would add about 90cM to the 3,600cM half-identical match total ((180cM in total to the 7200cM count but only for girls) . The Y chromosome and mtDNA strand are never included in the total matching length or shared DNA percentage count. mtDNA is so small anyway as to not be significant.

Fully Identical versus Half-Identical

Only full siblings have full-identical match regions. A full-identical match region is one where each chromosome is matching the other chromosome in that region; the same region on both chromosomes in the pair of the autosomes. (There are some special cases of endogenous populations with related parents that can have some fully-identical regions within the tester them-self. Meaning the region of each of their pair of chromosomes is matching fully.) Because results are reported unordered and only full-siblings have full-match regions, most tools simply report only half-identical matching. This simplifies the analysis for the data provided. Half-Identical matching under-reports the total matching between full siblings though. The average match for full siblings is reported as about 40% by half-identical tools and charts; as opposed to the actual and expected 50% like between parents and children. You cannot reliably use half-identical matching / reporting tools on full siblings unless you use corresponding phased results and sum the two independently measured match strengths. Phasing takes the un-ordered pairs of values and assigns them to the maternal or paternal contributed chromosome. Assuming parents are not related, only full siblings will have ANY fully-identical match regions. Similarly, half-siblings will have a match strength more like an Uncle/Aunt/Niece/Nephew and have NO fully-identical match regions. Thus a test for any full identical match regions between believed siblings is a way to determine if they are full or half-siblings. (Not to mention the half-identical match strength should be nearly twice as strong between full versus half-siblings.)

External References

1 Autosomal Match Spread from Ancestry.com help topic “How do we estimate relationships?”
2 White Paper on Matching from Ancestry.com (see specifically Figure 5.2 around page 40)
3 ISOGG Autosomal DNA Statistics page on ISOGG
4 Match Probability spreadsheet from Tim Janzen giving the probability that a given total matching segment length is related to a given number of generation events.
5 DNA Inheritance by Angela Cone (with a nice crossover explanation)
6 CentiMorgan in ISOGG Wiki