Conclusions from the use of autosomal DNA

 

Most of the relationships we sought to prove are too many generations ago to give reliable DNA matches through conventional means. The potential link between for example the English Lousadas and their 10th and 11th cousins among the USA Lousadas goes back 9 generations to different siblings of Amador de Lousada of Vinhais. Even 4th cousins do not show up well, for I have only one (25cM) segment in common with my 4th cousin Jeremy Lousada, and only 3 3cM matches with my 6th cousin John Griffiths. From the outset, we were well aware of the poor match-prediction powers of small (3cM) segment matches. Even at 7cM there is significant uncertainty as to their reliability, and only going to 15M gives this. Therefore we sought to rely on maximising the number of matches and then using statistical and related techniques to discern patterns lying within our datasets.  

As Qmatch was recommended to us for small matches by GEDmatch, and retaining our desire to accumulate many matches, we set about using Qmatch (set at 3cM, P=3) to compile all 2255 segment matches - mostly off-target matches and false positives of course -  from our 13 relative sample - see here the 1963 matches found before ELL was added to the sample of relatives. We found 46 RSBCs, 25 lefthand and 21 righthand, and most were new to us. We spent much time looking at RSBCs, how frequently they occur across chromosomes and in the match-rich areas on chromosomes. But they proved quite difficult to work with, and were relatively unproductive. At this point we checked Qmatch at the default 7cM P=7 settings, and were surprised to find this most worthwhile, for with our set of 12 relatives it yielded inter-branch matches that were totally absent with our 7 relative set.

We then moved from RSBCs to ASBs; but they proved misleading and just as we prematurely claimed in  'Fun with Autosomal DNA', we again thought that we may have established a genetic link between the USA Lousadas, the English Lousadas, the Barrows, Scott's wife (and hence the Fischls) and Randy's parents. The problem with ASBs was that instead of an ASB being associated with a unique crossover, in fact many unrelated crossovers can all report to the same ASB (in fact the same applies to RSBCs as well). This is because a crossover is only measured as lying between 2 particular SNPs - these SNPs being the first non-matching SNP beyond the crossover at one end of a segment and last SNP within the segment at the other end of the segment. Typically there are 5000 base-pair positions between each SNP so the potential for unrelated and (from the family viewpoint) spurious crossovers at the same ASB is large. That is, ASBs are not the amazingly precise way of penetrating the fog of unreliable small matches that was hoped. (An unresolved thought about ASBs, in the light of their appearance below is that the 3pASBs which generate the 3cm P=7 matches shown in Chart 1 may have an inbuilt strength that 4pASBs do not have in that the common relative in both abutting matches in a 3pASB is an assurance that the crossovers are genuine).

From this impasse it was timely advice from GEDmatch in February 2026 which assisted us. Thus, we came to use Qmatch to look again at all 3cM matches, not at P=3 but at P=7. These conditions give reasonable quality matches, almost as good as 7cM matches from some other providers. With our set of 13 relatives (and our comparison set of 13 randoms) we were able to arrive at the position summarised in Chart 1 as follows: 

 

 CHART 1

 

From Chart 1 we can see the futility of the 3cM P=3 results. For at this setting the randoms show more matches! While the RSBC totals indicate that a family signal may exist, the ASB numbers seem odd (and perhaps can only be explained by a combination of spurious crossovers and the much greater possible ancestor pool where there is no genealogy). But at 3cM P=7, the relatives show 187 more matches than the randoms, the strongest such signal we have seen in all our comparisons of relatives with randoms. Chart 2 shows the match-numbers for relative pairs. For example, among the 14 B/Je matches is one of 5cM on Cr10 which is presumably their Ancestry.com 7cM match. Missing matches (shown in grey) align with a family branch; JG misses DNA which would allow a match with 2 Barrow-Lousadas A and J whose specific ancestry may have been responsible though in different ways.

 

 

CHART  2

 

Comparing Chart 2 with the table of randoms' matches in Chart 4, we see that the randoms show a greater dispersion - with 4 people in less than 4% of matches (compared with 1 relative - J, who despite proven close Lousada genetic and genealogical links, has anomalously low match numbers), and 4 people in more than 10% of matches (compared with 1 relative). That is, as expected, family connections despite some stochastic phenomena also being present, are tighter. The presence of 5cM and 7cm matches in Chart 4 is indicated by colours as in Chart 2, but also with pale blue showing the pairs with 8 segment-matches (for the reason explained below). Our first task is to assess how well our grouping of relatives (indicated by lines across the above table) compares with random. We can see in Chart 3 that the mutual matching within each intrabranch match grouping comfortably exceeds that of the random set, and supporting this we have noted that a triangulation is shown by the 1st and 4th cousins A, Ju and J at Cr16 (3219600 - 6259081) within the Barrow-Lousadas.

CHART  3

 

 

The random set could contain its own (but unexpected/unknown) internal family links which must be allowed for when we consider the real match count and the false (off-target) match count. Chart 4 shows the random set with the yellow, green and grey colours as in Chart 2. The addition of the blue colour is explained next. 

 

CHART  4

The random set does indeed have structure with several obvious groupings. The central grouping seems to reflect several (perhaps 3) Ashkenasi families variously linked perhaps 4-8 generations ago. A second grouping is more distantly related perhaps with more diverse ancestry. The outer grouping is only remotely related. From this, we can make an estimate of the number of real segment-matches among the 373 in the random set - simply by augmenting the central group's 154 segment-matches by the 38 segment-matches in the 4 pairs showing 8 or more segment-matches. These 8 segment-matches are shown in blue, and this colour is also shown in Chart 5 to designate 8-segment matches. We arrived at 8 iteratively, and note that this gives us an estimate of 192 for the real segment-matches among the 373 in the randoms' set. As the relatives achieve 187 more segment-matches than the randoms (see Chart 1), we estimate the real relative segment-matches total 379 (that is 192 plus 187), so that the false segment-matches total 181 in each case. This enables us to depict the real segment-matches in the relatives' set and we show the result in Chart 5. For the white pairs give 133 segment-matches, and the small greens (less than 8) give us 39, and this totals 172 leaving room for a few false yellows (which we know there to be from the triangulation discussion below) within our estimate of 181 false matches. As we have a probability estimate covering the false positives (namely 32% or 181/560), 8 segment-matches are tantamount to proven matches since 0.32**8 = 0.0001 or 0.01%.

 

 

 

CHART  5 

 

 

There are 4 branches of relatives - English Lousada (MD), Barrow-Lousada (Ju, A, J), Barrow (E, JG, RM, RM, SW), and US Lousada (ELL, B, Je, TP). We extract from Chart 5 the level of interbranch matching and summarise this in Chart 6. From this we can make some observations:

 

 

 

CHART  6 

 

 

 

Triangulations show 2 or 3 branch matching - Ju/RM/Je at Cr2 (217m - 220m), B/Je/E at Cr5 (79m - 81m), J/TP/E at Cr10 (116m - 119m), RM/E/A at Cr17 (31978888 - 33622774) and RM/B/Je at Cr22 (25640628 - 26225384). But at the Cr2 site just noted the triangulations Ju/Je/TP and B/Je/TP are likely to be false (containing at least 2 non-Lousada matches as Ju does not match B here). And of the 5 remaining triangulations, only the 2nd triangulation (B/Je/E) is entirely composed of 8-segment matches and thus appears real along with the obvious Ju/J/A triangulation noted above. The absence of a triangulation can also be informative - on Cr18 (6.6m - 8.1m) each of the 1st cousins Ju and A has a good match with TP but don't match each other here which shows that at least one of the 2 7cM matches is in fact false. This data shows that false matches can occur in the low-match yellow pairs, which we allowed for above.

We may feel confident that despite the small segment sizes and remote ancestral connection, there is considerable interbranch matching which supports our genealogy (in which the various branches originate in the ancestral family of Amador de Lousada).

 

FOOTNOTE 

Finally, out of respect to those whose kit numbers went into generating the random sample, we comment further on Chart 4. This shows perhaps 3 family linkages around 4-8 generations ago (A/H/J/MW/DG) with a secondary group (N/S/M/P), together with an outlying group (C, MB, MMC, KB). The central 'family' group of 5 contains kits contributed from John Griffiths (A, H, J) and Julian Land (MW, DG). The secondary group of 4 consists of N contributed by John Griffiths and S, M, P contributed by Julian Land. The remotely-connected group of 4 consists of C contributed by John Griffiths and MB, MMC and KB contributed by Julian Land. The set of 13 randoms produced only 2 triangulations, on Cr1 at 158m - 160m (H, S, J) and on Cr8 at 11.3m - 12.6m (S, P, M). These triangulations mean little in this context but we note that the second has one 8-segment component S/P.