Half-1st cousin?

[MisterPC]

I have the following match from testing male 1st cousins both through the maternal line.

Shared CM: 503.33
Longest Block: 54.38
X-Match

Do these figures fit with a half-1st cousin rather than a full 1st cousin?

I've posted the results from David Pike's shared DNA comparison tool below.

Single Segment Matches of length at least 500 will be reported.
Double Segment Matches of length at least 250 will be reported.
No-Calls will be treated as matching SNPs.
Non-matching SNPs that are at least 150 SNPs away from the nearest non-matching SNP will be treated as matching.
Chr 6 has a Double Segment Match of length 352 from position 160622708 to position 161213047 ( 0.59 Mb)
Total Mb: 0.59

Chr 1 has a Single Segment Match of length 569 from position 45067435 to position 47745506 ( 2.68 Mb)
Chr 1 has a Single Segment Match of length 645 from position 92198162 to position 95052325 ( 2.85 Mb)
Chr 1 has a Single Segment Match of length 558 from position 147166377 to position 151820324 ( 4.65 Mb)
Chr 1 has a Single Segment Match of length 747 from position 209950681 to position 212885805 ( 2.94 Mb)
Chr 1 has a Single Segment Match of length 1799 from position 230771642 to position 236358092 ( 5.59 Mb)
Chr 2 has a Single Segment Match of length 619 from position 82957817 to position 85921761 ( 2.96 Mb) (2 non-matching SNPs treated as matching)
Chr 2 has a Single Segment Match of length 544 from position 129124530 to position 131731464 ( 2.61 Mb)
Chr 2 has a Single Segment Match of length 530 from position 135061348 to position 137529247 ( 2.47 Mb)
Chr 2 has a Single Segment Match of length 649 from position 142954841 to position 146382236 ( 3.43 Mb) (1 non-matching SNPs treated as matching)
Chr 2 has a Single Segment Match of length 553 from position 177576731 to position 179729341 ( 2.15 Mb)
Chr 2 has a Single Segment Match of length 572 from position 221772886 to position 223954609 ( 2.18 Mb) (1 non-matching SNPs treated as matching)
Chr 3 has a Single Segment Match of length 1946 from position 32275125 to position 41019888 ( 8.74 Mb)
Chr 3 has a Single Segment Match of length 4597 from position 75726925 to position 106217161 ( 30.49 Mb)
Chr 3 has a Single Segment Match of length 536 from position 123196692 to position 124919384 ( 1.72 Mb)
Chr 3 has a Single Segment Match of length 602 from position 128324161 to position 131460188 ( 3.14 Mb) (1 non-matching SNPs treated as matching)
Chr 3 has a Single Segment Match of length 4509 from position 148219044 to position 171284593 ( 23.07 Mb)
Chr 4 has a Single Segment Match of length 546 from position 45638847 to position 52904614 ( 7.27 Mb)
Chr 4 has a Single Segment Match of length 1586 from position 185338921 to position 190915650 ( 5.58 Mb)
Chr 5 has a Single Segment Match of length 715 from position 78069655 to position 80482514 ( 2.41 Mb) (1 non-matching SNPs treated as matching)
Chr 6 has a Single Segment Match of length 1568 from position 203878 to position 4490506 ( 4.29 Mb)
Chr 6 has a Single Segment Match of length 681 from position 25783315 to position 27318836 ( 1.54 Mb)
Chr 6 has a Single Segment Match of length 670 from position 30764907 to position 31142903 ( 0.38 Mb) (1 non-matching SNPs treated as matching)
Chr 6 has a Single Segment Match of length 1315 from position 31368588 to position 32338695 ( 0.97 Mb) (1 non-matching SNPs treated as matching)
Chr 6 has a Single Segment Match of length 8858 from position 130789807 to position 163340367 ( 32.55 Mb)
Chr 7 has a Single Segment Match of length 5588 from position 15481980 to position 32245755 ( 16.76 Mb)
Chr 7 has a Single Segment Match of length 1834 from position 47713313 to position 55123968 ( 7.41 Mb) (1 non-matching SNPs treated as matching)
Chr 7 has a Single Segment Match of length 537 from position 56259407 to position 66774157 ( 10.51 Mb) (1 non-matching SNPs treated as matching)
Chr 7 has a Single Segment Match of length 2730 from position 141421031 to position 152959100 ( 11.54 Mb)
Chr 8 has a Single Segment Match of length 548 from position 50037784 to position 53172242 ( 3.13 Mb)
Chr 8 has a Single Segment Match of length 883 from position 64006201 to position 68777877 ( 4.77 Mb)
Chr 8 has a Single Segment Match of length 556 from position 76566256 to position 79888451 ( 3.32 Mb) (1 non-matching SNPs treated as matching)
Chr 8 has a Single Segment Match of length 747 from position 121333937 to position 124426096 ( 3.09 Mb)
Chr 8 has a Single Segment Match of length 3344 from position 125094069 to position 136294660 ( 11.20 Mb)
Chr 10 has a Single Segment Match of length 514 from position 17188385 to position 18798319 ( 1.61 Mb)
Chr 10 has a Single Segment Match of length 502 from position 26901421 to position 28748263 ( 1.85 Mb)
Chr 10 has a Single Segment Match of length 8995 from position 30291963 to position 72258307 ( 41.97 Mb)
Chr 10 has a Single Segment Match of length 6515 from position 103071409 to position 125956605 ( 22.89 Mb) (1 non-matching SNPs treated as matching)
Chr 11 has a Single Segment Match of length 4517 from position 198510 to position 12292268 ( 12.09 Mb)
Chr 11 has a Single Segment Match of length 7261 from position 62172174 to position 92181793 ( 30.01 Mb)
Chr 11 has a Single Segment Match of length 651 from position 102666020 to position 105613109 ( 2.95 Mb)
Chr 12 has a Single Segment Match of length 652 from position 20803549 to position 21981946 ( 1.18 Mb)
Chr 12 has a Single Segment Match of length 602 from position 32719928 to position 40036338 ( 7.32 Mb) (1 non-matching SNPs treated as matching)
Chr 12 has a Single Segment Match of length 616 from position 85766847 to position 90078622 ( 4.31 Mb)
Chr 12 has a Single Segment Match of length 8936 from position 103771897 to position 133777645 ( 30.01 Mb) (2 non-matching SNPs treated as matching)
Chr 13 has a Single Segment Match of length 1043 from position 91955192 to position 95578584 ( 3.62 Mb)
Chr 13 has a Single Segment Match of length 717 from position 103151179 to position 104900372 ( 1.75 Mb)
Chr 14 has a Single Segment Match of length 3198 from position 23306048 to position 34589273 ( 11.28 Mb) (1 non-matching SNPs treated as matching)
Chr 15 has a Single Segment Match of length 661 from position 42414397 to position 46032151 ( 3.62 Mb) (1 non-matching SNPs treated as matching)
Chr 15 has a Single Segment Match of length 548 from position 46861963 to position 49090175 ( 2.23 Mb) (2 non-matching SNPs treated as matching)
Chr 15 has a Single Segment Match of length 3666 from position 53251140 to position 66462866 ( 13.21 Mb)
Chr 15 has a Single Segment Match of length 503 from position 74413634 to position 77255029 ( 2.84 Mb)
Chr 15 has a Single Segment Match of length 2700 from position 94852699 to position 102461162 ( 7.61 Mb)
Chr 16 has a Single Segment Match of length 5691 from position 69532417 to position 84867029 ( 15.33 Mb)
Chr 17 has a Single Segment Match of length 1642 from position 12344 to position 5467424 ( 5.46 Mb)
Chr 17 has a Single Segment Match of length 510 from position 41085683 to position 43417995 ( 2.33 Mb) (1 non-matching SNPs treated as matching)
Chr 18 has a Single Segment Match of length 2227 from position 10107749 to position 23000716 ( 12.89 Mb)
Chr 20 has a Single Segment Match of length 2951 from position 19411570 to position 36640207 ( 17.23 Mb)
Chr 20 has a Single Segment Match of length 507 from position 53438513 to position 55116737 ( 1.68 Mb) (1 non-matching SNPs treated as matching)
Chr 22 has a Single Segment Match of length 564 from position 37558356 to position 39618353 ( 2.06 Mb)
Total Mb: 487.71

[tlgarrison]

Half first cousins are about 425 cM. See: http://www.isogg.org/wiki/Autosomal_DNA_statistics. So that looks about right.

[MisterPC]

Half first cousins are about 425 cM. See: http://www.isogg.org/wiki/Autosomal_DNA_statistics. So that looks about right.

Thanks, I've seen that and it does seem to fall in the correct range.

I was hoping for a confirmation from the Megabase value from David Pike's utility as this is a NPE on my line so I need to be 100% sure.

[hansonrf]

It is "most likely" they are half, statistically, but no where near 100% probability. Before you hit someone with an npe claim, I would advise you to know the percentage chance your claim would be false. I do not have access to gobs of datasets but the variability I've seen tells me you would be unwise making such a statement.

[hfp43]

Half 1st-Cousin

Just to reinforce the note of caution: I have a half 1st-cousin with whom I share 801.60 total cM, with the largest shared segment at 93.20 cM. (His paternal grandparents were 1st cousins.)

[MisterPC]

It is "most likely" they are half, statistically, but no where near 100% probability. Before you hit someone with an npe claim, I would advise you to know the percentage chance your claim would be false. I do not have access to gobs of datasets but the variability I've seen tells me you would be unwise making such a statement.

Thanks for your input. That's the exact reason I posted the question though.

Do you know the percentages? Or have an example where the cM values are this low but were later proved to be full 1st cousins? This is far outside the range of an 1st cousin relationship that I've been able to find. I know the sisters are at least half-sisters.

[hansonrf]

Found an article:

http://gcbias.org/2013/12/02/how-man...with-a-cousin/

There is overlap, as I thought we would find. Half is around 20 IBD segments; range 10-35. Full is around 40 IBD segments; range 25-55.

[MisterPC]

Just to reinforce the note of caution: I have a half 1st-cousin with whom I share 801.60 total cM, with the largest shared segment at 93.20 cM. (His paternal grandparents were 1st cousins.)

Thanks for the figures, it always helps to see actual figures for other people's matches rather than just the theoretical ones posted online.

I think your figures reinforce the suggestion that this is a half-cousin relationship rather than give caution to it though.

Can anyone interpret the double matching segment figures? Shouldn't this also be higher for full 1st cousins?

[prairielad]

Double segment matching generally does not apply for 1st cousins unless both your paternal and maternal lines are related. 1st cousins generally only share 1 line(match only along the paternal or maternal chromosome), double segment matching would show when comparing two full siblings as they share both their paternal and maternal lines(will match on both maternal and paternal chromosomes).

[hfp43]

Notes of Caution

Just wanted to demonstrate that averages and known ranges don't always mesh with reality.

[MisterPC]

Double segment matching generally does not apply for 1st cousins unless both your paternal and maternal lines are related. 1st cousins generally only share 1 line(match only along the paternal or maternal chromosome), double segment matching would show when comparing two full siblings as they share both their paternal and maternal lines(will match on both maternal and paternal chromosomes).

Thanks, that makes sense. I keep forgetting that double segments is counting across chromosomes received from both parents, whereas single is either.

[MisterPC]

Just wanted to demonstrate that averages and known ranges don't always mesh with reality.

Thanks, it's much appreciated. What is the lowest cM values you've seen for 1st cousins?

I could test the sisters but even if they show in the half sister range will that prove any more than this result?

[hfp43]

1st Cousins Shared cM

The lowest I've seen for 1st cousins is 530 cM total.

[MisterPC]

The lowest I've seen for 1st cousins is 530 cM total.

Thanks, that's very close to my match . Did you also have a similar value to the longest shared segment of 54 cM?

How did you manage to differentiate the result to full 1st cousins?

[hfp43]

1st Cousins Shared cM

For 1st cousins, the "shortest longest" segment I've seen referenced is 50 cM.