Identical twin sisters?

[MisterPC]

My twin girls match as Parent/Child with the following values.

Shared CMs: 3384.30
Longest Block: 267.21
X-Match

Do these indicate identical twins?

We were told by the midwives when they were born that they weren't identical because they didn't share a placenta, but after getting these results I've researched this and seen that it is not true.

The girls look very very similar but not completely 100% identical.

I've matched up the raw data from the spreadsheets and after removing all the no-calls and matching SNPs, there are around 180 different SNPs out of the ~700,000. Assuming they are identical twins, is this due to the error rate of the sampling or is this where their DNA actually differs due to mutations?

The following data is the shared DNA comparison from David Pike's utility.

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 1 has a Double Segment Match of length 42655 from position 82154 to position 191836316 (191.75 Mb) (7 non-matching SNPs treated as matching)
Chr 1 has a Double Segment Match of length 490 from position 191887735 to position 194572285 ( 2.68 Mb)
Chr 1 has a Double Segment Match of length 14999 from position 194965856 to position 249218992 ( 54.25 Mb) (2 non-matching SNPs treated as matching)
Chr 2 has a Double Segment Match of length 56730 from position 18674 to position 243048760 (243.03 Mb) (8 non-matching SNPs treated as matching)
Chr 3 has a Double Segment Match of length 38758 from position 61495 to position 162689869 (162.63 Mb) (13 non-matching SNPs treated as matching)
Chr 3 has a Double Segment Match of length 7632 from position 162773033 to position 197838262 ( 35.07 Mb) (2 non-matching SNPs treated as matching)
Chr 4 has a Double Segment Match of length 8499 from position 71566 to position 35476300 ( 35.40 Mb) (4 non-matching SNPs treated as matching)
Chr 4 has a Double Segment Match of length 31149 from position 36007857 to position 190915650 (154.91 Mb) (11 non-matching SNPs treated as matching)
Chr 5 has a Double Segment Match of length 15062 from position 25328 to position 66845236 ( 66.82 Mb) (5 non-matching SNPs treated as matching)
Chr 5 has a Double Segment Match of length 26323 from position 66912233 to position 180693127 (113.78 Mb) (8 non-matching SNPs treated as matching)
Chr 6 has a Double Segment Match of length 47249 from position 203878 to position 170919470 (170.72 Mb) (8 non-matching SNPs treated as matching)
Chr 7 has a Double Segment Match of length 34482 from position 44935 to position 148796253 (148.75 Mb) (12 non-matching SNPs treated as matching)
Chr 7 has a Double Segment Match of length 2795 from position 149414709 to position 159119486 ( 9.70 Mb)
Chr 8 has a Double Segment Match of length 36362 from position 164984 to position 146293414 (146.13 Mb) (4 non-matching SNPs treated as matching)
Chr 9 has a Double Segment Match of length 32259 from position 46587 to position 141066491 (141.02 Mb) (11 non-matching SNPs treated as matching)
Chr 10 has a Double Segment Match of length 38464 from position 98087 to position 135477883 (135.38 Mb) (6 non-matching SNPs treated as matching)
Chr 11 has a Double Segment Match of length 36007 from position 198510 to position 134934063 (134.74 Mb) (8 non-matching SNPs treated as matching)
Chr 12 has a Double Segment Match of length 34903 from position 191619 to position 133777645 (133.59 Mb) (6 non-matching SNPs treated as matching)
Chr 13 has a Double Segment Match of length 12545 from position 19058717 to position 65044638 ( 45.99 Mb) (1 non-matching SNPs treated as matching)
Chr 13 has a Double Segment Match of length 1226 from position 65204339 to position 70493869 ( 5.29 Mb) (2 non-matching SNPs treated as matching)
Chr 13 has a Double Segment Match of length 13542 from position 70580104 to position 115103529 ( 44.52 Mb) (7 non-matching SNPs treated as matching)
Chr 14 has a Double Segment Match of length 22911 from position 19327823 to position 107287663 ( 87.96 Mb) (5 non-matching SNPs treated as matching)
Chr 15 has a Double Segment Match of length 21309 from position 20071673 to position 102461162 ( 82.39 Mb) (3 non-matching SNPs treated as matching)
Chr 16 has a Double Segment Match of length 22395 from position 88165 to position 90163275 ( 90.08 Mb) (1 non-matching SNPs treated as matching)
Chr 17 has a Double Segment Match of length 19890 from position 12344 to position 81046413 ( 81.03 Mb) (3 non-matching SNPs treated as matching)
Chr 18 has a Double Segment Match of length 21406 from position 13034 to position 78015180 ( 78.00 Mb) (6 non-matching SNPs treated as matching)
Chr 19 has a Double Segment Match of length 13717 from position 260912 to position 55870351 ( 55.61 Mb) (3 non-matching SNPs treated as matching)
Chr 19 has a Double Segment Match of length 1070 from position 55876074 to position 59097160 ( 3.22 Mb)
Chr 20 has a Double Segment Match of length 18156 from position 63244 to position 62912463 ( 62.85 Mb) (2 non-matching SNPs treated as matching)
Chr 21 has a Double Segment Match of length 10070 from position 10827533 to position 48100155 ( 37.27 Mb) (5 non-matching SNPs treated as matching)
Chr 22 has a Double Segment Match of length 10248 from position 16114244 to position 51211392 ( 35.10 Mb)
Total Mb: 2789.66

Chr 1 has a Single Segment Match of length 58228 from position 82154 to position 249218992 (249.14 Mb)
Chr 2 has a Single Segment Match of length 56730 from position 18674 to position 243048760 (243.03 Mb)
Chr 3 has a Single Segment Match of length 46402 from position 61495 to position 197838262 (197.78 Mb)
Chr 4 has a Single Segment Match of length 39745 from position 71566 to position 190915650 (190.84 Mb)
Chr 5 has a Single Segment Match of length 41397 from position 25328 to position 180693127 (180.67 Mb)
Chr 6 has a Single Segment Match of length 47249 from position 203878 to position 170919470 (170.72 Mb)
Chr 7 has a Single Segment Match of length 37417 from position 44935 to position 159119486 (159.07 Mb)
Chr 8 has a Single Segment Match of length 36362 from position 164984 to position 146293414 (146.13 Mb)
Chr 9 has a Single Segment Match of length 32259 from position 46587 to position 141066491 (141.02 Mb)
Chr 10 has a Single Segment Match of length 38464 from position 98087 to position 135477883 (135.38 Mb)
Chr 11 has a Single Segment Match of length 36007 from position 198510 to position 134934063 (134.74 Mb)
Chr 12 has a Single Segment Match of length 34903 from position 191619 to position 133777645 (133.59 Mb)
Chr 13 has a Single Segment Match of length 27382 from position 19058717 to position 115103529 ( 96.04 Mb)
Chr 14 has a Single Segment Match of length 22911 from position 19327823 to position 107287663 ( 87.96 Mb)
Chr 15 has a Single Segment Match of length 21309 from position 20071673 to position 102461162 ( 82.39 Mb)
Chr 16 has a Single Segment Match of length 22395 from position 88165 to position 90163275 ( 90.08 Mb)
Chr 17 has a Single Segment Match of length 19890 from position 12344 to position 81046413 ( 81.03 Mb)
Chr 18 has a Single Segment Match of length 21406 from position 13034 to position 78015180 ( 78.00 Mb)
Chr 19 has a Single Segment Match of length 14789 from position 260912 to position 59097160 ( 58.84 Mb)
Chr 20 has a Single Segment Match of length 18156 from position 63244 to position 62912463 ( 62.85 Mb)
Chr 21 has a Single Segment Match of length 10070 from position 10827533 to position 48100155 ( 37.27 Mb)
Chr 22 has a Single Segment Match of length 10248 from position 16114244 to position 51211392 ( 35.10 Mb)
Total Mb: 2791.66

[tlgarrison]

Identical twins would be a little over 6700 cM. See http://www.isogg.org/wiki/Autosomal_DNA_statistics

[MisterPC]

Identical twins would be a little over 6700 cM. See http://www.isogg.org/wiki/Autosomal_DNA_statistics

I think from what I understand about how Family Finder works, it is half identical by decent, so the maximum match possible is Parent/Child even with yourself at ~3400 cM.

Is this not your understanding?

[efgen]

I think from what I understand about how Family Finder works, it is half identical by decent, so the maximum match possible is Parent/Child even with yourself at ~3400 cM.

This is correct. Looks like identical twins to me. The ISOGG wiki does give a link to an article that you may be interested in, however:

Finding the needle in the haystack: Differentiating “identical” twins in paternity testing and forensics by ultra-deep next generation sequencing

http://www.sciencedirect.com/science...72497313002275

Elise

[tlgarrison]

I did not realize that it would not show higher but now looking at how it is structured it makes sense that it would not show more.

That makes me wonder about possible mistakes being made with people transferring their tests to FTDNA. I can see some sending the wrong individual and you would not realize it if the son uploaded his kit instead of his mothers. You would not even know it until you saw you always had same matches.

Thanks for asking this question

[JOlson]

I don't think you would have all those Single Segment matches if they were identical twins. Identical twins would appear to be the same individual in a DNA comparison. If they are loaded on GEDmatch, you will easily be able to see the difference by enabling the graphic on the one-to-one comparison. If it's solid green, they are identical. If there are signficant segments of yellow, they are not.

John

[efgen]

I don't think you would have all those Single Segment matches if they were identical twins. Identical twins would appear to be the same individual in a DNA comparison.

I wondered about this, but if you look at the position numbers given for the double and single segment matches, they're the same:

Chr 2 has a Double Segment Match of length 56730 from position 18674 to position 243048760 (243.03 Mb) (8 non-matching SNPs treated as matching)

Chr 2 has a Single Segment Match of length 56730 from position 18674 to position 243048760 (243.03 Mb)

So the entire chromosome 2 is a double-segment match (fully identical), but the tool is also saying that there's a single-segment match along the entire chromosome. I guess this should really say there's *at least* a single segment match (at least half-identical). Though I think it would be less confusing for it to not say there's a single-segment match on positions that actually have a double-segment match

Elise

[MisterPC]

I don't think you would have all those Single Segment matches if they were identical twins.

Double matches are matches on both chromosomes whereas single are matches on either. Therefore every double match is also a single match but not necessarily the other way round.

If I match one sister against herself I get similar results just with an extra 2Mb of double segment matches.

I'm convinced they are identical twins now (or I messed the samples up!), just a little confused of the very slight variation in DNA. What is the expected error rate in the sampling?

[1798]

Double matches are matches on both chromosomes whereas single are matches on either. Therefore every double match is also a single match but not necessarily the other way round.

If I match one sister against herself I get similar results just with an extra 2Mb of double segment matches.

I'm convinced they are identical twins now (or I messed the samples up!), just a little confused of the very slight variation in DNA. What is the expected error rate in the sampling?

Identical twins originate from one sperm and one egg and other twins don't.
The matching segments show matching SNPs not alleles.It will show a match with just one of the alleles. One set of alleles comes from each parent. My son matches me on the majority of his SNPs but he got half of them from his mother.So we have AA,GG,TT and CC for most of them but I know that he got one A etc. from her and one from me.

[Ann Turner]

I've matched up the raw data from the spreadsheets and after removing all the no-calls and matching SNPs, there are around 180 different SNPs out of the ~700,000. Assuming they are identical twins, is this due to the error rate of the sampling or is this where their DNA actually differs due to mutations?

It's more likely to be genotyping error. The mutation rate for SNPs is very, very low (about one or two changes per 100,000,000 bases) per generation. The algorithms for declaring segment matches take the error rate into account.