If Siblings Have Their Dna Tested For Genealogy Research Will Their Results Be Identical

How similar is one's DNA to their parents and siblings?

It's important to remember in these discussions that all human DNA is at least 90% the identical.Here's an answer I wrote to a similar question.William Halmeck's answer to How many genes do you share with a first cousin?Degree of relatedness is called Consanguinity, and is important when assessing inbreeding.You share, on average, 12.5% of your variant genome with your first cousin because there are three degrees of separation (or three meiotic divisions) between you and your first cousin. In actuality, you could share anywhere from 0% to 25% of your genome with a first cousin because of the nature of Meiosis. Let's look at siblings. Each parent gives you half your genome. Therefore, you share 50% of your genome with a parent. That's one degree of separation (or one meiotic division). Now, the same parents conceive your sibling. Each contributes half again, but this time meiosis divides the genes differently. Meiosis divides the chromosomes randomly every time. So, it is possible that you and your sibling share not a single gene at all (0%). Or, equally as possible, you and your sibling might share every gene (100%) or any amount between. We use the average between 0% and 100% to assign your sibling a coefficient of relatedness of 50%.Edit: Mike Wiser pointed out in his answer that consanguinity coefficients apply only to the part of the genome that differs among members of a species, and that most unrelated humans share over 90% of their genes.

If my brother and I get DNA tests done, will the results be very similar if we have the same parents?

One method for this is that you add the genetic distance of all segments (with at least 500 SNPs) which have >7 cM genetic distance. The result will be 2150–3070 for true siblings (both parents identic), and 1320–2134 for half-siblings. This data was statistically acquired, so there is in some cases an area where it is “unsure” (when for example you get a 2140 total). But usually for true siblings it is around 2600, for half-siblings around 1750, so this is pretty clear.I did the calculation once for 3 people where I knew that 2 of them were full siblings and another a half-sibling, and the result was exactly as expected.The data you can acquire by uploading your raw DNA sample to Page Redirection BTW. Or you could use the tools provided by FTDNA if you tested with them, they also provide this data. But most company’s sites also do the cM evaluation for you, so if you trust them they did not miscalculate you can just use what is displayed on their site.Be aware though to do this comparision you need to have the “okay” of all involved people (so from your brother).Asides from this you will notice a lot of “full identic segments” (green on gedmatch display) for true siblings, which are of a “longer” length, while for half-siblings the color in the display is mostly yellow, and the little green that exists is very short (just a few “green slashes”).You will also generally have longer sequences for full siblings.If you want to do this, but have trouble as to how to do this, feel free to comment to this post again, and what the problems are, and I might explain in more detail.You might also google for “Blaine T. Bettinger”, a scientist who did a lot of research in this area and there are some explanations from him available (especially charts of his statistics studies which show which cM sum could mean which degree of relation).Typical display for full siblings (that example is only 1 chromosome, the full data will of course be more of that):Typical display for half-siblings:(I did not take Chr.2 for this example as in the example I had that had more red than yellow or green - also showing the less close relation of course).Please be aware that all that what I wrote is about Autosomal testing, I am not so experienced on the other sort of tests (Y-DNA, MT-DNA). But I think for a sibling test you would do Autosomal Testing anyways.

Do siblings who share the same set of parents each have the same distribution of parental DNA? Can one sibling have more of one parent's DNA or would the parental DNA distribution be equal?

First question: No, they do not.Reason: Recombination. Before an egg precursor cell splits a full set of paired DNA (diploid) into two egg cells with one set of DNA (haploid), the complementary DNA are somewhat randomly cut up and chunks swapped between them.Since this happens separately for each cell before it splits, each egg has a different mix of DNA in the chromosomes.The same thing happens when sperm are created in the male.So your mother got one set of DNA from her mother, and the other from her father. But when the egg is split into a haploid cell, any given chromosome consists of a mix of DNA from the chromosomes she received from her mother and her father. So you cannot say that, say, a chromosome that you received from your mother, came from your grandmother. Instead, it will be a mix of DNA from both your grandmother and your grandfather, due to recombination.The only chromosome for which this is not true are the X andY from the father, in which case a couple's daughters share an X from the father, and the sons share the Y from the father. No other chromosomes are untouched by recombination.Second question: It depends. Number of chromosomes, or total amount of DNA?Because each egg cell has 23 chromosomes, and each sperm cell has 23 chromosomes, each sibling will have an equal number of chromosomes from each parent. However, the Y chromosome is substantially smaller than the X chromosome, so technically girls have more DNA from their father than boys do.There are the somewhat rare cases of a boy inheriting two Y chromosomes from the father. Other mutations/mistakes involving multiple chromosomes don't seem to survive to birth.Thank you, Dan Theman, for correcting something I’d said about XYY. Despite what I’d been told long ago and the “popular wisdom”, there does not seem to be any particular correlation between XYY syndrome and any particular difficulties in behavior, intelligence, learning, or aggressive behavior.XYY syndrome - Wikipedia

If my sister were to take an Ancestry.com DNA test, would the results be exactly like mine?

Your DNA is not identical, unless you are identical twins.  But, in practice, the kinds of matches you would get by atDNA matching services like at Ancestry would be nearly identical, especially for relations that are close enough to be relevant for genealogy, e.g., most-recent-ancestor within a time period where surnames and records exist.   This is because matches are based on things like the % of shared DNA and the length of shared segments.  So even though you each have a different mix of  DNA, when it comes to generated matches, your overall percentages will be nearly the same.In general I'd say that testing two siblings is a waste of money unless you suspect you are not really full-siblings.  You'll get more information from testing a parent.  Say you test your mother.  Then you know that any matches you have in common with her are on her side of the family, and any matches you don't share with her are on your father's side of the family.  (Assuming your parents are not close cousins!)   This is quite useful for interpreting the matches you do find.

My sister took Genetic Ancestry, would I get the same result if I take the test myself?

No. It would be similar, but there would be slight variances. Foe example, my brother and I took the same test, and our genomes came back as 52% similar.Now for the caveat. Based on historic research, about 15% of people seeking a genetic test for parental relationships discover that one or both of their parents is not, um, their genetic parent(s)! Forewarned is forearmed!

Do siblings have the same DNA?

The answer to this question is usually that siblings share 50% of their DNA.  But this is misleading.  Depending on how you count rearrangements, deletions and neutral codon assignment changes, all human genomes are between 99.0 and 99.9% identical[1] .  So the 50% difference between sibling genomes is really 50% of  that 0.1-1.0% difference, or 0.05-0.5%.It's usually less than that.  Not surprisingly, there are geographic variations in the frequencies of DNA variants[2] .  Most people's ancestors come from a single geographic region (although this pattern is breaking down), thus these ancestors are more related to each other than they are to a human being chosen at random from anywhere on the globe. Between the strikingly small degree of human genomic diversity, and the tendency to mate with (somewhat) related persons, most siblings share DNA identity of  99.5 - 99.95%.Footnotes[1] Human genome diversity: frequently asked questions.[2] Worldwide Human Relationships Inferred from Genome-Wide Patterns of Variation