X-DNA Inheritance Calculator
X-DNA inheritance calculator. See exactly which of your ancestors could have contributed to your X chromosome, with their Ahnentafel numbers and the average share each would have given — and how many of your ancestors are ruled out entirely. Works for men and women, up to twelve generations back.
X-DNA Inheritance Calculator
X-ancestors per generation
The ancestors themselves
| Sosa | Sex | Path from you | Average X share |
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Each share is an average over many families. A mother can pass on an X with no crossover in it, so in a real family an ancestor on this list may have given you a much larger share than shown, or none at all.
Look up an Ahnentafel number
How to use the X-DNA calculator
Say whether you are male or female
It changes everything. A man has no X-ancestors at all on his father’s side; a woman has them on both sides. At ten generations a woman has 144 and a man 89.
Choose how far back to look
The headline compares the ancestors who could have given you X against the full 2n at that generation. The gap widens fast: by ten generations fewer than one ancestor in ten is even eligible for a man (89 of 1,024), and about one in seven for a woman (144).
Read the list
Every eligible ancestor is listed with its Ahnentafel number, its sex, the mother-and-father path from you, and the share of your X it is expected to have contributed. The shares always add to 100%.
Test a specific ancestor
Have a Sosa number from your tree software? Enter it in the lookup and the tool gives the path it stands for and whether that person can have contributed to your X at all.
The chromosome that rules ancestors out
Most genetic genealogy works by inclusion: a shared segment tells you two people are related somewhere, and the work is finding out where. The X chromosome works the other way. It travels by rules strict enough that most of your family tree is disqualified from carrying it, and that makes an X match a useful filter on where to look.
The rules are short. A man has one X and it came entirely from his mother, because his father handed him a Y instead; he passes that X, unchanged, to every daughter and to no son. A woman has two X chromosomes, one from each parent, and passes a recombined mixture to every child. Everything else follows. Your father’s father cannot appear anywhere in your X ancestry, and nor can anyone above him: if you are a woman, that whole quarter of your tree is closed to your X; if you are a man, your father’s entire half is.
Why the count is a Fibonacci sequence
Follow the rules up a generation at a time and a pattern appears. Each man in the tree adds one ancestor above him who could have carried X — his mother. Each woman adds two, her mother and her father. So the count at any generation is the sum of the two counts before it, which is the Fibonacci rule exactly. A man’s numbers run 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89; a woman’s are the same sequence one step further along.
Set that against 2n and the contrast is stark. Ten generations back you have 1,024 ancestral slots. If you are a man, 89 of them could have given you part of your X and 935 could not. That is what a shared X segment can tell you that an autosomal one cannot: which lines it could have come down, a named and countable set. It tells you less about how far back. FamilyTreeDNA treats a 16 cM block on the X as the same probable number of shuffles as a 7 cM block on any other chromosome.
Ten generations back a man has 1,024 ancestors and 89 of them could have given him his X. The other 935 are ruled out before you start.
Ahnentafel numbers, and why they are worth learning
The list here gives every X-ancestor an Ahnentafel number, sometimes called a Sosa or Sosa-Stradonitz number. You are 1, your father is 2, your mother is 3, and the rule from there is simply that any person’s father is twice their number and their mother twice plus one. Every even number above 1 is a man; every odd number a woman. The scheme is older than DNA testing by a long way: Francis Galton set out the same numbering in Nature in 1883, in a note called “Arithmetic Notation of Kinship”.
It matters here because genealogy software speaks it. Once you know that ancestor 22 is your father’s mother’s mother’s father, you can carry the X constraint straight into your own files without redrawing anything.
Using it against a real match
FamilyTreeDNA reports X matches, and 23andMe includes the X chromosome when it compares members. If you share an X segment with someone and it came from a recent common ancestor, that ancestor has to sit on an X path in your tree and on an X path in theirs. So print the list on this page for your sex and depth, ask your match to do the same for theirs, and look for couples who appear on both.
That narrows where to look. It does not prove a line. X segments stay long for their age, so a shared one may come from further back than either tree reaches, and FamilyTreeDNA will not list someone as a match on X DNA alone: they must meet its minimum shared amount without counting the X. This page does not tell you how you are related to anyone.
Sources
- FamilyTreeDNA Help Center, X-Matches (updated 8 January 2026): the transmission rules, the 16 cM to 7 cM equivalence, and the two counting rules quoted above.
- 23andMe Customer Care, DNA Relatives: Detecting Relatives and Predicting Relationships (updated 13 October 2026): a woman inherits an X from each parent, a man from his mother only; X is compared alongside chromosomes 1 to 22.
- Francis Galton, “Arithmetic Notation of Kinship”, Nature 28, 435 (6 September 1883): the father-2, mother-3 numbering.
- The counts and shares are computed on this page from the two transmission rules; the Fibonacci pattern follows from them and is checked against every Ahnentafel number up to twelve generations in the tool’s test suite.
X-ancestors follow the Fibonacci sequence, and your father's father is never one
A man inherits his single X entirely from his mother.
A man’s father contributes none of his X — he gave the Y instead.
A woman’s paternal X comes wholly from her paternal grandmother.
The count of X-ancestors per generation follows the Fibonacci sequence.
Ten generations back a man has 89 X-ancestors out of 1,024.
A woman’s count at any generation equals a man’s one generation further back.
Ahnentafel slot 4, your father’s father, is never on an X path to you.
In Ahnentafel numbering a father is 2N and a mother 2N + 1, so the numbers double every generation.
Francis Galton described the same numbering in Nature in 1883.
A listed share is an average: a real ancestor on the list may have given you none of your X.
Frequently asked questions
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Two rules produce everything else. A man has one X and it came entirely from his mother, because his father gave him a Y instead; he passes that X unchanged to every daughter and to no son. A woman has two X chromosomes, one from each parent, and passes a recombined mixture of them to every child. Follow those rules up a family tree and most branches close off.
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Because of how the two rules compose. Each man in the tree contributes one X-ancestor in the generation above him — his mother — while each woman contributes two, her mother and her father. So the count at each generation is the previous count plus the one before it, which is the Fibonacci rule exactly. A man’s counts run 1, 1, 2, 3, 5, 8, 13, 21, 34; a woman’s begin one step further along.
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It is a numbering scheme for ancestors: you are 1, your father is 2, your mother is 3, and from then on any person’s father is twice their number and their mother twice plus one. Every even number above 1 is a man and every odd number a woman. It is also called Sosa or Sosa-Stradonitz numbering, and Francis Galton set out the same scheme in Nature in 1883, in a note called “Arithmetic Notation of Kinship”.
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Because it narrows where to look. An autosomal segment could have come down any line of your tree, but an X segment can only have reached you along the paths listed here. If you and a match share X DNA from a recent common ancestor, that ancestor has to sit on an X path in your tree and on an X path in theirs. It is a filter, not a proof. FamilyTreeDNA says a 16 cM block on the X represents the same probable number of shuffles as a 7 cM block on any other chromosome, because the X passes through men unbroken, so a long X segment can be older than it looks and the shared ancestor may sit further back than either tree reaches.
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They are expected shares, not measured ones. A man’s X comes half from his maternal grandmother and half from his maternal grandfather on average, because his mother recombined her two X chromosomes before passing one on. A woman’s paternal X, by contrast, is a fixed 100% from her paternal grandmother — her father had only the one X and passed it whole. The shares at any generation always add to 100%. Each one is an average over many families: a mother can pass on one of her two X chromosomes with no crossover in it, so in a real family an ancestor listed at 25% may have given you a much larger part of your X, or none of it.
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In women, yes: her two X chromosomes exchange material before one is passed on. In men, no — he has only one X and no second copy to recombine with, apart from the small pseudoautosomal regions at the tips. This is why an X passes from a father to his daughter intact, and why FamilyTreeDNA treats a 16 cM block on the X as equivalent, in shuffles, to a 7 cM block on any other chromosome.
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The rules are the same but the answers differ, and the difference is large. A woman has X-ancestors on both sides of her tree; a man has none at all on his father’s side, because he did not get an X from his father. At ten generations a woman has 144 X-ancestors where a man has 89, and none of a man’s are on his paternal side.
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Yes, as a filter. FamilyTreeDNA reports X matches, and 23andMe says it compares the X chromosome as well as chromosomes 1 to 22. FamilyTreeDNA sets two rules worth knowing: centimorgans on the X “do not contribute to overall shared cM”, and someone appears on your match list only if you share its minimum amount of DNA without counting the X. Use the list here to see which of your lines an X segment could have come down; a recent common ancestor has to be on an X path in your tree and in your match’s. This page does not say how you are related to anyone.
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Twelve generations, at which point a woman has 377 X-ancestors and a man 233, out of 4,096 ancestors each. Beyond that the shares become so small that the arithmetic stops being useful — and pedigree collapse means the real ancestors at that depth are far fewer than the theoretical count anyway.
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Method & sources
How it computes
Two transmission rules: a man has one X, from his mother, and passes it whole to daughters only; a woman has one X from each parent and passes a recombined one. Walking every Ahnentafel slot (father 2N, mother 2N + 1) up to 12 generations gives the eligible ancestors, whose count follows Fibonacci, and each one's average share of your X (half per step through a woman). Averages for an idealised pedigree: a real ancestor on the list may have given none.
What this tool implements
- X transmission as FamilyTreeDNA and 23andMe state it: a man's single X comes from his mother and passes to his daughters unrecombined and to no son; a woman inherits an X from each parent. Pseudoautosomal regions are not modelled.
- Ahnentafel (Sosa-Stradonitz) numbering: self 1, father of N 2N, mother of N 2N + 1, as set out by Galton (1883) Nature 28:435 "Arithmetic Notation of Kinship".
- Corrected 2026-10-14: the ladder stopped at 10 generations while the slider goes to 12; the Ahnentafel box read "1e21" as 1, "2.9" as 2 and numbers above 2^53 - 1 as different numbers (now digits only, up to 9,007,199,254,740,991); "fewer than one ancestor in ten" is now said of a man only (89 of 1,024; a woman 144); the About text said a man loses a quarter of his tree to his father's father where he loses his father's whole half; an X match is described as a filter, not a deduction, with FamilyTreeDNA's own counting rules.
Sources
- FamilyTreeDNA Help Center. X-Matches (updated 8 Jan 2026). https://help.familytreedna.com/hc/en-us/articles/9134064538895-X-…
- 23andMe Customer Care. DNA Relatives: Detecting Relatives and Predicting Relationships (updated 13 Oct 2026). https://support.23andme.org/hc/en-us/articles/42963245607063
- Galton F. Arithmetic Notation of Kinship. Nature. 1883;28:435. https://www.nature.com/articles/028435b0
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