Sex-Linked Inheritance Calculator

Background.

Haemophilia has a way of skipping generations. A man with the condition has healthy children, and years later one of his daughters has a son who bleeds the way his grandfather did. The explanation is the X chromosome. A father gives his single X to every daughter and his Y to every son, so his sons never receive the variant and his daughters always do. Those daughters are usually well, because their other X carries a working copy, but each son they have stands an even chance of inheriting it.

Because sons and daughters receive their X chromosomes by different routes, the usual Punnett square splits into two, one for each sex, and the odds differ between them. Choose the inheritance pattern and each parent's genotype, and you get the chance that a son is affected, the chance that a daughter is affected, the share of daughters who are heterozygous, the unaffected fractions, and the genotype ratios behind every figure.

Two patterns are covered. In X-linked recessive inheritance — haemophilia A and B, Duchenne muscular dystrophy, red-green colour vision deficiency — one copy of the variant is enough in a male, while a female needs two, which is why affected males far outnumber affected females. In X-linked dominant inheritance, such as X-linked hypophosphataemia, one copy is enough in either sex, and an affected father passes the condition to all of his daughters and none of his sons.

The results are exact Mendelian ratios for one X-linked gene with full penetrance, not a personal risk. Carrier women occasionally show symptoms, new mutations arise without warning, and decisions about a real family belong with a genetic counsellor.

What is sex-linked inheritance calculator?

An X-linked gene sits on the X chromosome, outside the short pseudoautosomal stretches that X and Y share. Females, with two X chromosomes, carry two copies and can be homozygous or heterozygous. Males carry one and are called hemizygous: there is no second copy to mask a variant, so whatever they inherit is expressed. Mothers pass one of their two X chromosomes at random to every child; fathers pass their X to daughters only. The resulting signatures are distinctive — in recessive pedigrees, affected men linked through unaffected women and no father-to-son transmission; in dominant pedigrees, affected fathers whose daughters are all affected.

How to use this calculator.

1. 01Choose X-linked recessive or X-linked dominant. The labels never change meaning — X^A is the dominant allele and X^a the recessive one — but the pattern decides which of them causes the condition, so under the dominant pattern an unaffected person carries X^a.
2. 02Set the mother's genotype: X^A X^A, X^A X^a or X^a X^a. If you only know the family history, estimate her carrier chance with the pedigree probability calculator first.
3. 03Set the father's genotype, X^A Y or X^a Y. A man has one X, so he cannot be heterozygous at an X-linked gene.
4. 04Read Affected sons and Affected daughters first: each is the chance for a child of that sex. The overall figure assumes equal numbers of sons and daughters.
5. 05Check Heterozygous daughters: under the recessive pattern they are carriers, usually unaffected; under the dominant pattern they are affected.

The formula.

Each sex gets its own two-box Punnett square. The mother contributes one of her two X chromosomes, each with probability 1/2. The father's contribution decides the child's sex: his X makes a daughter and his Y makes a son. So a son's genotype depends only on which X his mother passes, while a daughter's combines her mother's X with her father's, which is fixed. For each sex, P(outcome) = (matching boxes) ÷ 2, giving 0%, 50% or 100%. A son is affected whenever his one X carries the disease allele, under either pattern. A daughter is affected under the recessive pattern only if both of her X chromosomes carry it, and under the dominant pattern if either does — all the difference between the two patterns lies with the daughters. The overall figure is the plain average of the son and daughter results, (sons + daughters) ÷ 2, so it can also take 25% or 75%. Changing the father's genotype never changes any son's result, because the father's X never reaches a son. The model treats one gene with full penetrance in 46,XX and 46,XY children; it does not cover Y-linked or pseudoautosomal genes, new mutations, germline mosaicism, sex-chromosome aneuploidies such as 45,X or 47,XXY, or carrier women who show symptoms because of skewed X-inactivation.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Bin Shlhoob R., Tanaka J. & Pandya A. M. Genetics, X-Linked Inheritance. StatPearls, NCBI Bookshelf ID NBK557383, last update 22 March 2026. Primary source for every probability on this page: X-linked recessive carrier mother × unaffected father — 'each male offspring has a 50% chance of being affected, and each female offspring has a 50% chance of being a carrier'; affected father — 'none of his male offspring are affected, but all of his female offspring are carriers'; X-linked dominant — 'Affected males can transmit the mutant allele to all their daughters but not to their sons' and 'Affected females have a 50% chance of transmitting the mutant allele to each of their sons and daughters'. Free full text. Retrieved 2026-07-29.
2. [2]MedlinePlus Genetics, US National Library of Medicine. Inheritance Patterns, page last updated 19 April 2021. Independent confirmation of the affected-genotype rules: X-linked recessive — 'In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females, a variant would have to occur in both copies of the gene to cause the disorder'; X-linked dominant — 'a variant in one of the two copies of the gene in each cell is sufficient to cause the disorder. Females may experience less severe symptoms than males.' Free. Retrieved 2026-07-29.
3. [3]Politano L. (2025). Females with X-Linked Muscle Disorders: an underestimated patient population. Acta Myologica 44(1):33–36. doi:10.36185/2532-1900-1096, PMID 40183439. Peer-reviewed source for the manifesting-carrier caveat carried beside the heterozygous-daughters output: skewed X-chromosome inactivation is defined as 'more than 75% of an individual's cells choose one parent's X chromosome as the inactive X'; 'it is estimated that 1.5-23% of females have skewed X inactivation'; and 'about 2.5-18% of DMD/BMD carriers may present with varying degrees of muscle weakness'. Open access via PMC. Retrieved 2026-07-29.
4. [4]Gulani A. & Weiler T. Genetics, Autosomal Recessive. StatPearls, NCBI Bookshelf ID NBK546620, last update 1 May 2023. Used for the contrast with autosomal inheritance — the 25% / 50% / 25% carrier × carrier outcome that X-linked inheritance departs from because males are hemizygous. Free full text. Retrieved 2026-07-29.
5. [5]Genetic Alliance & New York–Mid-Atlantic Consortium for Genetic and Newborn Screening Services. Understanding Genetics, 'Inheritance Patterns'. NCBI Bookshelf ID NBK115561, 2009. Source for the pedigree signatures quoted on this page — X-linked recessive: 'Males are more frequently affected; affected males often present in each generation'; X-linked dominant: 'Females are more frequently affected because all daughters and no sons of an affected man will be affected'. Free full text. Retrieved 2026-07-29.